Now let’s bust the myth of weightlessness in space for good…
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Click here to go to next lesson on Binary System.
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A binary system exists when objects approach each other in size (and gravitational fields), the common point they rotate around (called the center of mass) lies outside both objects and they orbit around each other. Astronomers have found binary planets, binary stars, and even binary black holes.
The path of a planet around the Sun is due to the gravitational attraction between the Sun and the planet. This is true for the path of the Moon around the Earth, and Titan around Saturn, and the rest of the planets that have an orbiting moon.
Materials
- Soup cans or plastic containers with holes punched (like plastic yogurt containers, butter tubs, etc.)
- String
- Water
- Sand
- Rocks
- Pebbles
- Baking soda
- Vinegar
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Download Student Worksheet & Exercises
- Thread one end of the string through one of the holes and tie a strong knot. Really strong.
- Tie the other end through the other hole and tie off.
- Go outside.
- Fill your can partway with water.
- Move away from everyone before you start to swing your container in a gentle circle. As you spin faster and faster, notice where the water is inside the container.
- Now empty out the water and replace it with rocks. Spin again and fill out the data table.
- To make carbon dioxide gas, you’ll need to work with another lab team. Cover the bottom of your container with baking soda. Add enough vinegar so that the bubbles reach the top without overflowing. Wait patiently for the bubbles to subside. You now have a container filled with carbon dioxide gas (and a little sodium acetate, the leftovers from the reaction). Carefully pour this into the empty container from the other lab team. They can spin again and record their results. When they are done, borrow their container and give them yours so they can fill it for you.
What’s Going On?
The path of a planet around the Sun is due to the gravitational attraction between the Sun and the planet. This is true for the path of the Moon around the Earth, and Titan around Saturn, and the rest of the planets that have an orbiting moon.
Charon and Pluto orbit around each other due to their gravitational attraction to each other. However, Charon is not the moon of Pluto, as originally thought. Pluto and Charon actually orbit around each other. Pluto and Charon also are tidally locked, just like the Earth-Moon system, meaning that one side of Pluto is always faces the same side of Charon.
Imagine you have a bucket half full of water. Can you tilt a bucket completely sideways without spilling a drop? Sure thing! You can swing it by the handle, and even though it’s upside down at one point, the water stays put. What’s keeping the water inside the bucket?
Before we answer this, imagine you are a passenger in a car, and the driver is late for an appointment. They take a turn a little too fast, and you forgot to fasten your seat belts. The car makes a sharp left turn. Which way would you move in the car if they took this turn too fast? Exactly – you’d go sliding to the right. So, who pushed you?
No one! Your body wanted to continue in a straight line, but the car is turning, so the right side car door keeps pushing you to turn you in a curve – into the left turn. The car door keeps moving in your way, turning you into a circle. The car door pushing on you is called centripetal force. Centripetal means “center-seeking.” It’s the force that points toward the center of the circle you’re moving on. When you swing the bucket around your head, the bottom of the bucket is making the water turn in a circle and not fly away. Your arm is pulling on the handle of the bucket, keeping it turning in a circle and not letting it fly away. That’s centripetal force.
Think of it this way: If I throw a ball in outer space, does it go in a straight line or does it wiggle all over the place? Straight line, right? Centripetal force is the force needed to keep an object following a curved path.
Remember how objects will travel in a straight line unless they bump into something or have another force acting on them, such as gravity, drag force, and so forth? Well, to keep the bucket of water swinging in a curved arc, the centripetal force can be felt in the tension experienced by the handle (or your arm, in our case). Swinging an object around on a string will cause the rope to undergo tension (centripetal force), and if your rope isn’t strong enough, it will snap and break, sending the mass flying off in a tangent (straight) line until gravity and drag force pull the object to a stop. This force is proportional to the square of the speed – the faster you swing the object, the higher the force.
Exercises
- How is spinning the container like Pluto and Charon?
- What would happen if we cut the string while you are spinning? Which way would the container go?
- What happens if we triple the size of your container and what’s inside of it?
Click here to go to next lesson on What’s Up in the Sky?
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Today you get to learn how to read an astronomical chart to find out when the Sun sets, when twilight ends, which planets are visible, when the next full moon occurs, and much more. This is an excellent way to impress your friends.
The patterns of stars and planets stay the same, although they appear to move across the sky nightly, and different stars and planets can be seen in different seasons.
Materials:
- Printout of Stargazer’s Almanac
- Pencil
- Tape and scissors (optional)
- Ruler
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Download Student Worksheet & Exercises
- If your chart comes on two pages, you’ll need to cut the borders off at the top and bottom and tape them together so they fit perfectly.
- Use your ruler as a straight edge to help locate items as you read the chart.
- Print out copies of the almanac by clicking the image of the Skygazer’s Almanac. You can print it full-size on two pages, or size it to fit onto a single page. Since there’s a ton of information on it, it’s best read over two pages. This is an expired calendar to practice with.
- First, note the “hourglass” shape of the chart. Do you see how it’s skinnier in the middle and wider near the ends? Since it’s an astronomical chart that shows what’s up in the sky at night, the nights are shorter during the summer months, so the number of hours the stars are visible is a lot less than during the winter. You’ll find the hours of the night printed across the top and bottom of the chart (find it now) and the months and days of the year printed on the right and left side.
- Can you find the summer solstice on June 20? Use your finger and start on the left side between June 17 and June 24. The 20th is between those two dates somewhere. Here’s how you tell exactly…
- Look at the entire chart – do you see the little dots that make up little squares all over the chart, like a grid? Each dot in the vertical direction represents one day. There are eight dots on the vertical side of the box.
- Let’s say you want to find out what time Neptune rises on June 17. Go back to June 17, which has its own little set of dots. Follow the dots with your finger until you hit the line that says Neptune Rises. Stop and trace it up vertically to the top scale to read just after 11 p.m.
- Look again at the dot boxes. Each horizontal dot is 5 minutes apart, and every six dots there is a vertical line representing the half-hour. The line crosses between the second and third dot, so if you lived in a place where you can clearly see the eastern horizon and looked out at 11:07, you’d see Neptune just rising. Since Uranus and Neptune are so far away, though, you’d need a telescope to see them. So let’s try something you can find with your naked eye.
- Look at Oct 21. What time does Saturn set? (5:30p.m.).
- What other two planets set right afterward? (Mercury at 6:03 p.m. and Mars sets at 7:12 p.m.).
- When does Jupiter rise? (7:32 p.m.).
- What is Neptune doing that night of Oct. 21? (Neptune transits, or is directly overhead, at 8:07 p.m. and sets at 1:30 a.m.)
- What other interesting things happen on Oct. 21? (Betelgeuse, one of the bright stars in the constellation Orion, rises at 9:23 p.m. Sirius, the dog star, rises at 11:06 p.m. The Pleiades, also known as the Seven Sisters, are overhead at 1:42 a.m.)
- Let’s find out when the Moon rises on Oct. 21. You’ll find a half circle representing the Moon centered on 11:05 p.m. Which phase is the Moon at? First or third quarter? (First. You can tell if you look at the next couple of days to see if the Moon waxes or wanes. Large circles indicate one of the four main phases of the Moon.)
- When does the Sun rise and set for Oct. 21? First, find the nearest vertical set of dots and read the time (5:30 p.m.). Now subtract out the 5-minute dots until you get to the edge. You should read three dots plus a little extra, which we estimate to be 17 minutes. Sunset is at 5:13 p.m. on Oct 21.
- Note the fuzzy, lighter areas on both sides of the hourglass. That represents the twilight time when it’s not quite dark, but it’s not daylight either. There’s a thin dashed line that runs up and down the vertical, following the curve of the hourglass offset by about an hour and 35 minutes. That’s the official time that twilight ends and the night begins.
- Can you find a meteor shower? Look for a starburst symbol and find the date right in the center. Those are the peak times to view the shower, and it’s usually in the wee morning hours. The very best meteor showers are when there’s also a new Moon nearby.
- Notice how Mercury and Venus stay close by the edges of the twilight. You’ll find a half-circle symbol representing the day that they are furthest from the Sun as viewed from the Earth, which is the best date to view it. For Venus, the * indicates the day that it’s the brightest.
- What do you think the open circle means at sunset on May 20? (New Moon)
- Students that spot the “Sun slow” or “Sun fast” marks on the chart always ask about it. It’s actually rather complicated to explain, but here’s the best way to think about it. Imagine that the vertical timeline running down the center means noon, not midnight. Do you see a second line weaving back and forth across the noon line throughout the year? That’s the line that shows the when the Sun crosses the meridian. On Feb 5, the Sun crosses that meridian at 12:14, so it’s “running slow,” because it “should have” crossed the meridian at noon. This small variation is due to the axis tilt of the Earth. Note that it never gets much more than 15 minutes fast or slow. The wavy line that represents this effect is called the Equation of Time. We’ll be using that later when we make our own sundials and have to correct for the Sun not being where it’s supposed to be.
- Look at Mars and Saturn both setting around the same time on Aug. 14. When two event lines cross, you’ll find nearby an open circle with a line coming from the top right side, accompanied by a set of arrows pointing toward each other. This means conjunction, and is a time when you can see two objects at once. Usually the symbol isn’t right at the intersection, because one of the objects is rising or setting and isn’t clearly visible. On Aug. 14, you’ll want to view them a little before they set, so the symbol is moved to a time where you can see them both more clearly.
- Important to note: If your area uses daylight savings time, you’ll need to add one hour to the times shown on the chart.
- Time corrections for advanced students: This chart was made for folks living on the 40o north latitude and 90o west longitude lines (which is Peoria, Ill.).
- If you live near the standardized longitudes for Eastern Time (75o), Central (90o), Mountain (105o) or Pacific (120o), then you don’t have to correct the chart times you read. However, if you live a little west or east of these standardized locations, you need a correction, which looks like this:
- For every degree west, add four minutes to the time you read off the chart.
- For every degree east, subtract four minutes from the time.
- For example, if you lived in Washington, D.C. (which is 77o longitude), note that this is 2o west of the Eastern Time, so you’d add 8 minutes to the time you read off the chart. Memorize your particular adjustment and always use it.
- If your latitude isn’t 40o north, then you need to adjust the rise and set times like this:
- If you live north of 40o, then the object you are viewing will be in the sky for longer than the chart shows, as it will rise earlier and set later.
- If you live south of 40o, then the object you are viewing will be in the sky for less time than the chart shows, as it will rise later and set earlier.
- The easiest way to calculate this is to note what time an object should rise, and then watch to see when it actually appears against a level horizon. This is your correction for your location.
What’s Going On?
This is one of the finest charts I’ve ever used as an astronomer, as it has so much information all in one place. You’ll find the rise and set times for all eight planets, peak times for annual meteor showers, moon phases, sunrise and set times, and it gives an overall picture of what the evening looks like over the entire year. Kids can clearly see the planetary movement patterns and quickly find what they need each night. I keep one of these posted right by the door for everyone to view all year long.
- If you live near the standardized longitudes for Eastern Time (75o), Central (90o), Mountain (105o) or Pacific (120o), then you don’t have to correct the chart times you read. However, if you live a little west or east of these standardized locations, you need a correction, which looks like this:
Exercises
- Is Mercury visible during the entire year?
- In general, when and where should you look for Venus?
- When is the best time to view a meteor shower?
- Which date has the most planets visible in the sky?
Click here to go to next lesson on Build a Solar System.
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What would happen if our solar system had three suns? Or the Earth had two moons? You can find out all these and more with this lesson on orbital mechanics. Instead of waiting until you hit college, we thought we'd throw some university-level physics at you... without the hard math.
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To get you experienced with the force of gravity without getting lost in the math, there's an excellent computer program that allows you to see how multi-object systems interact. Most textbooks are limited to the interaction between a very large object, like the Earth, and much smaller objects that are very close to it, like the Moon. This seriously cuts out most of the interesting solar systems that are out there in the real universe.
The University of Colorado at Boulder designed a great system to do the hard math for you. Don't be fooled by the simplistic appearance - the physics behind the simulation is rock-solid... meaning that the results you get are exactly what scientists would predict to happen.
How do I design a solar system?
Go to the My Solar System simulation on the PhET website and carefully follow the instructions for each activity. Answer the questions and record your results before going on to the next activity. Click here to RUN the simulation. If that link doesn't work try this alternative.
Here's what you should see and do:
Download Student Worksheet & Exercises
Exercises:
- What effect does changing the mass of orbiting planet have on the diameter of the orbit?
- What effect does changing the speed have on a planet’s orbit?
- What happens to the planet's orbit when you increase the initial distance between the planet and the Sun?
- Find the mass values needed for a stable orbit. Circle the values on the table that make a stable orbit.
- Why don’t a feather and a brick hit the ground at the same time?
Click here to go to next lesson on Retrograde Motion.
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If you watch the moon, you’d notice that it rises in the east and sets in the west. This direction is called ‘prograde motion’. The stars, sun, and moon all follow the same prograde motion, meaning that they all move across the sky in the same direction.
However, at certain times of the orbit, certain planets move in ‘retrograde motion’, the opposite way. Mars, Venus, and Mercury all have retrograde motion that have been recorded for as long as we’ve had something to write with. While most of the time, they spend their time in the ‘prograde’ direction, you’ll find that sometimes they stop, go backwards, stop, then go forward again, all over the course of several days to weeks.
Here are videos I created that show you what this would look like if you tracked their position in the sky each night for an year or two.
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Mercury and Venus Retrograde Motion
This is a video that shows the retrograde motion of Venus and Mercury over the course of several years. Venus is the dot that stays centered throughout the video (Mercury is the one that swings around rapidly), and the bright dot is the sun. Note how sometimes the trace lines zigzag, and other times they loop. Mercury and Venus never get far from the sun from Earth’s point of view, which is why you’ll only see Mercury in the early dawn or early evening.
Retrograde Motion of Mars
You’ve probably heard of epicycles people used to use to help explain the retrograde motion of Mars. Have you ever wondered what the fuss was all about? Here’s a video that traces out the path Mars takes over the course of several years. Do you see our Moon zipping by? The planets, Sun, and Moon all travel along line called the ‘ecliptic’, as they all are in about the same plane.
Download Student Worksheet & Exercises
Exercises
- During which of the months does Mars appear to move in retrograde?
- Why does Mars appear to move backward?
- Which planets have retrograde motion?
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Click here to go to next lesson on Potential Energy in the Stars.
In our physics problems so far, we’ve kept the objects close to the earth so that the acceleration due to gravity g remains constant, and we defined the potential energy of an object on the surface of the earth to be zero. What if we look up and see three stars in a system and want to find out the gravitational potential energy of the system?
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Globular clusters have a HUGE amount of gravitational potential energy.
This image above is known as Messier 13 (M13) or NGC 6205, also called the Hercules Globular Cluster, is a home to 300,000 stars in the constellation of Hercules. Can you start to see how there’s an enormous amount of gravitational potential energy in the universe?
Click here to go to next lesson on Escape Speed.
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Let’s determine the gravitational potential energy of the earth-moon system as well as the speed needed to escape the Earth’s gravitational pull:
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Click here to go to next lesson on Conservation of Mechanical Energy.
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Here’s how you put it all together and figure out the total energy of the system. This is useful when you’re trying to figure out something that you can’t otherwise solve for… let me show you with a set of videos here. Remember, for satellites the only force we have on the object is due to gravity, so the external work force term always goes to zero like this:
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Click here to go to next lesson on Geosynchronous Orbit.
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A geosynchronous orbit is an orbit that a satellite has when viewed from the earth, looks like the satellite is stationary.
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Click here to go to next lesson on Gravitational Potential Energy.
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How do you find the gravitational potential energy between two objects that are really far apart, like two stars or two galaxies? Here’s how:
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Click here to go to next lesson on Rocketship Races.
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Imagine you and I are racing rocketships in orbit around the Earth. I can slow down and still beat you around the Earth. Want to see how?
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Click here to go to next lesson on Equivalence Principle.
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Einstein once said: “I was sitting in a chair in the patent office at Bern when all of the sudden a thought occurred to me: If a person falls freely, he will not feel his own weight. I was startled. This simple thought made a deep impression on me. It impelled me toward a theory of gravitation.”
This led Einstein to develop his general theory of relativity, which interprets gravity not as a force but as the curvature of space and time. This topic is out of the scope for our lesson here, but you can explore more about it in this lesson.
The fundamental principle for relativity is the principle of equivalence, which says that if you were locked up in a box, you wouldn’t tell the difference between being in a gravitational field and accelerating (with an acceleration value equal to g) in a rocket.
The same thing is also true if you were either locked in a box, floating in outer space or in an elevator shaft experiencing free-fall. Any experiments you could do in either of those cases wouldn’t be able to tell you what was really happening outside your box. The way a ball drops is exactly the same in either case, and you would not be able to tell if you were falling in an elevator shaft or drifting in space.
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Have you ever seen an icicle? They usually grown downward toward the center of the earth (the direction of free-fall). But icicles on a car wheel grow radially outward because the wheel spins and flings the water toward the outside of the wheel where it freezes into spikes. The icicles can’t tell if the wheel is rotating and that’s why they grow radially, or it’s at rest at the gravitational field is in the radial direction!
Here’s a questions for you: these two astronauts (below) are inside the space station, which is currently in orbit around the earth. Which astronaut is upside down? Can you tell?
The principle of equivalence has some consequences! Navigation systems for ships, airplanes, missiles and submarines rely on acceleration information to calculate their velocity and position. However, the instruments that measure acceleration also react with unexpected variations in the earth’s gravitational field, and there’s no direct way to separate these two effect to avoid errors.
Highlights for Kepler’s Laws:
- The Law of Orbits: All planets move in elliptical orbits, with the sun at one focus.
- The Law of Areas: A line joining the planet to the sun sweeps out equal areas in equal times.
- The Law of Periods: The square of the period of any planet is proportional tot he cube of the semi-major axis of its orbit.
Yay! You completed this set of lessons on circular motion! Now it’s time for you to work your own physics problems!
Download your Circular Motion Problem Set here.
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Gravity is the reason behind books being dropped and suitcases feeling heavy. It’s also the reason our atmosphere sticks around and oceans staying put on the surface of the earth. Gravity is what pulls it all together, and we’re going to look deeper into what this one-way attractive force is all about.
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Galileo was actually one of the first people to do science experiment on gravity.Galileo soon figured out that objects could be the same shape and different weights (think of a golf ball and a ping pong ball), and they will still fall the same. It was only how they interacted with the air that caused the fall rate to change. By studying ramps (and not just dropping things), he could measure how long things took to drop using not a stopwatch but a water clock (imagine having a sink that regularly dripped once per second).
Whenever I teach a class about gravity, I’ll drop something (usually something large). After the heads whip around, I ask the hard question: “Why did it fall?”
You already know the answer – gravity. But why? Why does gravity pull things down, not up? And when did people first start noticing that we stick to the surface of the planet and not float up into the sky? No one can tell you why gravity is – that’s just the way the universe is wired. Gravitation is a natural thing that happens when you have mass.
Would it sound strange to you if I said that gravity propagates at the speed of light? If we suddenly made the sun disappear, the Earth’s orbit wouldn’t be instantaneously affected… it would take time for that information to travel to the earth. What does that mean? By the end of this section, you’ll be able to tell me about it. Let’s get started!
So far, saying the force of gravity is pretty comfortable. When you throw a ball high in the air, the force of gravity slows it down and as it falls back to the earth the force of gravity speeds the ball up. The force of gravity causes an acceleration during this flight, and is called the acceleration of gravity. The acceleration of gravity g is the acceleration experienced by an object when the only force acting on it is the force of gravity. This value of g is the same no matter how massive the object is. It’s always 9.81 m/s2.
Johannes Kepler, a German mathematician and astronomer in the 1600s, was one of the key players of his time in astronomy. Among his best discoveries was the development of three laws of planetary orbits. He worked for Tycho Brahe, who had logged huge volumes of astronomical data, which was later passed onto to Kepler. Kepler took this information to design and develop his ideas about the movements of the planets around the Sun. We’re going to go into deeper discussion about Kepler’s Laws in the next section, but here they are in a nutshell:
- The Law of Orbits: All planets move in elliptical orbits, with the sun at one focus.
- The Law of Areas: A line joining the planet to the sun sweeps out equal areas in equal times.
- The Law of Periods: The square of the period of any planet is proportional tot he cube of the semi-major axis of its orbit.
Did you notice that while Kepler’s Laws describe the motion of the planets around the sun, they don’t say why these paths are there? Kepler only hinted at an interaction between the sun and the planets to drive their motion, but not between the planets themselves, and it really was only a teensy hint.
Newton wasn’t satisfied with this explanation. He was determined to figure out the cause for the elliptical motion, especially since it wasn’t a circle or a straight line (remember Newton’s First Law: Objects in motion tend to stay in motion unless acted upon by an unbalanced force?) And circular motion needs centripetal force to keep the object following a curved path. So what force was keeping the planets in an ellipse around the sun?
Click here to go to next lesson on Inverse Square Law.
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One of Newton’s biggest contributions was figuring out how to show that gravity was the same force that caused both objects like an apple to fall to the earth at a rate of 9.81 m/s2 AND the moon being accelerated toward the earth but at a different rate of 0.00272 m/s2. If these are both due to the same force of gravity, why are they different numbers then? Why is the acceleration of the moon 1/3600th the acceleration of objects near the surface of the earth? It has to do with the fact that gravity decreases the further you are from an object. The moon is in orbit about 60 times further from the earth’s center than an object on the surface of the earth, which indicates that gravity is proportional to the inverse of the square of the distance (also called the inverse square law). So the force of gravity acts between any two objects and is inversely proportional to the square of the distance between the two centers. The further apart the objects are, the less they force of gravity is between the two of them. If you separate the objects by twice the distance, the gravitational force goes down by a factor of 4.
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Click here to go to next lesson on Applying the Inverse Square Law.
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All objects are attracted to each other with a gravitational force. You need objects the size of planets in order to detect this force, but everything, everywhere has a gravitational field and force associated with it. If you have mass, you have a gravitational attractive force. Newton’s Universal Law of Gravitation is amazing not because he figured out the relationship between mass, distance, and gravitational force (which is pretty incredible in its own right), but the fact that it’s universal, meaning that this applies to every object, everywhere.
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Newton suggested that every particle everywhere attracts every other particle with a force given by the following equation. If you have mass, then this force applies to you. Newton’s Universal Law of Gravitation is: 
That G is called the universal gravitation constant and is determined by doing experiments.
Click here to go to next lesson on Cavendish Experiment.
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Lord Henry Cavendish in 1798 (about a century after Newton) performed experiments with a torsion balance to figure out the value of G. It’s a very small number, so Cavendish had to carefully calibrate his experiment! The reason the number is so small is because we don’t see the effects of gravity until objects are very massive, like a moon or a planet in size.
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Cavendish used an experiment where two small lead spheres were fastened to the ends of a rod which had a very fine string (actually a quartz fiber) attached to the middle so it could be lifted off the ground. This is called a torsion balance, meaning that you can carefully measure the twist in the string by measuring how much the rod spins around. (Torsion balances can be made from other materials that have a stiffer spring constant value, like metal rods.) Back to his experiment: Cavendish placed two large lead spheres next to the smaller spheres, which moved the larger spheres and exerted a torque on the rod, and Cavendish was able to calculate the value of G.
The value of G is always the same, everywhere you go and any situation you apply it to. Once you know the masses and distances between objects, you can always calculate the force due to gravity with this one equation.
Although Newton’s Law of Gravitation applies only to particles, you can apply it to real objects as long as the sizes of the objects is small when you compare it to the distances between them.
You can concentrate an objects mass by shrinking it down to a particle using the idea of the center of mass like this:
Let’s practice it now…
Click here to go to next lesson on Star Wobble.
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How do astronomers find planets around distant stars? If you look at a star through binoculars or a telescope, you’ll quickly notice how bright the star is, and how difficult it is to see anything other than the star, especially a small planet that doesn’t generate any light of its own! Astronomers look for a shift, or wobble, of the star as it gets gravitationally “yanked” around by the orbiting planets. By measuring this wobble, astronomers can estimate the size and distance of larger orbiting objects.
Doppler spectroscopy is one way astronomers find planets around distant stars. If you recall the lesson where we created our own solar system in a computer simulation, you remember how the star could be influenced by a smaller planet enough to have a tiny orbit of its own. This tiny orbit is what astronomers are trying to detect with this method.
Materials
- Several bouncy balls of different sizes and weights, soft enough to stab with a toothpick
- Toothpicks
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Download Student Worksheet & Exercises
- Does your ball have a number written on it? If so, that’s the weight, and you can skip measuring the weight with a scale.
- If not, weigh each one and make a note in the data table.
- Take the heaviest ball and spin it on the table. Can you get it to spin in place? That’s like a Sun without any planets around it.
- Insert a toothpick into the ball. Now insert the end of the toothpick into the smallest weight ball. Now spin the original ball. What happened?
What’s Going On?
Nearly half of the extrasolar (outside our solar system) planets discovered were found by using this method of detection. It’s very hard to detect planets from Earth because planets are so dim, and the light they do emit tends to be infrared radiation. Our Sun outshines all the planets in our solar system by one billion times.
This method uses the idea that an orbiting planet exerts a gravitational force on the Sun that yanks the Sun around in a tiny orbit. When this is viewed from a distance, the star appears to wobble. Not only that, this small orbit also affects the color of the light we receive from the star. This method requires that scientists make very precise measurements of its position in the sky.
Exercises
- For homework tonight, find out how many extrasolar planets scientists have detected so far.
- Also for homework, find out the names (they will probably be a string of numbers and letters together) of the 3 most recent extrasolar planet discoveries.
Click here to go to next lesson on If the Earth Gained Weight, Would You?
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Weight is nothing more than a measure of how much gravity is pulling on you. This is why you can be “weightless” in space. You are still made of stuff, but there’s no gravity to pull on you so you have no weight. The larger a body is, the more gravitational pull (or in other words the larger a gravitational field) it will have.
The Moon has a fairly small gravitational field (if you weighed 100 pounds on Earth, you’d only be 17 pounds on the Moon). The Earth’s field is fairly large and the Sun has a HUGE gravitational field (if you weighed 100 pounds on Earth, you’d weigh 2,500 pounds on the Sun!).
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As a matter of fact, the dog and I both have gravitational fields! Since we are both bodies of mass, we have a gravitational field which will pull things toward us. All bodies have a gravitational field. However, my mass is so small that the gravitational field I have is miniscule. Something has to be very massive before it has a gravitational field that noticeably attracts another body.
So what’s the measurement for how much stuff you’re made of? Mass. Mass is basically a weightless measure of how much matter makes you you. A hamster is made of a fairly small amount of stuff, so she has a small mass. I am made of more stuff, so my mass is greater than the hamster’s. Your house is made of even more stuff, so its mass is greater still.
So, here’s a question. If you are “weightless” in space, do you still have mass? Yes, the amount of stuff you’re made of is the same on Earth as it is in your space ship. Mass does not change, but since weight is a measure for how much gravity is pulling on you, weight will change.
Click here to go to next lesson on Turning the Sun into a Black Hole.
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At some point in the future you may ask yourself this question, “How can gravity pull harder (put more force on some things, like bowling balls) and yet accelerate all things equally?”
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Now that we have studied Newton’s laws, you can see that the above statement doesn’t make any sense at all! More force equals more acceleration is basically Newton’s Second law. The explanation for this is inertia.
Click here to go to next lesson on Gravity and Inertia.
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If you could stand on the Sun without being roasted, how much would you weigh? The gravitational pull is different for different objects. Let’s find out which celestial object you’d crack the pavement on, and which your lightweight toes would have to be careful about jumping on in case you leapt off the planet.
Weight is nothing more than a measure of how much gravity is pulling on you. Mass is a measure of how much stuff you’re made out of. Weight can change depending on the gravitational field you are standing in. Mass can only change if you lose an arm.
Materials
- Scale to weigh yourself
- Calculator
- Pencil
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Download Student Worksheet & Exercises
- We need to talk about the difference between weight and mass. In everyday language, weight and mass are used interchangeably, but scientists know better.
- Mass is how much stuff something is made out of. If you’re holding a bowling ball, you’ll notice that it’s hard to get started, and once it gets moving, it needs another push to get it to stop. If you leave the bowling ball on the floor, it stays put. Once you push it, it wants to stay moving. This “sluggishness” is called inertia. Mass is how much inertia an object has.
- Every object with mass also has a gravitational field, and is attracted to everything else that has mass. The amount of gravity something has depends on how far apart the objects are. When you step on a bathroom scale, you are reading your weight, or how much attraction is between you and the Earth.
- If you stepped on a scale in a spaceship that is parked from any planets, moons, black holes, or other objects, it would read zero. But is your mass zero? No way. You’re still made of the same stuff you were on Earth, so your mass is the same. But you’d have no weight.
- What is your weight on Earth? Let’s find out now.
- Step on the scale and read the number. Write it down.
- Now, what is your weight on the Moon? The correction factor is 0.17. So multiply your weight by 0.17 to find what the scale would read on the Moon.
- For example, if I weigh 100 pounds on Earth, then I’d weight only 17 pounds on the Moon. If the scale reads 10 kg on Earth, then it would read 1.7 kg on the Moon.
What’s Going On?
Weight is nothing more than a measure of how much gravity is pulling on you. This is why you can be “weightless” in space. You are still made of stuff, but there’s no gravity to pull on you so you have no weight. The larger a body is, the more gravitational pull (or in other words the larger a gravitational field) it will have.
The Moon has a fairly small gravitational field (if you weighed 100 pounds on Earth, you’d only be 17 pounds on the Moon).The Earth’s field is fairly large and the Sun has a HUGE gravitational field (if you weighed 100 pounds on Earth, you’d weigh 2,500 pounds on the Sun!).
As a matter of fact, the dog and I both have gravitational fields! Since we are both bodies of mass, we have a gravitational field which will pull things toward us. All bodies have a gravitational field. However, my mass is so small that the gravitational field I have is miniscule. Something has to be very massive before it has a gravitational field that noticeably attracts another body.
So what’s the measurement for how much stuff you’re made of? Mass. Mass is basically a weightless measure of how much matter makes you you. A hamster is made of a fairly small amount of stuff, so she has a small mass. I am made of more stuff, so my mass is greater than the hamster’s. Your house is made of even more stuff, so its mass is greater still. So, here’s a question. If you are “weightless” in space, do you still have mass? Yes, the amount of stuff you’re made of is the same on Earth as it is in your space ship. Mass does not change, but since weight is a measure for how much gravity is pulling on you, weight will change.
Did you notice that I put weightless in quotation marks? Wonder why?
Weightlessness is a myth! Believe it or not, one is never weightless. A person can be pretty close to weightless in very deep space, but the astronauts in a space ship actually do have a bit of weight.
Think about it for a second. If a space ship is orbiting the Earth, what is it doing? It’s constantly falling! If it wasn’t moving forward at tens of thousands of miles an hour it would hit the Earth. It’s moving fast enough to fall around the curvature of the Earth as it falls but, indeed, it’s falling as the Earth’s gravity is pulling it to us.
Otherwise the ship would float out to space. So what is the astronaut doing? She’s falling, too! The astronaut and the space ship are both falling to the Earth at the same rate of speed and so the astronaut feels weightless in space. If you were in an elevator and the cable snapped, you and the elevator would fall to the Earth at the same rate of speed. You’d feel weightless! (Don’t try this at home!)
Either now, or at some point in the future you may ask yourself this question, “How can gravity pull harder (put more force on some things, like bowling balls) and yet accelerate all things equally?” When we get into Newton’s laws in a few lessons, you’ll realize that doesn’t make any sense at all. More force equals more acceleration is basically Newton’s Second law.
Well, I don’t want to take too much time here since this is a little deeper then we need to go but I do feel some explanation is in order to avoid future confusion. The explanation for this is inertia. When we get to Newton’s First law we will discuss inertia. Inertia is basically how much force is needed to get something to move or stop moving.
Now, let’s get back to gravity and acceleration. Let’s take a look at a bowling ball and a golf ball. Gravity puts more force on the bowling ball than on the golf ball. So the bowling ball should accelerate faster since there’s more force on it. However, the bowling ball is heavier so it is harder to get it moving. Vice versa, the golf ball has less force pulling on it but it’s easier to get moving. Do you see it? The force and inertia thing equal out so that all things accelerate due to gravity at the same rate of speed!
Gravity had to be one of the first scientific discoveries. Whoever the first guy was to drop a rock on his foot, probably realized that things fall down! However, even though we have known about gravity for many years, it still remains one of the most elusive mysteries of science. At this point, nobody knows what makes things move toward a body of mass.
Why did the rock drop toward the Earth and on that guy’s foot? We still don’t know. We know that it does, but we don’t know what causes a gravitational attraction between objects. Gravity is also a very weak force. Compared to magnetic forces and electrostatic forces, the gravitational force is extremely weak. How come? No one knows. A large amount of amazing brain power is being used to discover these mysteries of gravity. Maybe it will be you who figures this out!
Exercises
- Of the following objects, which ones are attracted to one another by gravity?
a) Apple and Banana b) Beagle and Chihuahua c) Earth and You d) All of the above - True or False: Gravity accelerates all things differently
- True or False: Gravity pulls on all things differently
- If I drop a golf ball and a golf cart at the same time from the same height, which hits the ground first?
- There is a monkey hanging on the branch of a tree. A wildlife biologist wants to shoot a tranquilizer dart at the monkey to mark and study him. The biologist very carefully aims directly at the shoulder of the monkey and fires. However, the gun makes a loud enough noise that the monkey gets scared, lets go of the branch and falls directly downward. Does the dart hit where the biologist was aiming, or does it go higher or lower then he aimed? (This, by the way, is an old thought problem.)
Click here to go to next lesson on Rockets and Gravity.
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What happens to gravity if you’re in a rocket moving up through the atmosphere to a satellite in orbit?
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Does it remain the same or does it change during your flight? Here’s information about the earth:
- Radius: 3,959 miles (6,371 km)
- Distance from Sun: 92,960,000 miles (149,600,000 km)
- Mass: 5.972E24 kg
The International Space Station, an object about 72 meters long by 108 meters wide and 20 meters high stays in an orbital altitude of between 330 km (205 miles) and 410 km (255 miles), and moves at a rate of 27,724 kph(17,227 mph).
Click here to go to next lesson on Gravity for Different Objects.
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First we’re going to assume the earth is like a ball in that it’s a perfect sphere, and also that the density of the earth is even and it depends only how far from the center of the earth you are. Let’s also assume the earth isn’t rotating. Once we have these things in mind, then the magnitude of the force of gravity acting on an object goes like this…
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Click here to go to next lesson on Earth’s Crust is not Uniform.
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These are the scientific concepts students learn, separated by grade level according to both the national standards for science and Aurora’s personal experience in working with kids for nearly two decades. The scientific concepts are organized into categories within each grade level. You’ll find some areas span more than one grade level, so you will see some experiments listed for multiple grade levels.
PRE-K & K
Material properties, introduction to forces and motion, plants and animals, and basic principles of earth science.
First Grade
States of matter, weather, sound energy, light waves, and experimenting with the scientific method.
Second Grade
Chemical reactions, polymers, rocks and minerals, genetic traits, plant and animal life cycles, and Earth's resources.
Third Grade
Newton's law of motion, celestial objects, telescopes, measure the climate of the Earth and discover the microscopic world of life.
Fourth Grade
Electricity and magnetism, circuits and robotics, rocks and minerals, and the many different forms of energy.
Fifth Grade
Chemical elements and molecules, animal and plant biological functions, heat transfer, weather, planetary and solar astronomy.
Sixth Grade
Heat transfer, convetion currents, ecosystems, meteorology, simple machines, and alternative energy.
Seventh Grade
Cells, genetics, DNA, kinetic and potential, thermal energy, light and lasers, and biological structures.
Eighth Grade
Acceleration, forces projectile motion chemical reactions, deep space astronomy, and the periodic table.
High School (Advanced)
Alternative energy, astrophysics, robotics, chemistry, electronics, physics and more. For high school & advanced 5-8th students.
Teaching Resources
Tips and tricks to getting the science education results you want most for your students.
Science Fair Projects
Hovercraft, Light Speed, Fruit Batteries, Crystal Radios, R.O.V Underwater Robots and more!
There are three main differences between assuming the earth is round, uniformly dense, and not rotating as we did before.
First, the crust is not uniform. There are lumps and clumps everywhere that vary the density add up to make small variations in the force of gravity that we can actually measure with objects in free-fall motion. It’s actually how scientists find pockets of oil in the earth. They measure the surface gravity and plot it out, and if there’s a large enough deviation, it means there’s something interesting underground.
This image of the Mors salt dome in Denmark was studied for radioactive waste disposal. It’s a surface gravity survey that measures the acceleration due to gravity that shows something interesting is underground! The dots are the places where gravity was actually measured. You can read more about how gravity is measured from advanced lecture notes here. The unit of measurement for these deviations is called the “milligal” for Galileo, where 1 gal = 1,000 mgal = 1 cm/s3.
Click here to go to next lesson on The Earth is not a Sphere.
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The second problem with our assumption sis that the earth is not a sphere. It’s flattened a bit at the poles and bulges out at the equator. The ring around equator is larger than a ring around the poles by 21 km, which makes the poles closer to the center of the earth than the equator! Free-fall at the poles is slightly more than free-fall at the equator.
But before you book a trip to skydive in Ecuador, Colombia, Brazil, Sao Tome, Gabon, the Republic of the Congo, the Democratic Republic of the Congo, Uganda, Kenya, Somalia, Maldives, Indonesia or Kiribati, let’s talk about the assumption we made… The earth really does rotate. That’s not a surprise. How does this affect the value of g then? The bottom line is that gravity changes with altitude from 9.78 to 9.84 m/s2., mostly due to the earth spinning, but some to the earth not being a perfect sphere.
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Here are the Scientific Concepts to remember about Gravity:
- Gravity is a force that attracts things to one another.
- All bodies (objects) have a gravitational field.
- The larger a body is, the greater the strength of the gravitational field.
- Bodies must be very, very large before they exert any noticeable gravitational field.
- Gravity accelerates all things equally. Which means all things speed up the same amount as they fall.
- Gravity does not care what size things are or whether things are moving. All things are accelerated towards the Earth at the same rate of speed.
- Gravity does pull on things differently. Gravity is pulling greater on objects that weigh more.
- Weight is a measure of how much gravity is pulling on an object.
- Mass is a measure of how much matter (how many atoms) make up an object.
Click here to go to next lesson on Planetary and Satellite Motion.
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Circular motion is a little different from straight-line motion in a few different ways. Objects that move in circles are roller coasters in a loop, satellites in orbit, DVDs spinning in a player, kids on a merry go round, solar systems rotating in the galaxy, making a left turn in your car, water through a coiled hose, and so much more.
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Click here to go to next lesson on Introduction to Circular Motion.
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Imagine driving your car in a circle, like you would when take a clover-leaf type of freeway exit, or make a right turn on a green light. Here’s how the forces play out during the motion:
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Click here to go to next lesson on Calculating Average Speed.
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For any object that goes in a circle (or you can approximate it to a circle), you’ll want to use this approach when solving problems. You can feel the effect of circular motion if you’ve ever been in a car that suddenly turns right or left. You feel a push to the opposite side, right? If you are going fast enough and you take the turn hard enough, you can actually get slammed against the door. So my question to you is: who pushed you? Let’s find out!
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Click here to go to next lesson on Acceleration in a Circle.
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An object that moves in a circle with constant speed (like driving your car in a big circle at 30 mph) is called uniform circular motion. Although the speed is constant (30 mph), the velocity, which is a vector and made up of speed and direction, is not constant. The velocity vector has the same speed (magnitude), but the direction keeps changing as your car moves around the circle. The direction is an arrow that’s tangent to the circle as long as the car is moving on a circular path. This means that the tangent arrow is constantly changing and pointing in a new direction.
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It’s a common assumption that if the speed is constant, then there’s no acceleration… right?
Nope!
If the velocity is constant, then there’s no acceleration. But for circular motion, it’s speed, not velocity that is constant. Velocity is changing as the car turns a corner because the direction is changing, which also means that there is acceleration also! An accelerating object changes it’s velocity… it can be changing it’s speed, direction, or both. So objects moving in a circle are accelerating because they are always changing their direction.
Click here to go to next lesson on Accelerometer.
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A common misconception in science is that centrifugal and centripetal force (or acceleration) are the same thing. These two terms constantly throw students into frenzy, mostly because there is no clear definition in most textbooks. Here’s the scoop: centripetal and centrifugal force are NOT the same thing!
This experiment is mostly for Advanced Students, but here's a quick lesson you can do with your younger students...
[am4show have='p8;p9;p12;p39;p92;' guest_error='Guest error message' user_error='User error message' ] Before we jump in, let's recap what we've learned so far. A ball sitting still has a position you can chart on a map (latitude, longitude, and altitude), but no velocity or acceleration, because it's not moving. When you kick the ball, that's when it gets interesting. The second your toe touches the ball, things start to change. Velocity is a change of position. If you kick the ball ten feet, and it takes five seconds to go the distance, the average speed of the ball is 2 feet per second (about 1.4 MPH).
The trickier part of this scenario has to do with acceleration, which is the change in velocity. When you drive on the freeway at a constant 65 MPH, your acceleration is zero. Your speed does not change, so you have no acceleration. Your position is constantly changing, but you have constant speed. For example, when you enter the freeway, your speed changes from zero to 65 MPH in, say, ten seconds. Your acceleration is greatest when your foot first hits the gas – when your speed changes the most – when you’re moving from zero to a higher speed.
There's an interesting effect that happens when you travel in a curve. You can feel the effect of a different type of acceleration when you suddenly turn your car to the right – you will feel a push to the left. If you are going fast enough and you take the turn hard enough, you can get slammed against the door. So - who pushed you?
Think back to the first law of motion. An object in motion tends to stay in motion unless acted upon by an external force. This is the amazing part – the car is the external force. Your body was the object in motion, wanting to stay in motion in a straight line. The car turns, and your body still tries to maintain its straight path, but the car itself gets in the way. When you slam into the car door, the car is turning itself into your path, forcing you to change direction.
This effect is true when you travel in a car or in a roller coaster. It's the reason the water stays in the bucket when you swing it over your head. Physical motion is everywhere, challenging toddlers learning to walk as well as Olympic downhill skiers to go the distance. Here's a quick experiment you can do right now to wrap your head around this idea: Here's what you need to find for these experiments:
Now let's take a deeper look at centripetal, centrifugal, and how you can measure the g-force when taking a sharp turn in your car:
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Centrifugal (translation = “center-fleeing”) force has two different definitions, which also causes confusion. The inertial centrifugal force is the most widely referred to, and is purely mathematical, having to do with calculating kinetic forces using reference frames, and is used with Newton’s laws of motion. It’s often referred to as the ‘fictitious force’.
Reactive centrifugal force happens when objects move in a curved path. This force is actually the same magnitude as centripetal force, but in the opposite direction, and you can think of it as the reaction force to the centripetal force. Think of how you stand on the Earth… your weight pushes down on the Earth, and a reaction force (called the “normal” force) pushes up in reaction to your weight, keeping you from falling to the center of the Earth. A centrifugal governor (spinning masses that regulate the speed of an engine) and a centrifugal clutch (spinning disk with two masses separated by a spring inside) are examples of this kind of force in action.
What about the fictitious (inertial) centrifugal force? Well, if you imagine being inside the car as it is banking with the windows blacked out, you suddenly feel a magical ‘push’ toward the door away from the center of the bend. This “push” is the fictitious force invoked because the car’s motion and acceleration is hidden from you (the observer) in the reference frame moving within the car. Okay, enough talk for now. Let's make two acceleration-measuring instruments (called 'accelerometers') so you can jump, run, swing, and zoom around to find out how many g's you can pull. You're going to make a cork-accelerometer and a g-force ring, both of which are used by my students when I teach mechanical engineering dynamics.
Download Student Worksheet & Exercises
To use the accelerometer: invert the bottle and try to make the cork move about. Remember – it is measuring acceleration, which is the change in speed. It will only move when your speed changes. You can do this experiment in a car while doing your other vehicle experiment: Why Bother With Seatbelts?. The trouble with this accelerometer is that there are no measurements you can take - it's purely visual. This next activity is more accurate at measuring the number of g-s you pull in a sharp turn (whether in a vehicle or in a roller coaster!)
Get about a foot of tubing – the bigger the diameter, the easier it will be to read. Also get a barbed union (plastic barbs work just fine). Fill your tube halfway with COLORED water (it’s impossible to read when it’s clear). Blue, green, red… your choice of food dye additive. Make an O-shape using your barb to water-seal the junction. Grab hold of one side and hold the circle vertical, with the barb-end pointing to the sky. The water should fill the bottom half, and air fills the top half. Make sure there are equal amounts of water and air in your tube. Make a mark on the tube where the water meets the air with a black marker. This is your 0-g reading (relative, of course). No acceleration. Not a whole lot of fun.
Now, for your 1-g mark – measure up 45 degrees from the first mark. (If the top of the circle is 90 degrees, and the 0-g mark is zero degrees, find the halfway point and label it).
The 2-g mark is 22.5 degrees up from the 1-g mark.
3-g mark is 11.25 degrees up from the last mark. And 4-g is 5.6 degrees up from the last mark. (See a pattern? You can prove this mathematically in college, and it’s kind of fun to figure out!)
Now, next time mom drives around town, hold the tube in your hand so that the water line starts at the zero mark. When she pulls a turn, see how far it sloshes up and tell her how many g’s she pulled. We also used to have contests to see who could pull the most g’s while spinning in a circle. Have fun!
This experiment is mostly for Advanced Students, but here's a quick lesson you can do with your younger students...
[am4show have='p8;p9;p12;p39;p92;' guest_error='Guest error message' user_error='User error message' ] Before we jump in, let's recap what we've learned so far. A ball sitting still has a position you can chart on a map (latitude, longitude, and altitude), but no velocity or acceleration, because it's not moving. When you kick the ball, that's when it gets interesting. The second your toe touches the ball, things start to change. Velocity is a change of position. If you kick the ball ten feet, and it takes five seconds to go the distance, the average speed of the ball is 2 feet per second (about 1.4 MPH).
The trickier part of this scenario has to do with acceleration, which is the change in velocity. When you drive on the freeway at a constant 65 MPH, your acceleration is zero. Your speed does not change, so you have no acceleration. Your position is constantly changing, but you have constant speed. For example, when you enter the freeway, your speed changes from zero to 65 MPH in, say, ten seconds. Your acceleration is greatest when your foot first hits the gas – when your speed changes the most – when you’re moving from zero to a higher speed.
There's an interesting effect that happens when you travel in a curve. You can feel the effect of a different type of acceleration when you suddenly turn your car to the right – you will feel a push to the left. If you are going fast enough and you take the turn hard enough, you can get slammed against the door. So - who pushed you?
Think back to the first law of motion. An object in motion tends to stay in motion unless acted upon by an external force. This is the amazing part – the car is the external force. Your body was the object in motion, wanting to stay in motion in a straight line. The car turns, and your body still tries to maintain its straight path, but the car itself gets in the way. When you slam into the car door, the car is turning itself into your path, forcing you to change direction.
This effect is true when you travel in a car or in a roller coaster. It's the reason the water stays in the bucket when you swing it over your head. Physical motion is everywhere, challenging toddlers learning to walk as well as Olympic downhill skiers to go the distance. Here's a quick experiment you can do right now to wrap your head around this idea: Here's what you need to find for these experiments:
- bucket
- water
- outdoor area
- you
- clear tubing (about 12-18" long)
- nylon or metal barbed union that fits inside the tubing
- empty soda bottle
- clean wine cork
- string
Bucket Splash
Fill a bucket half-full with water. Grasp the handle and swing it over your head in a circle in the vertical direction. Try spinning around while holding the handle out in front of your chest to swing it in the horizontal plane. Vary your spin speed to find the minimum!Now let's take a deeper look at centripetal, centrifugal, and how you can measure the g-force when taking a sharp turn in your car:
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For Advanced Students:
Centripetal (translation = “center-seeking” ) is the force needed to keep an object following a curved path. Remember how objects will travel in a straight line unless they bump into something or have another force acting on it (gravity, drag force, etc.)? Well, to keep the bucket of water swinging in a curved arc, the centripetal force can be felt in the tension experienced by the handle (or your arm, in our case). Swinging an object around on a string will cause the rope to undergo tension (centripetal force), and if your rope isn’t strong enough, it will snap and break, sending the mass flying off in a tangent (straight) line until gravity and drag force pull the object to a stop. This force is proportional to the square of the speed - the faster you swing the object, the higher the force.Centrifugal (translation = “center-fleeing”) force has two different definitions, which also causes confusion. The inertial centrifugal force is the most widely referred to, and is purely mathematical, having to do with calculating kinetic forces using reference frames, and is used with Newton’s laws of motion. It’s often referred to as the ‘fictitious force’.
Reactive centrifugal force happens when objects move in a curved path. This force is actually the same magnitude as centripetal force, but in the opposite direction, and you can think of it as the reaction force to the centripetal force. Think of how you stand on the Earth… your weight pushes down on the Earth, and a reaction force (called the “normal” force) pushes up in reaction to your weight, keeping you from falling to the center of the Earth. A centrifugal governor (spinning masses that regulate the speed of an engine) and a centrifugal clutch (spinning disk with two masses separated by a spring inside) are examples of this kind of force in action.
Say WHAT?!?
Don't worry if these ideas make your brain turn into a pretzel. Most college students take three courses in this before it makes sense to them. Here's one more example: imagine driving a car along a banked turn. The road exerts a centripetal force on the car, keeping the car moving in a curved path (the “banked” turn). If you neglected to buckle your seat belt and the seats have a fresh coat of Armor-All (making them slippery), then as the car turns along the banked curve, you get “shoved” toward the door. But who pushed you? No one – your body wanted to continue in a straight line but the car keeps moving in your path, turning your body in a curve. The push of your weight on the door is the reactive centrifugal force, and the car pushing on you is the centripetal force.What about the fictitious (inertial) centrifugal force? Well, if you imagine being inside the car as it is banking with the windows blacked out, you suddenly feel a magical ‘push’ toward the door away from the center of the bend. This “push” is the fictitious force invoked because the car’s motion and acceleration is hidden from you (the observer) in the reference frame moving within the car. Okay, enough talk for now. Let's make two acceleration-measuring instruments (called 'accelerometers') so you can jump, run, swing, and zoom around to find out how many g's you can pull. You're going to make a cork-accelerometer and a g-force ring, both of which are used by my students when I teach mechanical engineering dynamics.
Cork Accelerometer
Fill an empty soda bottle to the top with water. Modify the soda bottle cap as follows: attach a string 8-10" long to a clean wine cork. Hot glue the free end of the string to the inside of the cap. Place the cork and string inside the bottle and screw on the top (try to eliminate the air bubbles). The cork should be free to bob around when you hold the bottle upside-down.G-Force Ring Accelerometer
One more university-level gadget for demonstrating the fascinating world of physical dynamics. This quick homemade device roughly measures acceleration in “g’s”. We used it to measure the g-force on roller coasters at Six Flags Magic Mountain, and it worked just as well as the expensive ones you buy in scientific catalogs!The 2-g mark is 22.5 degrees up from the 1-g mark.
3-g mark is 11.25 degrees up from the last mark. And 4-g is 5.6 degrees up from the last mark. (See a pattern? You can prove this mathematically in college, and it’s kind of fun to figure out!)
Now, next time mom drives around town, hold the tube in your hand so that the water line starts at the zero mark. When she pulls a turn, see how far it sloshes up and tell her how many g’s she pulled. We also used to have contests to see who could pull the most g’s while spinning in a circle. Have fun!
Advanced Students:
Advanced Students: Download your G-Force lab here. Exercises- Which accelerometer was better at giving a visual representation of accelerating?
- Which one do you prefer? Why?
- What activity did you do that created the most acceleration?
- What does that tell you about acceleration?
Click here to go to next lesson on Rotating Candles.
[/am4show]Now this next experiment is a little dangerous (we’re going to be spinning flames in a circle), so I found a video by MIT that has a row of five candles sitting on a rotating platform (like a “lazy susan”) so you can see how it works.
The candles are placed inside a dome (or a glass jar) so that when we spin them, they aren’t affected by the moving air but purely by acceleration. So for this video above, a row of candles are inside a clear dome on a rotating platform. When the platform rotates, air inside the dome gets swung to the outer part of the dome, creating higher density air at the outer rim, and lower density air in the middle. The candle flames point inwards towards the middle because the hot gas in the flames always points towards lower density air. Source: http://video.mit.edu
Now you’re beginning to understand how an object moving in a circle experiences acceleration, even if the speed is constant.
So what direction is the acceleration vector?
It’s pointed straight toward the center of that circle.
Velocity is always tangent to the circle in the direction of the motion, and acceleration is always directed radially inward. Because of these two things, the acceleration that arises from traveling in a circle is called centripetal acceleration (a word created by Sir Isaac Newton). There’s no direct relationship between the acceleration and velocity vectors for a moving particle.
Click here to go to next lesson on Centripetal Force.
Do you remember when I asked you “Who pushed you?” when you were riding in a car that took a sharp turn? Well, the answer has to do with centripetal force. Centripetal (translation = “center-seeking” ) is the force needed to keep an object following a curved path.
Remember how objects will travel in a straight line unless they bump into something or have another force acting on it like gravity, friction, or drag force? Imagine a car moving in a straight line at a constant speed. You’re inside the car, no seat belt, and the seat is slick enough for you to slide across easily. Now the car turns and drives again at constant speed but now on a circular path. When viewed from above the car, we see the car following a circle, and we see you wanting to keep moving in a straight line, but the car wall (door), moves into your path and exerts a force on you to keep you moving in a circle. The car door is pushing you into the circle.
According to Newton’s second law of motion, if you are experiencing an acceleration you must also be experiencing a net force (F=ma). The direction of the net force is in the same direction as the acceleration, so for the example with you inside the car, there’s an inward force acting on you (from the car door) keeping you moving in a circle.
If you have a bucket of water and you’re swinging it around your head, in order to keep a bucket of water swinging in a circle, the centripetal force can be felt in the tension experienced by the handle. Swinging an object around on a string will cause the rope to undergo tension (centripetal force), and if your rope isn’t strong enough, it will snap and break, sending the mass flying off in a tangential straight line until gravity and drag force pull the object to a stop.
This force is proportional to the square of the speed, meaning that the faster you swing the object, the higher the magnitude of the force will be.
Remember Newton’s First Law? The law of inertia? It states that objects in motion tend to stay in motion with the same speed and direction unless acted upon by an unbalanced/external force. Which means that objects naturally want to continue going their straight and merry way (like you did in a straight line when you were inside the car) until an unbalanced force causes it to turn speed up or stop. Can you see how an unbalanced force is required for objects to move in a circle? There has to be a force pushing on the object, keeping in on a circular path because otherwise, it’ll go off in a straight line!
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Click here to go to next lesson on Earth-Satellite System.
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Every object moving in a circle will experience a force pushing or pulling it toward the center of the circle. Whether it’s a car making a turn and the friction force from the road are acting on the wheels of the car, or a bucket is swung around your head and the tension of the rope keeps it moving in a circle, they all have to have a force keeping them moving in that circle, and that force is called centripetal force. Without it, objects could never change their direction. Because centripetal force is tangent to the velocity vector, the force can change the direction of an object without changing the magnitude.
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Click here to go to next lesson on Another Earth-Satellite System Example.
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Let's do another example of a satellite system:[am4show have='p9;p58;' guest_error='Guest error message' user_error='User error message' ]
There's a lot of confusion around the difference between centripetal and centrifugal force. The confusion usually starts with a thought like this: "When I go on a ride, I am getting thrown to the outside (if it's like a fast merry-go-round) or being squashed down in my seat (if it's like a roller coaster loop), but either way I am being pushed away from the center of the circle."
Can you imagine thinking that? Lots of people do! Now let's see if we can punch a few holes in that thought so you can really see how it's not true at all.
First of all, without the inward force pushing on you to keep you in a circle, you would be going off in a straight line and not around the loop of the roller coaster. The track is exerting a force on you, pushing you toward the center of the circle.
Now here's a question for you: just because you feel like you're being thrown, does that mean there has to be a force causing this? Is there any other way to explain that sensation? (Think Newton's Laws!)
Imagine again yourself in a car making a turn. If we had a video camera above the car, you'd see you wanting to continue in a straight line, but the car is now moving into your path and exerting a force on you, pushing you into a circle. That's when you hit the door.
The trick to really seeing this is to get out of yourself and into a different perspective! Einstein made a famous observation that described if you were in a rocket (without windows so you can't view the outside world), you would not be able to tell if the rocket was in space accelerating, or if it were standing still and you were experiencing the same amount of acceleration due to gravity on a planet. Because of F=ma, you can experience these two situations and still feel the same and not be able to tell which is which!
Okay, so are you ready to tackle centrifugal force?
Click here to go to next lesson on Centrifugal Force.
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Centrifugal (translation = “center-fleeing”) force has two different definitions, which causes even more confusion. The inertial centrifugal force is the most widely referred to, and is purely mathematical, having to do with calculating kinetic forces using reference frames, and is used with Newton’s laws of motion. It’s often referred to as the ‘fictitious force’.
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Reactive centrifugal force happens when objects move in a curved path. This force is actually the same magnitude as centripetal force, but in the opposite direction, and you can think of it as the reaction force to the centripetal force. Think of how you stand on the Earth. Your weight pushes down on the Earth, and a reaction force (the “normal” force) pushes up in reaction to your weight, keeping you from falling to the center of the Earth.
A centrifugal governor (spinning masses that regulate the speed of an engine) and a centrifugal clutch (spinning disk with two masses separated by a spring inside) are examples of this kind of force in action.
Here’s anther example: imagine driving a car along a banked turn. The road exerts a centripetal force on the car, keeping the car moving in a curved path (the “banked” turn). If you neglected to buckle your seat belt and the seats have a fresh coat of Armor-All (making them slippery), then as the car turns along the banked curve, you get “shoved” toward the door. But who pushed you? No one – your body wanted to continue in a straight line but the car keeps moving in your path, turning your body in a curve. The push of your weight on the door is the reactive centrifugal force, and the car pushing on you is the centripetal force.
What about the fictitious (inertial) centrifugal force? Well, if you imagine being inside the car as it is banking with the windows blacked out, you suddenly feel a magical ‘push’ toward the door away from the center of the bend. This “push” is the fictitious force invoked because the car’s motion and acceleration is hidden from you (the observer) in the reference frame moving within the car.
James Watt invented a “centrifugal governor”, which is a closed loop mechanical device you’ll find in lawn mowers, cruise controls, and airplane propellers to automatically control the speed of these things. The heavy brass balls spin around, and the faster they go, the more they rise up, which increases the rotational energy of this device, and since it’s connected to the throttle of something like a lawn mower engine, it can be carefully set to maintain the same speed or output power of the engine. It’s an automatic feedback system that is purely mechanical. Source: Mirko Junge, Science Museum London.
For circular motion, there are a couple of equations we will need to tackle physics problems that involve speed (v), acceleration (a), and force (F):
Here’s the equation for calculating centripetal force:
There’s another equation that relates the rotational speed (w) with the velocity like this:
Here’s a cool experiment you can do that will really show you how objects that move in a circle experience centripetal force. You can lift at least 10 balls by using only one! All you need are balls, fishing line or dental floss, and an old pen.
Video note: Oops! I made a mistake. Around 5:00 it should be sqrt(10rg) not sqrt(20rg).
Let’s calculate the velocity of the above experiment using our new circular motion equations. Let’s say you timed yourself, and you can get one ball to lift five identical balls when the one ball swings around once every second. Let’s calculate the acceleration, force, and speed.
The net force acting on the ball is directed inwards. There might be more than one force acting on an object moving in a circle, but it’s the net force that adds them all up. The net force is proportional to the square of the speed (look at the equations again!). So if your speed increases three times, then the force increases by a factor of nine.
Click here to go to next lesson on Circular Motion with a Car.
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Let’s do a sample problem involving a car:
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Click here to go to next lesson on Circular Motion with a Kid.
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Now let’s do a similar problem but this time with a kid instead of a car:
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Click here to go to next lesson on Favorite Amusement Park Ride.
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I love amusement park rides, even though I know what’s going on from the science side of things! Here’s one that’s always been a favorite of mine:
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Click here to go to next lesson on Circular Motion and Friction.
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Before we go any further, we need to take a look at how friction gets handled in these types of problems:
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Click here to go to next lesson on Swinging Bucket.
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You’ve come so far with your analysis that I really want to give you the “real way” to solve these types of problems. Normally, this method isn’t introduced to you until your second year in college, and that’s only if you’re an engineer taking Statics and Dynamics classes (the next level after this course).
Here’s a step-by-step method that really puts all the pieces we’ve been working on all together into one:
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Do you see how easy that is? It puts the FBD (free body diagram) together with the MAD (mass-acceleration diagram) and uses Newton’s Laws to solve for the things you need to know. Using pictures and equations, you can solve anything now!
Click here to go to next lesson on Clothoid Loops.
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Let’s do something fun now… want to know about the physics of real roller coasters?
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It’s really important to know how much centrifugal force people experience, whether its in cars or roller coasters! In fact roller coaster loops used to be circular, but now they use clothoid loops instead to keep passengers happy during their ride so they don’t need nearly the acceleration that they’d need for a circular loop (which means less g-force so passengers don’t black out).
Click here to go to next lesson on Roller Coaster Maneuvers.
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There’s two main types of maneuvers a roller coaster can do that’s easy for us to analyze with uniform circular motion: camel-backs are the first one:
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…and loops are the second type of maneuver:
Click here to go to next lesson on Roller Coaster Activity.
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We’re going to build monster roller coasters in your house using just a couple of simple materials. You might have heard how energy cannot be created or destroyed, but it can be transferred or transformed (if you haven’t that’s okay – you’ll pick it up while doing this activity).
Roller coasters are a prime example of energy transfer: You start at the top of a big hill at low speeds (high gravitational potential energy), then race down a slope at break-neck speed (potential transforming into kinetic) until you bottom out and enter a loop (highest kinetic energy, lowest potential energy). At the top of the loop, your speed slows (increasing your potential energy), but then you speed up again and you zoom near the bottom exit of the loop (increasing your kinetic energy), and you’re off again!
Here’s what you need:
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- marbles
- masking tape
- 3/4″ pipe foam insulation (NOT neoprene and NOT the kind with built-in adhesive tape)
To make the roller coasters, you’ll need foam pipe insulation, which is sold by the six-foot increments at the hardware store. You’ll be slicing them in half lengthwise, so each piece makes twelve feet of track. It comes in all sizes, so bring your marbles when you select the size. The ¾” size fits most marbles, but if you’re using ball bearings or shooter marbles, try those out at the store. (At the very least you’ll get smiles and interest from the hardware store sales people.) Cut most of the track lengthwise (the hard way) with scissors. You’ll find it is already sliced on one side, so this makes your task easier. Leave a few pieces uncut to become “tunnels” for later roller coasters.
Read for some ‘vintage Aurora’ video? This is one of the very first videos ever made by Supercharged Science:
Download Student Worksheet & Exercises
Tips & Tricks
Loops Swing the track around in a complete circle and attach the outside of the track to chairs, table legs, and hard floors with tape to secure in place. Loops take a bit of speed to make it through, so have your partner hold it while you test it out before taping. Start with smaller loops and increase in size to match your entrance velocity into the loop. Loops can be used to slow a marble down if speed is a problem.
Camel-Backs Make a hill out of track in an upside-down U-shape. Good for show, especially if you get the hill height just right so the marble comes off the track slightly, then back on without missing a beat.
Whirly-Birds Take a loop and make it horizontal. Great around poles and posts, but just keep the bank angle steep enough and the marble speed fast enough so it doesn’t fly off track.
Corkscrew Start with a basic loop, then spread apart the entrance and exit points. The further apart they get, the more fun it becomes. Corkscrews usually require more speed than loops of the same size.
Jump Track A major show-off feature that requires very rigid entrance and exit points on the track. Use a lot of tape and incline the entrance (end of the track) slightly while declining the exit (beginning of new track piece).
Pretzel The cream of the crop in maneuvers. Make a very loose knot that resembles a pretzel. Bank angles and speed are the most critical, with rigid track positioning a close second. If you’re having trouble, make the pretzel smaller and try again. You can bank the track at any angle because the foam is so soft. Use lots of tape and a firm surface (bookcases, chairs, etc).
Troubleshooting Marbles will fly everywhere, so make sure you have a lot of extras! If your marble is not following your track, look very carefully for the point of departure – where it flies off.
-Does the track change position with the weight of the marble, making it fly off course? Make the track more rigid by taping it to a surface.
-Is the marble jumping over the track wall? Increase your bank angle (the amount of twist the track makes along its length).
-Does your marble just fall out of the loop? Increase your marble speed by starting at a higher position. When all else fails and your marble still won’t stay on the track, make it a tunnel section by taping another piece on top the main track. Spiral-wrap the tape along the length of both pieces to secure them together.
HOT TIPS for ULTRA-COOL PARENTS: This lab is an excellent opportunity for kids to practice their resilience, because we guarantee this experiment will not work the first several times they try it. While you can certainly help the kids out, it’s important that you help them figure it out on their own. You can do this by asking questions instead of rushing in to solve their problems. For instance, when the marble flies off the track, you can step back and say:
“Hmmm… did the marble go to fast or too slow?”
“Where did it fly off?”
“Wow – I’ll bet you didn’t expect that to happen. Now what are you going to try?”
Become their biggest fan by cheering them on, encouraging them to make mistakes, and try something new (even if they aren’t sure if it will work out).
Check out this cool roller coaster from one of our students!
Exercises
- What type of energy does a marble have while flying down the track of a roller coaster?
- What type of energy does the marble have when you are holding it at the top of the track?
- At the top of a camel back hill, which is higher for the marble, kinetic or potential energy?
- At the top of an inverted loop, which energy is higher, kinetic or potential energy?
Click here to go to next lesson on Speed Skating.
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You can find circular motion everywhere, including football, car racing, ice skating, and baseball. An ice skater spins on ice, or a competition speed skater makes a turn… they are both examples of circular motion. A turn happens when there’s a force component directed inward from the circular path. Let me show you a couple of examples:
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Click here to go to next lesson on Unbanked Car Turn without Friction.
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What about your car along a circular path? Let’s take a look at two different examples. The first is an unbanked turn with friction:
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Click here to go to next lesson on Banked Car Turn with Friction.
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The second example is a banked turn, like NASCAR racing:
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Click here to go to next lesson on Universal Gravitation.
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We’ve already studied the different types of forces and learned how to draw free body diagrams. We’re going to use those concepts to put forces into two different categories: internal and external forces. Internal forces include forces due to gravity, magnetism, electricity, and springs. External forces include applied, normal, tension, friction, drag and air resistance forces.
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Click here to go to next lesson on Mechanical Energy Relationship .
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The reason why we put the forces into two different categories will be obvious when we start solving physics problems, but for now, you can think of it like this: when total amount of work is done on an object is done by only internal forces, energy will change forms (like going from kinetic to potential energy), and the total amount of mechanical energy is conserved, and the forces are also conserved. When the total amount of work done is done by an external force, the forces are not conserved and the object with either gain or lose energy.
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As Phillip holds the ball at the top of the building, the ball has 100 Joules of potential energy (the number is just an example). When he drops it, the potential energy of the ball drops since the height of the ball gets less and less. At the same time, however, kinetic energy increases because the speed of the ball increases. All the way down, the sum of the two energies equals 100. No energy gets lost, it only gets transferred. Energy is conserved.
Now here’s a question you may be asking yourself, “If energy is neither created or destroyed in a closed system then why doesn’t a pendulum swing forever?”
That’s a very good question. Energy is neither created or destroyed, but it can be transferred into non- useful energy.
In the case of a pendulum, every swing loses a little bit of energy,which is why each swing goes slightly less high (achieves slightly less PE) than the swing before it. Where does that energy go? To heat. The second law of thermodynamics states basically that eventually all energy ends up as heat. If you could measure it, you’d find that the string, and the weight have a slightly higher temperature then they did when they started due to friction. The energy of your pendulum is lost to heat!
If you could prevent the loss of energy to useless energy, you could create a perpetual motion machine. A machine that works forever! There have been a lot of folks who have spent a lot of time trying to make a perpetual motion machine. So far, they have all failed. A perpetual motion machine is one that is said to be 100% energy efficient. In other words all the energy that goes into it goes to useful energy.
Your pendulum could be said to be about 90% efficient.Very little energy is converted into useless energy. In most systems, energy is converted to useless heat and sound energy.
As Phillip holds the ball at the top of the building, the ball has 100 Joules of potential energy (the number is just an example). When he drops it, the potential energy of the ball drops since the height of the ball gets less and less. At the same time, however, kinetic energy increases because the speed of the ball increases. All the way down, the sum of the two energies equals 100. No energy gets lost, it only gets transferred. Energy is conserved.
Now here’s a question you may be asking yourself, “If energy is neither created or destroyed in a closed system then why doesn’t a pendulum swing forever?”
That’s a very good question. Energy is neither created or destroyed, but it can be transferred into non- useful energy.
In the case of a pendulum, every swing loses a little bit of energy,which is why each swing goes slightly less high (achieves slightly less PE) than the swing before it. Where does that energy go? To heat. The second law of thermodynamics states basically that eventually all energy ends up as heat. If you could measure it, you’d find that the string, and the weight have a slightly higher temperature then they did when they started due to friction. The energy of your pendulum is lost to heat!
If you could prevent the loss of energy to useless energy, you could create a perpetual motion machine. A machine that works forever! There have been a lot of folks who have spent a lot of time trying to make a perpetual motion machine. So far, they have all failed. A perpetual motion machine is one that is said to be 100% energy efficient. In other words all the energy that goes into it goes to useful energy.
Your pendulum could be said to be about 90% efficient.Very little energy is converted into useless energy. In most systems, energy is converted to useless heat and sound energy.
Click here to go to next lesson on Energy Exchange between Kinetic and Potential Energy.
[/am4show]This is a nit-picky experiment that focuses on the energy transfer of rolling cars. You’ll be placing objects and moving them about to gather information about the potential and kinetic energy.
We’ll also be taking data and recording the results as well as doing a few math calculations, so if math isn’t your thing, feel free to skip it.
Here’s what you need:
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- a few toy cars (or anything that rolls like a skate)
- a board, book or car track
- measuring tape
The setup is simple. Here’s what you do:
1. Set up the track (board or book so that there’s a nice slant to the floor).
2. Put a car on the track.
3. Let the car go.
4. Mark or measure how far it went.
Download Student Worksheet & Exercises
As you lifted the car onto the track you gave the car potential energy. As the car went down the track and reached the floor the car lost potential energy and gained kinetic energy. When the car hit the floor it no longer had any potential energy only kinetic.
If the car was 100% energy efficient, the car would keep going forever. It would never have any energy transferred to useless energy. Your cars didn’t go forever did they? Nope, they stopped and some stopped before others. The ones that went farther were more energy efficient. Less of their energy was transferred to useless energy than the cars that went less far.
Where did the energy go? To heat energy, created by the friction of the wheels, and to sound energy. Was energy lost? NOOOO, it was only changed. If you could capture the heat energy and the sound energy and add it to the the kinetic energy, the sum would be equal to the original amount of energy the car had when it was sitting on top of the ramp.
For K-8 grades, click here to download a data sheet.
For Advanced Students, click here for the data log sheet. You’ll need Microsoft Excel to use this file.
Exercises
- Where is the potential energy greatest?
- Where is the kinetic energy greatest?
- Where is potential energy lowest?
- Where is kinetic energy lowest?
- Where is KE increasing, and PE is decreasing?
- Where is PE increasing and KE decreasing?
Click here to go to next lesson on Inclined Plane.
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What’s an inclined plane? Jar lids, spiral staircases, light bulbs, and key rings. These are all examples of inclined planes that wind around themselves. Some inclined planes are used to lower and raise things (like a jack or ramp), but they can also used to hold objects together (like jar lids or light bulb threads).
Here’s a quick experiment you can do to show yourself how something straight, like a ramp, is really the same as a spiral staircase.
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Here’s what you need to find:
- sheet of paper
- short dowel or cardboard tube from a coat hanger
- tape
- ruler
Cut a right triangle out of paper so that the two sides of the right angle are 11” and 5 ½” (the hypotenuse – the side opposite the right angle – will be longer than either of these). Find a short dowel or use a cardboard tube from a coat-hanger. Roll the triangular paper around the tube beginning at the short side and roll toward the triangle point, keeping the base even as it rolls.
Notice that the inclined plane (hypotenuse) spirals up as a tread as you roll. Remind you of screw threads? Those are inclined planes. If you have trouble figuring out how to do this experiment, just watch the video clip below:
Download Student Worksheet & Exercises
Inclined planes are simple machines. It’s how people used to lift heavy things (like the top stones for a pyramid).
Here’s another twist on the inclined plane: a wedge is a double inclined plane (top and bottom surfaces are inclined planes). You have lots of wedges at home: forks, knives, and nails just name a few.
When you stick a fork in food, it splits the food apart. You can make a simple wedge from a block of wood and drive it under a heavy block (like a tree stump or large book) with a kid on top.
Exercises
- What is one way to describe energy?
- The amount of atoms moving around at any given moment
- Electrons flowing from one area to another
- The ability to do work
- The square root of the speed of an electron
- Work is when something moves when:
- Force is applied
- Energy is used
- Electrons are lost or gained
- A group of atoms vibrate
- Name two simple machines:
- Name one example of a simple machine:
Click here to go to next lesson on Energy Exchange.
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When you toss down a ball, gravity pulls on the ball as it falls (creating kinetic energy) until it smacks the pavement, converting it back to potential energy as it bounces up again. This cycles between kinetic and potential energy as long as the ball continues to bounce.
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But note that when you drop the ball, it doesn’t rise up to the same height again. If the ball did return to the same height, this means you recovered all the kinetic energy into potential energy and you have a 100% efficient machine at work. But that’s not what happens, is it? Where did the rest of the energy go? Some of the energy was lost as heat and sound. (Did you hear something when the ball hit the floor?)
Click here to go to next lesson on Elastic Potential Energy.
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Note: Do the pendulum experiment first, and when you’re done with the heavy nut from that activity, just use it in this experiment.
You can easily create one of these mystery toys out of an old baking powder can, a heavy rock, two paper clips, and a rubber band (at least 3″ x 1/4″). It will keep small kids and cats busy for hours.
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Here’s what you get:
- can with a lid
- heavy rock or large nut
- two paper clips
- rubber band
You’ll need two holds punched through your container – one in the lid and the bottom. Thread your rubber band through the heavy washer and tie it off (this is important!). Poke the ends of the rubber band through one of the holes and catch it on the other side with a paper clip. (Just push a paper clip partway through so the rubber band doesn’t slip back through the hole.) Do this for both sides, and make sure that your rubber band is a pulled mildly-tight inside the can. You want the hexnut to dangle in the center of the can without touching the sides of the container.
Download Student Worksheet & Exercises
Now for the fun part… gently roll the can on a smooth floor away from you. The can should roll, slow down, stop, and return to you! If it doesn’t, check the rubber band tightness inside the can.
The hexnut is a weight that twists up the rubber band as the can rolls around it. The kinetic energy (the rolling motion of the can) transforms into potential (elastic) energy stored in the rubber band the free side twists around. The can stops (this is the point of highest potential energy) and returns to you (potential energy is being transformed into kinetic). The farther the toy is rolled the more elastic potential energy it stores.
Exercises
- Explain in your own words two types of energy transfer:
- True or false: All energy in a system is lost to heat.
- True
- False
Click here to go to next lesson on Pendulums and Energy Transfer.
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This is a very simple yet powerful demonstration that shows how potential energy and kinetic energy transfer from one to the other and back again, over and over. Once you wrap your head around this concept, you’ll be well on your way to designing world-class roller coasters.
For these experiments, find your materials:
- some string
- a bit of tape
- a washer or a weight of some kind
- set of magnets (at least 6, but more is better)
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Here’s what you do:
1. Make the string into a 2 foot or so length.
2. Tie the string to the washer, or weight.
3. Tape the other end of the string to a table.
4. Lift the weight and let go, causing the weight to swing back and forth at the end of the pendulum.
Download Student Worksheet & Exercises
Watch the pendulum for a bit and describe what it’s doing as far as energy goes. Some questions to think about include:
- Where is the potential energy greatest?
- Where is the kinetic energy greatest?
- Where is potential energy lowest?
- Where is kinetic energy lowest?
- Where is KE increasing, and PE is decreasing?
- Where is PE increasing and KE decreasing?
- Where did the energy come from in the first place?
Remember, potential energy is highest where the weight is the highest.
Kinetic energy is highest were the weight is moving the fastest. So potential energy is highest at the ends of the swings. Here’s a coincidence, that’s also where kinetic energy is the lowest since the weight is moving the least.
Where’s potential energy the lowest? At the middle or lowest part of the swing. Another coincidence, this is where kinetic energy is the highest! Now, wait a minute…coincidence or physics? It’s physics right?
In fact, it’s conservation of energy. No energy is created or destroyed, so as PE gets lower KE must get higher. As KE gets higher PE must get lower. It’s the law…the law of conservation of energy! Lastly, where did the energy come from in the first place? It came from you. You added energy (increased PE) when you lifted the weight.
(By the way, you did work on the weight by lifting it the distance you lifted it. You put a certain amount of Joules of energy into the pendulum system. Where did you get that energy? From your morning Wheaties!)
Chaos Pendulum
For this next experiment, we’ll be using magnets to add energy into the system by having a magnetic pendulum interact with magnets carefully spaced around the pendulum. Watch the video to learn how to set this one up. You’ll need a set of magnets (at least one of them is a ring magnet so you can easily thread a string through it), tape, string, and a table or chair. Are you ready?
Exercises
- Why can we never make a machine that powers itself over and over again?
- Energy is mostly lost to heat.
- Energy is completely used up.
- Energy is unlimited, but is absorbed by neighboring air molecules.
- None of these
- In the pendulum, as kinetic energy increases, potential energy ______________.
- Increases
- Decreases
- As potential energy decreases, kinetic energy _________________.
- Increases
- Decreases
Click here to go to next lesson on Potato Cannon.
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This experiment is for Advanced Students.There are several different ways of throwing objects. This is the only potato cannon we’ve found that does NOT use explosives, so you can be assured your kid will still have their face attached at the end of the day. (We’ll do more when we get to chemistry, so don’t worry!)
These nifty devices give off a satisfying *POP!!* when they fire and your backyard will look like an invasion of aliens from the French Fry planet when you’re done. Have your kids use a set of goggles and do all your experimenting outside.
Here’s what you need:
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- potatoes
- an acrylic tube (clear is best so you can see what’s happening inside!)
- wooden dowel
- washer (this is your ‘hand-saver’)
Where is the potential energy the greatest? How much energy did your spud have at this point? Hmmm… let’s see if we can get a few actual numbers with this experiment. In order to calculate potential energy at the highest point of travel, you’ll need to figure out how high it went.
Here are instructions for making your own height-gauge:
Once you get your height gauge working right, you’ll need to track your data. Start a log sheet in your journal and jot down the height for each launch. Let’s practice a sample calculation:
If you measured an angle of 30 degrees, and your spud landed 20 feet away, we can assume that the spud when highest right in the middle of its flight, which is halfway (10 feet). Use basic trigonometry to find the height 45 degrees up at a horizontal distance ten feet away to get:
height = h = (10′) * (tan 30) = 5.8 feet
(Convert this to meters by: (5.8 feet) * (12 inches/foot) / (39.97 inches/meter) = 1.8 meters)
I measured the mass of my spud to be 25 grams (which is 0.025 kg).
Now, let’s calculate the potential energy:
PE = mgh = (0.025 kg) * (1.8 meters) * (10 m/s2) = 0.44 Joules
How fast was the spud going before it smacked into the ground? Set PE = KE to solve for velocity:
mgh = 0.5 mv2 gives v = (2gh)1/2
Plug in your numbers to get:
v = [(2) * (10) * (1.8)]2 = 6 m/s (or about 20 feet per second). Cool!
Click here to go to next lesson on Bobsleds.
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Bobsleds use the low-friction surface of ice to coast downhill at ridiculous speeds. You start at the top of a high hill (with loads of potential energy) then slide down a icy hill til you transform all that potential energy into kinetic energy. It’s one of the most efficient ways of energy transformation on planet Earth. Ready to give it a try?
This is one of those quick-yet-highly-satisfying activities which utilizes ordinary materials and turns it into something highly unusual… for example, taking aluminum foil and marbles and making it into a racecar.
While you can make a tube out of gift wrap tubes, it’s much more fun to use clear plastic tubes (such as the ones that protect the long overhead fluorescent lights). Find the longest ones you can at your local hardware store. In a pinch, you can slit the gift wrap tubes in half lengthwise and tape either the lengths together for a longer run or side-by-side for multiple tracks for races. (Poke a skewer through the rolls horizontally to make a quick-release gate.)
Here’s what you need:
- aluminum foil
- marbles (at least four the same size)
- long tube (gift wrapping tube or the clear protective tube that covers fluorescent lighting is great)
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If you’re finding that the marbles fall out before the bobsled reaches the bottom of the slide, you need to either crimp the foil more closely around the marbles or decrease your hill height.
Check to be sure the marbles are free to turn in their “slots” before launching into the tube – if you’ve crimped them in too tightly, they won’t move at all. If you oil the bearings with a little olive oil or machine oil, your tube will also get covered with oil and later become sticky and grimy… but they sure go faster those first few times!
Download Student Worksheet & Exercises
Exercises Answer the questions below:
- Potential energy is energy that is related to:
- Equilibrium
- Kinetic energy
- Its system
- Its elevation
- If an object’s energy is mostly being used to keep that object in motion, we can say it has what type of energy?
- Kinetic energy
- Potential energy
- Heat energy
- Radiation energy
- True or False: Energy is able to remain in one form that is usable over and over again.
- True
- False
Click here to go to next lesson on Roller Coasters.
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We’re going to build monster roller coasters in your house using just a couple of simple materials. You might have heard how energy cannot be created or destroyed, but it can be transferred or transformed (if you haven’t that’s okay – you’ll pick it up while doing this activity).
Roller coasters are a prime example of energy transfer: You start at the top of a big hill at low speeds (high gravitational potential energy), then race down a slope at break-neck speed (potential transforming into kinetic) until you bottom out and enter a loop (highest kinetic energy, lowest potential energy). At the top of the loop, your speed slows (increasing your potential energy), but then you speed up again and you zoom near the bottom exit of the loop (increasing your kinetic energy), and you’re off again!
Here’s what you need:
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- marbles
- masking tape
- 3/4″ pipe foam insulation (NOT neoprene and NOT the kind with built-in adhesive tape)
To make the roller coasters, you’ll need foam pipe insulation, which is sold by the six-foot increments at the hardware store. You’ll be slicing them in half lengthwise, so each piece makes twelve feet of track. It comes in all sizes, so bring your marbles when you select the size. The ¾” size fits most marbles, but if you’re using ball bearings or shooter marbles, try those out at the store. (At the very least you’ll get smiles and interest from the hardware store sales people.) Cut most of the track lengthwise (the hard way) with scissors. You’ll find it is already sliced on one side, so this makes your task easier. Leave a few pieces uncut to become “tunnels” for later roller coasters.
Read for some ‘vintage Aurora’ video? This is one of the very first videos ever made by Supercharged Science:
Download Student Worksheet & Exercises
Tips & Tricks
Loops Swing the track around in a complete circle and attach the outside of the track to chairs, table legs, and hard floors with tape to secure in place. Loops take a bit of speed to make it through, so have your partner hold it while you test it out before taping. Start with smaller loops and increase in size to match your entrance velocity into the loop. Loops can be used to slow a marble down if speed is a problem.
Camel-Backs Make a hill out of track in an upside-down U-shape. Good for show, especially if you get the hill height just right so the marble comes off the track slightly, then back on without missing a beat.
Whirly-Birds Take a loop and make it horizontal. Great around poles and posts, but just keep the bank angle steep enough and the marble speed fast enough so it doesn’t fly off track.
Corkscrew Start with a basic loop, then spread apart the entrance and exit points. The further apart they get, the more fun it becomes. Corkscrews usually require more speed than loops of the same size.
Jump Track A major show-off feature that requires very rigid entrance and exit points on the track. Use a lot of tape and incline the entrance (end of the track) slightly while declining the exit (beginning of new track piece).
Pretzel The cream of the crop in maneuvers. Make a very loose knot that resembles a pretzel. Bank angles and speed are the most critical, with rigid track positioning a close second. If you’re having trouble, make the pretzel smaller and try again. You can bank the track at any angle because the foam is so soft. Use lots of tape and a firm surface (bookcases, chairs, etc).
Troubleshooting Marbles will fly everywhere, so make sure you have a lot of extras! If your marble is not following your track, look very carefully for the point of departure – where it flies off.
-Does the track change position with the weight of the marble, making it fly off course? Make the track more rigid by taping it to a surface.
-Is the marble jumping over the track wall? Increase your bank angle (the amount of twist the track makes along its length).
-Does your marble just fall out of the loop? Increase your marble speed by starting at a higher position. When all else fails and your marble still won’t stay on the track, make it a tunnel section by taping another piece on top the main track. Spiral-wrap the tape along the length of both pieces to secure them together.
HOT TIPS for ULTRA-COOL PARENTS: This lab is an excellent opportunity for kids to practice their resilience, because we guarantee this experiment will not work the first several times they try it. While you can certainly help the kids out, it’s important that you help them figure it out on their own. You can do this by asking questions instead of rushing in to solve their problems. For instance, when the marble flies off the track, you can step back and say:
“Hmmm… did the marble go to fast or too slow?”
“Where did it fly off?”
“Wow – I’ll bet you didn’t expect that to happen. Now what are you going to try?”
Become their biggest fan by cheering them on, encouraging them to make mistakes, and try something new (even if they aren’t sure if it will work out).
Check out this cool roller coaster from one of our students!
Exercises
- What type of energy does a marble have while flying down the track of a roller coaster?
- What type of energy does the marble have when you are holding it at the top of the track?
- At the top of a camel back hill, which is higher for the marble, kinetic or potential energy?
- At the top of an inverted loop, which energy is higher, kinetic or potential energy?
Click here to go to next lesson on Including Friction in your Calculations.
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Most real life situations do involve friction. But how does it fit in with kinetic and potential energy equations? Here’s how:
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Click here to go to next lesson on Springs.
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Springs are used a lot in physics problems, because you can model things like car suspension systems, springs on door hinges, or even how flexible (or elastic) a material is by modeling it as a spring on paper for your analysis. Here's how:
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Springs in Physics Problems Now that you understand how to set up spring type problems, here's how to solve one using the conservation of energy equation:
Click here to go to next lesson on Springs and the Conservation of Energy.
[/am4show]Springs can launch projectiles huge distances, and they’re really easy to model on paper using the conservation of energy:
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Click here to go to next lesson on Car Suspension Problem.
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Here’s how you can model a car suspension system using a simple spring model and a couple of energy equations:
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Click here to go to next lesson on Elevator Nightmare.
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A lot of people’s worst nightmare is an elevator cable breaking while they are in the elevator. Let’s find out exactly how bad this type of accident can be from a physics perspective:
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Click here to go to next lesson on Speedy Waterslide.
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Do you like water slides? Did you know that you can find your speed that you hit the water without even knowing the shape of the slide? Here’s how…
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Click here to go to next lesson on Gravity Go-Karts.
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Nothing says summer time fun than a home-built go-kart that can race down the driveway with just as much thrill as two story roller coasters.
A go-karts (also called “go-cart”) can be gravity powered (without a motor) or include electric or gas powered motors. The gravity powered kind are also known as Soap Box Derby racers, and are the simplest kind to make since all you need is wheels, a frame, and a good hill (and a helmet!).
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Materials:
Hardware Bits and Pieces:
- 4-6″ wheels that do NOT swivel
- 3-5′ rope
- two door hinges
- One 4-1/2″ threaded hex head bolt with 4 washers and 2 nuts
- 2 heavy duty eye hooks
- Box of 1-1/2″ long coarse threaded wood or drywall screws
- Six 3″ long coarse threaded wood or drywall screws
Wood:
- one piece that is 4″ x 1″ x 24″
- one piece that is 6″ x 2″ x 8 feet cut into three pieces (one 4′ long piece and two pieces that are 2′ long each)
Tools:
- Crescent wrench, open end wrenches, or socket wrench
- Saw for cutting wood to size
- Drill and drill bits (also a 1/2″ bit)
- Measuring tape
- Pencil
Make sure you wear your HELMET and get someone to help you with the power tools!
The go-kart we’re going to make is long enough to hold two passengers, so feel free to shorten it up a bit if you’re only needing it for one passenger. You’ll need only a couple of tools like a drill and a saw, and also some experienced adult help and you’ll be off and riding this vehicle in under two hours, from start to finish.
After you’ve got this working, you’re probably going to be more than a little popular, especially with younger kids that might be too small to ride safely. Here’s a smaller version you can build them with only a few parts. You’ll not only get points for making something really cool, but it’ll keep them busy so you can ride your new go-kart!
And yes, you must INSIST that everyone wears helmets, or you’ll take the wheels off. Helmet hair is way more fashionable than squashed brain cells.
Click here to go to next lesson on Shooting the Sand.
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If you’ve ever thrown a ball down into the sand, you know it can bury itself below the surface. Here’s how you figure out the non-conservative forces into the equation of the sand exerting a force on the ball as it slows down and stops deep in the sand.
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Click here to go to next lesson on Friction Energy.
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Friction can be tricky to deal with, especially since it’s a non-conservative force (meaning that you can’t recover the energy from it for a useful purpose the way you can with potential and kinetic energy).
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Click here to go to next lesson on Driving with Physics in Mind.
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Have you learned how to drive yet, or are you excited to learn? Here’s a question on the driver’s test that is really kind of scary from a physics point of view, but it will make a lot of sense once you see how it works. And might even keep you from speeding, now that you understand what can happen if you lock up your brakes while going too fast.
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Click here to go to next lesson on Light Speed Particles.
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How do you calculate the energies of particles going near the speed of light? It’s a little tricky, but you can do it if you have the right equation. Since the kinetic energy equation comes from Newton’s Laws of Motion, which don’t apply to particles moving near the speed of light, we have to add a correction factor from Einstein’s Theory of Relativity in order to compensate and make the equations accurate. Here’s the equation for particles going close to light speed:
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Yay! You’ve completed this set of lessons! Now it’s time for you to work your own physics problems.
Download your Work, Energy and Power Problem Set here.
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Energy is the ability to do work. Work happens when something moves a distance against a force. Although it seems a little hard to comprehend, this is truly one of the most useful concepts in physics.
[am4show have='p9;p58;' guest_error='Guest error message' user_error='User error message' ] I’m willing to bet you spend a lot of your time moving things a distance against a force. Do you ever climb stairs, walk, ride a bicycle, or lift a fork to your mouth to eat? Of course you do. Each one of those things requires you to move something a distance against a force. You’re using energy and you’re doing work.
[am4show have='p9;p58;' guest_error='Guest error message' user_error='User error message' ] I’m willing to bet you spend a lot of your time moving things a distance against a force. Do you ever climb stairs, walk, ride a bicycle, or lift a fork to your mouth to eat? Of course you do. Each one of those things requires you to move something a distance against a force. You’re using energy and you’re doing work.
Click here to go to next lesson on What is Work?
[/am4show]Work is not that hard… it’s force that can be difficult. Imagine getting up a 10-step flight of stairs without a set of stairs. Your legs don’t have the strength or force for you to jump up… you’d have to climb up or find a ladder or a rope. The stairs allow you to, slowly but surely, lift yourself from the bottom to the top. Now imagine you are riding your bike and a friend of yours is running beside you.
Who’s got the tougher job? Your friend, right? You could go for many miles on your bike but your friend will tire out after only a few miles. The bike is easier (requires less force) to do as much work as the runner has to do. Now here’s an important point, you and your friend do about the same amount of work.
You also do the same amount of work when you go up the stairs versus climbing up the rope. The work is the same, but the force needed to make it happen is much different. Don’t worry if that doesn’t make sense now. As we move forward, it will become clearer. Before we start solving physics problems, we first have to accurately define a couple of terms we’re going to be using a lot that you might already have a different definition for.
Here are three concepts we’re going to be working with in this section:
- Work
- Energy
- Power
Energy is the ability to do work. Work is done on an object when a force acts on it so the object moves somewhere. It can be a large or small displacement, but as long as it’s not in its original position when it’s done, work is said to be done on the object. An example of work is when an apple falls off the tree and hits the ground. The apple falls because the gravitational force is acting on it, and it went from the tree to the ground. If you carry a heavy box up a flight of stairs, you are doing work on the box.
An example of what is not work is if you push really hard against a brick wall. The wall didn’t go anywhere, so you didn’t do any work at all (even though your muscles may not agree!). Mathematically, work is a vector, and is defined as the force multiplied by the distance like this: W = F d
If there’s an angle between the force and displacement vectors, then you’ll need to also multiply by the cosine of the angle between the two vectors. This is an important concept: Notice that the force has to cause the displacement. If you’re carrying a heavy box across the room (no stairs) at a constant speed, then you are not doing work on the box.
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The box is traveling in the horizontal direction at a constant speed. You are holding the heavy box up in the vertical direction. The force you are applying to the box is not causing it to be displaced in the same direction. There has to be a component of the force in the horizontal direction if you’re doing work on the box ((Remember F=ma? Constant speed means no acceleration!) Mathematically, the work equation would have angle between the force and the displacement vectors at 90 degrees, and the cosine of 90 degrees is zero, thus cancelling the work out to zero.
Click here to go to next lesson on Units for Energy.
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We’ll cover power in a little bit, but first we need to have a unit of measurement for work. The units for work and energy are the same, but note that energy and work are not the same. (Remember, energy is the ability to do work.)
For energy, a couple of units are the Joule (J) and the calorie (cal or Cal). A Joule is the energy needed to lift one Newton one meter. A Newton is a unit of force. One Newton is about the amount of force it takes to lift 100 grams or 4 ounces or an apple.
It takes about 66 Newtons to lift a 15-pound bowling ball and it would take a 250-pound linebacker about 1000 Newtons to lift himself up the stairs! So, if you lifted an apple one meter (about 3 feet) into the air you would have exerted one Joule of energy to do it.
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The calorie is generally used to talk about heat energy, and you may be a bit more familiar with it due to food and exercise. A calorie is the amount of energy it takes to heat one gram of water one degree Celsius. Four Joules are about one calorie. A 100-gram object takes about one Newton of force to lift. Since it took one Newton of force to lift that object, how much work did we do? Remember work = force x distance so in this case work = 1 Newton x 20 meters or work = 20 Joules.
Click here to go to next lesson on Moving Against a Force.
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This experiment is for Advanced Students. We’re going to really get a good feel for energy and power as it shows up in real life. For this experiment, you need:
- Something that weighs about 100 grams or 4 ounces, or just grab an apple.
- A meter or yard stick
This might seem sort of silly but it’s a good way to get the feeling for what a Joule is and what work is.
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Download Student Worksheet & Exercises
1. Grab your 100 gram object, put it on a table.
2. Now lift it off the table straight up until you lift it one meter (one yard).
3. Lift it up and down 20 times.
A 100 gram object takes about one Newton of force to lift. Since it took one Newton of force to lift that object, how much work did we do? Remember work = force x distance so in this case work = 1 Newton x 20 meters or work = 20 Joules.
You may ask “but didn’t we move it 40 meters, 20 meters up and 20 down?” That’s true, but work is moving something against a force. When you moved the object down you were moving the object with a force, the force of gravity. Only in lifting it up, are you actually moving it against a force and doing work. Four Joules are about 1 calories so we did 5 calories of work.
“Wow, I can lift an apple 20 times and burn 5 calories! Helloooo weight loss!” Well…not so fast there Richard Simmons. When we talk about calories in nutrition we are really talking about kilo calories. In other words, every calorie in that potato chip is really 1000 calories in physics. So as far as diet and exercise goes, lifting that apple actually only burned .005 calories of energy,…rats.
It is interesting to think of calories as the unit of energy for humans or as the fuel we use. The average human uses about 2000 calories (food calories that is, 2,000,000 actual calories) a day of energy. Running, jumping, sleeping, eating all uses calories/energy. Running 15 minutes uses 225 calories. Playing soccer for 15 minutes uses 140 calories. (Remember those are food calories, multiply by 1000 to get physics calories). This web site has a nice chart for more information: Calories used in exercise.
Everything we eat refuels that energy tank. All food has calories in it and our body takes those calories and converts them to calories/energy for us to use. How did the food get the energy in it? From the sun! The sun’s energy gives energy to the plants and when the animals eat the plants they get the energy from the sun as well.
So, if you eat a carrot or a burger you are getting energy from the sun! Eating broccoli gives you about 50 calories. Eating a hamburger gives you about 450 calories! We use energy to do things and we get energy from food. The problem comes when we eat more energy than we can use. When we do that, our body converts the energy to fat, our body’s reserve fuel tank. If you use more energy then you’ve taken in, then your body converts fat to energy. That’s why exercise and diet can help reduce your weight.
Let’s take the concept of work a little bit farther. If Bruno carries a 15 pound bowling ball up a 2 meter (6 foot) flight of stairs, how much work does he do on the bowling ball? It takes 66 Newtons of force to lift a 15 pound bowling ball 1 meter. Remember work = force x distance.
So, work = 66 Newtons x 2 meters. In this case, Bruno does 132 Joules of work on that bowling ball. That’s interesting, but what if we wanted to know how hard poor Bruno works? If he took a half hour to go up those stairs he didn’t work very hard, but if he did it in 1 second, well then Bruno’s sweating!
That’s the concept of power. Power is to energy like miles per hour is to driving. It is a measure of how much energy is used in a given span of time. Mathematically it’s Power = work/time. Power is commonly measured in Watts or Horsepower. Let’s do a little math and see how hard Bruno works.
In both cases mentioned above Bruno, does 132 Joules of work, but in the first case he does the work in 30 minutes (1800 seconds) and in the last case he does it in 1 second. Let’s first figure out Bruno’s power in Watts. A Watt is 1 Joule/second so:
For the half hour Bruno’s Power = 132 Joules/1800 seconds = .07 Watts
For the second Bruno’s Power = 132 joules/1 second = 132 Watts
You can see that the faster you exert energy the more power you use. Another term for power is horsepower. You may have heard the term horsepower in car ads. The more powerful car can exert more energy faster, getting the car moving faster. A Dodge Viper has 450 horsepower which can accelerate a 3,300 pound car from 0 to 60 mph in 4.1 seconds…WOW!
One horsepower is 745 Watts or one Watt is .001 horsepower. So converting Watts to horsepower poor Bruno exerts:
.07 x .001 = .00007 horsepower over the half hour
132 x .001 = .132 horsepower over the second (not exactly a Dodge Viper!)
Exercises
- If something has a weight of 2 Newtons and is moved half a meter, how many Joules of energy are used? Show your work.
- What is the source of all this energy we’re working with here?
- It doesn’t count as work when you move the apple back down. Why not?
Click here to go to next lesson on Energy of Food.
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A peanut is not a nut, but actually a seed. In addition to containing protein, a peanut is rich in fats and carbohydrates. Fats and carbohydrates are the major sources of energy for plants and animals.
The energy contained in the peanut actually came from the sun. Green plants absorb solar energy and use it in photosynthesis. During photosynthesis, carbon dioxide and water are combined to make glucose. Glucose is a simple sugar that is a type of carbohydrate. Oxygen gas is also made during photosynthesis.
The glucose made during photosynthesis is used by plants to make other important chemical substances needed for living and growing. Some of the chemical substances made from glucose include fats, carbohydrates (such as various sugars, starch, and cellulose), and proteins.
Photosynthesis is the way in which green plants make their food, and ultimately, all the food available on earth. All animals and nongreen plants (such as fungi and bacteria) depend on the stored energy of green plants to live. Photosynthesis is the most important way animals obtain energy from the sun.
Oil squeezed from nuts and seeds is a potential source of fuel. In some parts of the world, oil squeezed from seeds-particularly sunflower seeds-is burned as a motor fuel in some farm equipment. In the United States, some people have modified diesel cars and trucks to run on vegetable oils.
Fuels from vegetable oils are particularly attractive because, unlike fossil fuels, these fuels are renewable. They come from plants that can be grown in a reasonable amount of time.
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Materials
- Shelled peanut
- Small pair of pliers
- Match or lighter
- Sink
Download Student Worksheet & Exercises
Procedure
ASK AN ADULT TO HELP YOU WITH THIS EXPERIMENT. DO NOT DO THIS EXPERIMENT BY YOURSELF. The fuel from the peanut can flare up and burn for a longer time than expected.
Close the drain in the kitchen sink. Fill the sink with water until the bottom of the sink is just covered.
Using a small pair of pliers, hold the peanut over the sink containing water. Ask an adult to hold the flame of a lit match or lighter directly under the peanut. When the peanut starts to burn, the lit match or lighter can be removed.
Allow the peanut to burn for one minute. MAKE SURE AN ADULT REMAINS PRESENT AND MAKE SURE TO HOLD THE PEANUT OVER THE SINK. To extinguish the burning peanut, drop it into the water. After you have extinguished the peanut, allow it to cool and then examine it carefully.
Observations
How long does it take for the peanut to start to burn? Does the peanut burn with a clean flame or a sooty flame? What color is the flame? What color does the peanut turn when it burns? Did the size of the peanut change after it has burned for several minutes?
Discussion
You should find that the peanut ignites and burns after a lit match or lighter is held under it for a few seconds. Although you only let the peanut burn for one minute as a safety measure, the peanut would burn for many minutes.
In this experiment, when the peanut burns, the stored energy in the fats and carbohydrates of the peanut is released as heat and light. When you eat peanuts, the stored energy in the fats and carbohydrates of the peanut is used to fuel your body.
Other Things to Try
Hold one end of a piece of uncooked spaghetti in a pair of pliers. Ask an adult to hold the flame of a lit match or lighter under the other end of the spaghetti. When the spaghetti starts to burn, place it in an aluminum pie pan that is in the sink. Make sure to extinguish the burning spaghetti with water when you are finished with the experiment. How does the burning of the spaghetti compare with the burning of the peanut?
Exercises
- What is the process called where plants get food from the sun?
- Osteoporosis
- Photosynthesis
- Chlorophyll
- Metamorphosis
- Where does all life on the planet get its food?
- List two ways that we could use the energy in a peanut:
Click here to go to next lesson on Bomb Calorimeter.
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This experiment is for advanced students. Did you know that eating a single peanut will power your brain for 30 minutes? The energy in a peanut also produces a large amount of energy when burned in a flame, which can be used to boil water and measure energy.
Peanuts are part of the bean family, and actually grows underground (not from trees like almonds or walnuts). In addition to your lunchtime sandwich, peanuts are also used in woman’s cosmetics, certain plastics, paint dyes, and also when making nitroglycerin.
What makes up a peanut? Inside you’ll find a lot of fats (most of them unsaturated) and antioxidants (as much as found in berries). And more than half of all the peanuts Americans eat are produced in Alabama. We’re going to learn how to release the energy inside a peanut and how to measure it.
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Materials:
- raw peanuts
- chemistry stand with glass test tube and holder (watch video)
- flameproof surface (large ceramic tile or cookie sheet)
- paper clip
- alcohol burner or candle with adult help
- fire extinguisher
Download Student Worksheet & Exercises
What’s Going On? There’s chemical energy stored inside a peanut, which gets transformed into heat energy when you ignite it. This heat flows to raise the water temperature, which you can measure with a thermometer. You should find that your peanut contains 1500-2100 calories of energy! Now don’t panic… this isn’t the same as the number of calories you’re allowed to eat in a day. The average person aims to eat around 2,000 Calories (with a capital “C”). 1 Calorie = 1,000 calories. So each peanut contains 1.5-2.1 Calories of energy (the kind you eat in a day). Do you see the difference?
But wait… did all the energy from the peanut go straight to the water, or did it leak somewhere else, too? The heat actually warmed up the nearby air, too, but we weren’t able to measure that. If you were a food scientist, you’d use a nifty little device known as a bomb calorimeter to measure calorie content. It’s basically a well-insulated, well-sealed device that catches nearly all the energy and flows it to the water, so you get a much more accurate temperature reading. (Using a bomb calorimeter, you’d get 6.1-6.8 Calories of energy from one peanut!)
How do you calculate the calories from a peanut?
Let’s take an example measurement. Suppose you measured a temperature increase from 20 °C to 100 °C for 10 grams of water, and boiled off 2 grams. We need to break this problem down into two parts – the first part deals with the temperature increase, and the second deals with the water escaping as vapor.
The first basic heat equation is this:
Q = m c T
Q is the heat flow (in calories)
m is the mass of the water (in grams)
c is the specific heat of water (which is 1 degree per calorie per gram)
and T is the temperature change (in degrees)
So our equation becomes: Q = 10 * 1 * 80 = 800 calories.
If you measured that we boiled off 2 grams of water, your equation would look like this for heat energy:
Q = L m
L is the latent heat of vaporization of water (L= 540 calories per gram)
m is the mass of the water (in grams)
So our equation becomes: Q = 540 * 2 = 1080 calories.
The total energy needed is the sum of these two:
Q = 800 calories + 1080 calories = 1880 calories.
Click here to go to next lesson on Back to work!
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We’re going to learn how to calculate the amount of work done by forces by looking at how the force acts on the object, and if it causes a displacement. Have you spotted the three things you need to know in order to calculate the work done?
- Force
- Displacement
- Angle between the force and displacement vector (called theta)
The easiest way to do this is to show you by working a set of physics problems. So take out your notebook and a pencil, and do these problems right along with me. Here we go!
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Click here to go to next lesson on Work done by Friction.
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How do we calculate the work done by friction? Here’s a classic problem that shows you how to handle friction forces in your physics problems.
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Click here to go to next lesson on How much work in climbing stairs?.
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Ever get out of breath while climbing stairs? How much work do you think you did? Let’s find out…
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Click here to go to next lesson on Non-conservative Forces.
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Work done by friction is never conserved, since it’s turned into heat or sound, and we can’t get that back. It’s a non-conservative force. Other forces like gravity and speed are said to be conservative, since we can transfer that energy to a different form for a useful purpose. When you pull back a swing and then let go, you’re using the energy created by the gravitational force on the swing and transforming it into the forward motion of the swing as it moves through its arc. Energy from friction forces cannot be recovered, so we say that it’s an external energy, or work done by an external force.
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Click here to go to next lesson on Kinetic Energy.
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All the different forms of energy (heat, electrical, nuclear, sound, and so forth) can be broken down into two main categories: potential and kinetic energy. Kinetic energy is the energy of motion. Kinetic energy is an expression of the fact that a moving object can do work on anything it hits; it describes the amount of work the object could do as a result of its motion. Whether something is zooming, racing, spinning, rotating, speeding, flying, or diving… if it’s moving, it has kinetic energy.
How much energy it has depends on two important things: how fast it’s going and how much it weighs. A bowling ball cruising at 100 mph has a lot more kinetic energy than a cotton ball moving at the same speed.
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Click here to go to next lesson on Bow and Arrow Problem.
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Imagine an arrow is shot from a bow and by the time it hits an apple it is traveling with 10 Joules of kinetic energy (kinetic energy is the energy of motion). What’s meant by kinetic energy is that when it hits something, it can do that much work on whatever is hit.
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Soooo, back to the arrow... if the arrow hits that apple it can exert 10 Joules of energy on that apple. It takes about 1 Newton of force to move that apple so the arrow can move the apple 10 meters. One Joule equals one Newton x one meter so 10 Joules would equal one Newton x 10 meters.
It could also exert a force of 10 Newtons over one meter or any other mathematical calculation you’d like to play with there. (This, by the way, is completely hypothetical. With the apple example we are conveniently ignoring a bunch of stuff like the fact that the arrow would actually pierce the apple, and that there’s friction, heat, sound, and a variety of other forces and energies that would take place here.)
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Click here to go to next lesson on Freefall Pennies.
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Here’s a fun experiment that uses a penny in free fall to practice calculating kinetic energy.
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Energy changes to other forms of energy all the time. The electrical energy coming out of a wall socket transfers to light energy in the lamp. The chemical energy in a battery transfers to electrical energy which transfers to sound energy in your personal stereo. In the case of the ball falling, gravitational potential energy transfers to kinetic energy, the energy of motion.
Click here to go to next lesson on Potential Energy.
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Think of potential energy as the “could” energy. The battery “could” power the flashlight. The light “could” turn on. I “could” make a sound. That ball “could” fall off the wall. That candy bar “could” give me energy. Potential energy is the energy that something has that can be released. Objects can store energy as a result of their position.
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Click here to go to next lesson on Elastic Energy.
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There are many different kinds of potential energy. We’ve already worked with gravitational potential energy, so let’s take a look at elastic potential energy.
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Materials: a rubber band
A simple way to demonstrate elastic energy is to stretch a rubber band without releasing it. The stretch in the rubber band is your potential energy. When you let go of the rubber band, you are releasing the potential energy, and when you aim it toward a wall, it’s converted into motion (kinetic energy).
Here’s another fun example: the rubber band can also show how every is converted from one form to another. If you place the rubber band against a part of you that is sensitive to temperature changes (like a cheek or upper lip), you can sense when the band heats up. Simply stretch and release the rubber band over and over, testing the temperature as you go. Does it feel warmer in certain spots, or in just one location?
Click here to go to next lesson on Energy Transforms.
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The rubber band can also show how energy is converted from one form to another. If you place the rubber band against a part of you that is sensitive to temperature changes (like a cheek or upper lip), you can sense when the band heats up. Simply stretch and release the rubber band over and over, testing the temperature as you go. Does it feel warmer in certain spots, or in just one location?
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There are other ways to store potential energy. For example, the battery has the potential energy to light the bulb of the flashlight if the flashlight is turned on and the energy is released from the battery. Your legs have the potential energy to make you hop up and down if you want to release that energy (like you do whenever it’s time to do science!). The fuel in a gas tank has the potential energy to make the car move. Those are all ways to store potential energy.
Click here to go to next lesson on WackaPOW!
[/am4show]In this experiment, you’re looking for two different things: first you’ll be dropping objects and making craters in a bowl of flour to see how energy is transformed from potential to kinetic, but you’ll also note that no matter how carefully you do the experiment, you’ll never get the same exact impact location twice.
To get started, you’ll need to gather your materials for this experiment. Here’s what you need:
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- several balls of different weights no bigger then the size of a baseball (golf ball, racket ball, ping pong ball, marble etc. are good choices)
- fill a good size container or mixing bowl with flour or corn starch (or any kind of light powder)
- If you’re measuring your results, you’ll also need a tape measure (or yard stick) and a spring scale (or kitchen scale).
Are you ready?
1. Fill the container about 2 inches or so deep with the flour.
2. Weigh one of the balls (If you can, weigh it in grams).
3. Hold the ball about 3 feet (one meter) above the container with the flour.
4. Drop the ball.
5. Whackapow! Now take a look at how deep the ball went and how far the flour spread. (If all your balls are the same size but different weights it’s worth it to measure the size of the splash and the depth the ball went. If they are not, don’t worry about it. The different sizes will effect the splash and depth erratically.
6. Try it with different balls. Be sure to record the mass of each ball and calculate the potential energy for each ball.
Each one of the balls you dropped had a certain amount of potential energy that depended on the mass of the ball and the height it was dropped from. As the ball dropped the potential energy changed to kinetic energy until, “whackapow”, the kinetic energy of the ball collided with and scattered the flour. The kinetic energy of the ball transferred kinetic energy and heat energy to the flour.
For Advanced Students:
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Calculate the gravitational potential energy of the ball. Take the mass of the ball, multiply it by 10 m/s2 and multiply that by 1 meter. For example, if your ball had a mass of 70 grams (you need to convert that to kilograms so divide it by 1000 so that would be .07 grams) your calculation would be
PE=.07 x 10 x 1 = .7 Joules of potential energy.
So, how much kinetic energy did the ball in the example have the moment it impacted the flour? Well, if all the potential energy of the ball transfers to kinetic energy, the ball has .7 Joules of kinetic energy.
Create a table in your science journal or use ours. (You’ll need Microsoft Excel to use this file.)
Click here to go to next lesson on Power.
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We didn’t finish with our three concepts of energy, work, and power yet! The important concept of Power is work done over time, and is measured in watts (W), which is a Joule per second (J/s).
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Work doesn’t have anything to do with time, but power does. Sometimes work is done slow, and other times faster. Someone hiking a mountain can reach the peak way before a rock climber, even though they are both traveling the same vertical distance. A hiker in our example has a higher power rating than a rock climber. Power is the rate that work is done.
Click here to go to next lesson on Power is Scalar.
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Is power a vector or a scalar quantity? Power is a scalar, but it’s made up of two vector quantities of force and velocity like this:
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Click here to go to next lesson on What size engine do you need?.
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What if you’re wanting to get a motor for a winch on the front of your jeep? What size motor do you need? Here’s how to calculate the minimum power so you don’t spend more cash than you need to for a motor that will still do the job. (Near the end of the video below, I’ll show you how to convert watts to horsepower.)
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I love to water ski (no kidding!). Here’s a neat problem about how to determine some things about the boat and deal with weird units like knots in your calculations.
Click here to go to next lesson on Work-Energy Relationship.
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This is a recording of a recent live teleclass I did with thousands of kids from all over the world. I've included it here so you can participate and learn, too!
Spark together electric motors, build homemade burglar alarms, wire up circuits and build your own robot from junk! Create your own whizzing, hopping, dancing, screeching, swimming, crawling, wheeling, robot during class. We'll cover hot topics in electricity, magnetism, electrical charges, robot construction, sensors and more.
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Download the worksheet for this teleclass HERE.
Robots need electricity to make the motors move, the LEDs light up, the buzzers to sound, and more. When you move electrons around, that’s what creates the electricity. When you rub a balloon on your head for example, you’re picking off the electrons from the atoms in your hair and sticking them on the balloon. There's a static charge on your head due to the extra electrons.
The electrons have a negative charge, and so just like the north and south poles of a magnet attract each other, the negative charge of the electron is attracted to positive charges. That’s why your batteries have plus and minus signs on them. Electricity is when the electric charge is moving around inside the wires in the circuit.
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Spark together electric motors, build homemade burglar alarms, wire up circuits and build your own robot from junk! Create your own whizzing, hopping, dancing, screeching, swimming, crawling, wheeling, robot during class. We'll cover hot topics in electricity, magnetism, electrical charges, robot construction, sensors and more.
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- AA battery case (Jameco 216081)
- Two AA batteries for your battery case**
- A couple of LEDs (You can use any under 3V, but if you need recommendations, try Jameco 2-lead LEDs or 3-lead LEDs)
- Set of 10 alligator wires (Jameco 10444)
- One or two 1.5-3V DC motors (Jameco 231925)
- Index card
- 6 brass fasteners
- Two large paper clips
- One foam block (2” cube or larger)
- 1 wooden clothespin
- 10 wooden skewers
- Drill & bit the size of the motor shaft diameter
- Hot glue gun, razor and adult assistant
- OPTIONAL: Buzzer (Jameco 24872)
Download the worksheet for this teleclass HERE.
Key Concepts
A robot not only moves but it can also interact with its environment and it does that by using sensors, like light detectors that can see light, you can have motion detectors that can sense movement, touch sensors, pressure sensors, infrared light sensors, proximity sensors, water detectors, spit sensors, detecting all different kinds of stuff!Robots need electricity to make the motors move, the LEDs light up, the buzzers to sound, and more. When you move electrons around, that’s what creates the electricity. When you rub a balloon on your head for example, you’re picking off the electrons from the atoms in your hair and sticking them on the balloon. There's a static charge on your head due to the extra electrons.
The electrons have a negative charge, and so just like the north and south poles of a magnet attract each other, the negative charge of the electron is attracted to positive charges. That’s why your batteries have plus and minus signs on them. Electricity is when the electric charge is moving around inside the wires in the circuit.
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