Special Science Teleclass: Flying Machines

Sunday September 7, 2014

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!


Soar, zoom, fly, twirl, and gyrate with these amazing hands-on classes which investigate the world of flight. Students created flying contraptions from paper airplanes and hangliders to kites! Topics we will cover include: air pressure, flight dynamics, and Bernoulli’s principle.


Materials:


  • 5 sheets of 8.5×11” paper
  • 2 index cards
  • 2 straws
  • 2 small paper clips
  • Scissors, tape
  • Optional: ping pong ball and a small funnel

[am4show have=’p8;p9;p11;p38;p93;’ guest_error=’Guest error message’ user_error=’User error message’ ]



Key Concepts

While the kids are playing with the experiments see if you can get them to notice these important ideas. When they can explain these concepts back to you (in their own words or with demonstrations), you’ll know that they’ve mastered the lesson.


  1. Air pressure is all around us. Air pushes downward and creates pressure on all things.
  2. Air pressure changes all the time.
  3. Higher pressure always pushes.
  4. The faster air travels over a surface, the less time it has to push down on that surface and create pressure. Fast moving air creates low pressure regions. (Bernoulli’s Law).
  5. The four fundamental forces on an airplane are lift, weight, thrust, and drag.

What’s Going On?

There’s air surrounding us everywhere, all at the same pressure of 14.7 pounds per square inch (psi). You feel the same force on your skin whether you’re on the ceiling or the floor, under the bed or in the shower.


An interesting thing happens when you change a pocket of air pressure – things start to move. This difference in pressure causes movement that creates winds, tornadoes, airplanes to fly, and some of the experiments we’re about to do together.


An important thing to remember is that higher pressure always pushes stuff around. While lower pressure does not “pull,” we think of higher pressure as a “push”. The higher pressure inside a balloon pushes outward and keeps the balloon in a round shape.


Weird stuff happens with fast-moving air particles. When air moves quickly, it doesn’t have time to push on a nearby surface, such as an airplane wing. The air just zooms by, barely having time to touch the surface, so not much air weight gets put on the surface. Less weight means less force on the area. You can think of “pressure” as force on a given area or surface. Therefore, a less or lower pressure region occurs wherever there is fast air movement.


There’s a reason airplane wings are rounded on top and flat on the bottom. The rounded top wing surface makes the air rush by faster than if it were flat. When you put your thumb over the end of a gardening hose, the water comes out faster when you decrease the size of the opening. The same thing happens to the air above the wing: the wind rushing by the wing has less space now that the wing is curved, so it zips over the wing faster, and creates a lower pressure area than the air at the bottom of the wing.


The Wright brothers figured how to keep an airplane stable in flight by trying out a new idea, watching it carefully, and changing only one thing at a time to improve it. One of their biggest problems was finding a method for generating enough speed to get off the ground. They also took an airfoil (a fancy word for “airplane wing”), turned it sideways, and rotated it around quickly to produce the first real propeller that could generate an efficient amount of thrust to fly an aircraft.  Before the Wright brothers perfected the airfoil, people had been using the same “screw” design created by Archimedes in 250 BC.  This twist in the propeller was such a superior design that modern propellers are only 5% more efficient than those created a hundred years ago by the two brilliant Wright brothers.


Questions to Ask

When you’ve worked through most of the experiments ask your kids these questions and see how they do:


  1. Higher pressure does which? (a) pushes (b) pulls (c) decreases temperature (d) meows (e) causes winds, storms, and airplanes to fly
  2. The tips on the edge of a paper airplane wing provide more lift by: (a) flapping a lot
    (b) destroying wingtip vortices that kill lift (c) getting stuck in a tree more easily (d) decreasing speed
  3. In the ping pong ball and funnel experiment, the ball stayed in the funnel was because:         (a) you couldn’t blow hard enough (b) you glued it into the funnel (c) the ball had a hole in it  (d) the fast blowing caused a low-pressure region around the ball, causing the surrounding atmospheric pressure to be a higher pressure, thus pushing the ball into the funnel
  4. If your plane takes a nose dive, you should try (a) changing the elevators by pinching the edges (b) change the dihedral angle (c) change how you throw it (d) all of the above
  5. What are the four forces that act on every airplane in flight?
  6. Draw a quick sketch of your plane viewed from the front with a positive dihedral.
  7. If you were designing your own “Flying Paper Machine Kit”, what would be inside the box?
  8. What’s the one thing you need to remember about higher pressure?
  9. What keep an airplane from falling?
  10. Where is the low pressure area on an airplane wing?

Answers:


1 (a, e) 2 (b) 3 (d) 4 (d) 5 (lift, weight, thrust, drag) 8 (higher pressure pushes) 9 (lift) 10 (top surface)


[/am4show]


Special Science Teleclass: Rocketry & Spaceflight

Monday September 1, 2014
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! Blast your imagination with this super-popular class on rocketry! Kids learn about fin design, hybrid and solid-state rocketry, and how rockets make it into space without falling out of orbit. This class is taught by a real live rocket scientist (me!). We'll launch rockets during the class, too! [am4show have='p8;p9;p11;p38;p95;' guest_error='Guest error message' user_error='User error message' ] Materials:
  • straw
  • paperclip
  • rubber band
  • index card
  • popsicle stick
  • scissors
  • masking tape
  • water
  • alka-seltzer tablets (generic brands work fine)
  • film canister or small container with tight-fitting lid
  • OPTIONAL: Small toy car
Below you will find an older version of the same teleclass. We are making a different experiment during class, so the materials you will need are a little different: Materials:
  • 2L soda bottle
  • 1/2" PVC pipe
  • duct tape
  • pen or pencil
  • index cards
  • sheets of paper
  • bicycle inner tube

Key Concepts

A rocket has a few parts different from an airplane. One of the main differences is the absence of wings. Rockets utilize fins, which help steer the rocket, while airplanes use wings to generate lift. Rocket fins are more like the rudder of an airplane than the wings. Another difference is the how rockets get their speed. Airplanes generate thrust from a rotating blade, whereas rockets get their movement by squeezing down a high-energy gaseous flow and squeezing it out a tiny exit hole. If you've ever used a garden hose, you already know how to make the water stream out faster by placing your thumb over the end of the hose. You're decreasing the amount of area the water has to exit the hose, but there's still the same amount of water flowing out, so the water compensates by increasing its velocity. This is the secret to rocket nozzles - squeeze the flow down and out a small exit hole to increase velocity. The rockets we're about to build get their thrust by generating enough pressure and releasing that pressure very quickly. You will generate pressure both by pumping and by chemical reaction, which generates gaseous products.

What's Going On?

For every action, there is equal and opposite reaction. If flames shoot out of the rocket downwards, the rocket itself will soar upwards. It's the same thing if you blow up a balloon and let it go-the air inside the balloon goes to the left, and the balloon zips off to the right (at least, initially). Your rocket generates a high pressure by squeezing the air into a very small space and using Bernoulli's Principles in action! As you stomp on the rocket, the air pressure leaves the bottle pretty quickly, pushing the paper rocket out of the way as it zooms out of the tube. By narrowing the exit diameter, you allow the air to speed up as it exits, creating a higher launch for your rocket. You can modify your rocket body design. Add more fins, tilt the fins at a angle, or try no fins at all! You can add a more steeply slanted  nose. You can cut the rocket body in half or make it twice as long.  There's so many things you can test out, change, or modify with this simple activity! You can also add canards (glider-type wings) to either side of the rocket body right under the nose and turn it into a glider when it starts to fall back to Earth!

Questions to Ask

  1. Does it matter how many fins you use?
  2. What happens if there's an air leak in the system?
  3. How can you make the rocket fly even higher? Name three different ways.
  4. Is the center of pressure before or aft of the center of gravity on your rocket?
  5. For stable flight, how many fins do you ideally need?
  6. How can you make the rocket spin as it launches?
[/am4show]

Simple Hovercraft

Wednesday February 10, 2010
Hovercraft transport people and their stuff across ice, grass, swamp, water, and land. Also known as the Air Cushioned Vehicle (ACV), these machines use air to greatly reduce the sliding friction between the bottom of the vehicle (the skirt) and the ground. This is a great example of how lubrication works – most people think of oil as the only way to reduce sliding friction, but gases work well if done right.

In this case, the readily-available air is shoved downward by the pressure inside of balloon. This air flows down through the nozzle and out the bottom, under the CD, lifting it slightly as it goes and creating a thin layer for the CD to float on.

Although this particular hovercraft only has a 'hovering' option, I'm sure you can quickly figure out how to add a 'thruster' to make it zoom down the table! (Hint - you will need to add a second balloon!)

Here's what you need:

[am4show have='p8;p9;p10;p37;p151;p92;' guest_error='Guest error message' user_error='User error message' ]
  • 7-9" balloon
  • water bottle with a sport-top (see video for a visual - you can also use the top from liquid dish soap)
  • old CD
  • paper cup (or index card)
  • thumbtack
  • hot glue gun
  • razor with adult help




Download Student Worksheet here.

There's air surrounding us everywhere, all at the same pressure of 14.7 pounds per square inch (psi). You feel the same force on your skin whether you're on the ceiling or the floor, under the bed or in the shower. An interesting thing happens when you change a pocket of air pressure - things start to move.

This difference in pressure causes movement that creates winds, tornadoes, airplanes to fly, and the air to rush out of a full balloon. An important thing to remember is that higher pressure always pushes stuff around. While lower pressure does not "pull," we think of higher pressure as a "push".

The stretchy balloon has a higher pressure inside than the surrounding air, and the air is allowed to escape out the nozzle which is attached to the water bottle cap through tiny holes (so the whole balloon doesn't empty out all at once and flip over your hovercraft!) The steady stream of air flows under the CD and creates a cushion of air, raising the whole hovercraft up slightly... which makes the hovercraft easy to slide across a flat table.

Want to make an advanced model Hovercraft using wires, motors, and leftovers from lunch? Then click here.

[/am4show] [am4show have='p9;p39;' guest_error='Guest error message' user_error='User error message' ] Advanced students: Download your Hovercraft Lab here. [/am4show]

Unit 1: Mechanics (Force) Video

Friday August 14, 2009

There are four forces in the universe that make everything move, shift, explode, zoom, wiggle, and dance. As two of these forces require a nuclear reactor in your garage, we’ll just focus on the other two for now: the electromagnetic and the gravitational.


This video gets you started on the right foot. We’ll outline what’s coming up for this week and how to get the most out of our lesson together. Enjoy!


[am4show have=’p8;p9;p11;p38;’ guest_error=’Guest error message’ user_error=’User error message’ ]



[/am4show]


Detecting the Gravitational Field

Friday August 14, 2009

Ok, sort of a silly experiment I admit. But here’s what we’re going for – there is an invisible force acting on you and the ball. As you will see in later lessons, things don’t change the way they are moving unless a force acts on them. When you jump, the force that we call gravity pulled you back to Earth. When you throw a ball, something invisible acted on the ball forcing it to slow down, turn around, and come back down. Without that force field, you and your ball would be heading out to space right now!
[am4show have=’p8;p9;p11;p38;p92;’ guest_error=’Guest error message’ user_error=’User error message’ ]
Here’s what you need:


  • you
  • the Earth (or any planet that’s convenient)
  • a ball


Download Student Worksheet & Exercises


Here’s what you do:


1. Jump!
2. Carefully observe whether or not you come back down.
3. Take the ball and throw it up.
4. Again, watch carefully. Does it come down?


Gravity is probably the force field you are most familiar with. If you’ve ever dropped something on your foot you are painfully aware of this field! Even though we have known about this field for a looooong time, it still remains the most mysterious field of the four.


What we do know is that all bodies, from small atoms and molecules to gigantic stars, have a gravitational field. The more massive the body, the larger its gravitational field. As we said earlier, gravity is a very weak force, so a body really has to be quite massive (like moon or planet size) before it has much of a gravitational field. We also know that gravity fields are not choosy. They will attract anything to them.


All types of bodies, from poodles to Pluto, will will attract and be attracted to any other type of body. One of the strangest things about gravity is that it is only an attractive force. Gravity, as far as we can tell, only pulls things towards it. It does not push things away. All the other forces are both attractive (pull things towards them) and repulsive (push things away). (Gravity will be covered more deeply in a later lesson.)


Exercises 


  1. What did you determine about gravity and how it affects the rate of falling?
  2. Did changing the object affect the rate of falling? Why or why not?
  3. Did changing the variable affect the rate of falling?  Why or why not?

[/am4show]


[am4show have=’p9;p39;’ guest_error=’Guest error message’ user_error=’User error message’ ]


Advanced students: Download your Gravitational Force Lab here.


[/am4show]


Detecting the Magnetic Field

Friday August 14, 2009

Remember, there are four different kinds of forces: strong nuclear force,
electromagnetism, weak nuclear force, and gravity. There are also four basic force fields that you come into contact with all the time. They are the gravitational field, the electric field, the magnetic field, and the electromagnetic field. Notice that those four force fields really only use two of the four different kinds of force: electromagnetism and gravity. Let’s take a quick look at what causes these four fields and what kind of objects they can affect, starting with the magnetic field.


Here’s what you need:
[am4show have=’p8;p9;p11;p38;p72;p92;’ guest_error=’Guest error message’ user_error=’User error message’ ]


  • you
  • a compass

You’re probably fairly familiar with magnetic fields. If you’ve ever stuck a magnet to a refrigerator, you’ve taken advantage of magnetic fields. Sticking a magnet to a refrigerator is one of those every day experiences that should just be absolutely flabbergasting. There you are holding an “I’d Rather be Relative” magnet and it sticks to the fridge! But wait a minute, if you put it on the wall… it falls off! How does it “know” what to stick to? Not only does it stick to the fridge, it also pushes some things away, attracts other things and couldn’t care less about still other things. What’s that all about?! We rarely think about what magnets do but, wow, the things they do are weird!


Magnetic fields come from objects that have a surplus of electrons all moving in the same direction. This can be an electric wire with current running through it or one of several special types of metals. Iron, nickel and cobalt are the most common metals that can be magnetic. Magnetic fields can only affect objects that can be magnetic themselves. That’s why a magnet can attract an iron nail, but it can’t attract an aluminum can. The iron nail can be magnetic, but the aluminum cannot. Magnets can also be attractive or repulsive. Two magnets with the same kind of poles facing one another will push themselves apart. Two magnets with opposite poles facing one another will pull themselves together.



 
Download Student Worksheet & Exercises


Using a compass and the Earth, you can do a simple experiment to detect the magnetic field of our planet. (If you don’t have a compass, just slide a magnet along the length of a needle several times (make sure you only swipe in one direction!) then stick it through a cork or bit of foam. Float the needle-foam thing in a cup of water.)


1. Look at the compass


2. Walk anywhere and keep your eye on the compass.


3. Turn around in circles and keep your eye on the compass (don’t get too dizzy).


Again a very simple little activity, but I hope you can see the point. No matter where you went or what you did, that needle always pointed the same direction! The Earth’s magnetic force field, another strange and mysterious force, always pushes that needle in the same direction. It’s invisible and you can’t feel it…but the needle can!


 Exercises 


  1. Why does the needle need the foam?
  2.  Why do we use water?
  3. What are the forces in a magnetic field?

[/am4show]


The Electromagnetic Field

Friday August 14, 2009

iStock_000002030797XSmallThe electromagnetic field is a bit strange. It is caused by either a magnetic field or an electric field moving. If a magnetic field moves, it creates an electric field. If an electric field moves, it creates a magnetic field.


[am4show have=’p8;p9;p11;p38;p92;’ guest_error=’Guest error message’ user_error=’User error message’ ]
A moving electric field creates a moving magnetic field, which creates a moving electric field, which creates a moving magnetic field and on and on. Pretty strange huh? So an electromagnetic field is both an electric field and a magnetic field all rolled into one. Light, radio waves, and microwaves are examples of electromagnetic waves created by moving self-creating electronic and magnetic fields.


Until 1820, magnetic fields and electric fields were thought of as two completely separate things. A fellow by the name of Hans Christian Ørsted was preparing for a lecture when he noticed that a compass needle jumped when he flipped a switch that caused electricity to flow through some wires. This chance observation caused him to investigate further and discover that electric fields create magnetic fields and vice-versa.


Thus, through a completely random observation, electromagnetism was discovered. Without that discovery we wouldn’t have electric engines, radios, cell phones, television or Filbert the Flounder electric toothbrushes! Hooray for Ørsted!


A Summary of Force Fields and Objects

Type of Force Field Objects That Create Force Objects Are Affected By Force Force Field Can Attract and Repel
Gravitational Any object Any object Only attract
Magnetic Moving Electrons A metal that can be magnetic Attract and repel
Electric An electrically charged body Any body Attract and repel
Electromagnetic A moving magnetic field or a moving electric field A magnetic or electric field Attract and repel

The rest of this experiment (below) is for advanced students: (Hint: you need to have access to upper level content.)
[/am4show][am4show have=’p9;p38;p90;p44;’ guest_error=’Guest error message’ user_error=’User error message’ ]


Measuring the Earth’s Magnetic Pulse

When you stare at a compass, the needle that indicates the magnetic field from the Earth appears to stand still, but we’re going to find how it fluctuates and moves by creating a super-sensitive instrument using everyday materials (for comparison, you would spend over $100 for a scientific instrument that does the same thing).


Today you get to learn how to amplify tiny pulses in the Earth’s magnetic field using a laser and a couple of magnets. It’s a very cool experiment, but it does take patience to make it work right.


Materials


  • Index card or scrap of cardboard
  • 2 small mirrors
  • 2 rare earth magnets
  • Nylon filament (thin nylon thread works, too)
  • 4 doughnut magnets
  • Laser pointer (any kind will work – even the cheap key-chain type)
  • Clean glass jar (pickle, jam, mayo, etc… any kind of jar that’s heavy so it won’t knock over easily)
  • Wooden spring-type clothespin
  • Hot glue gun, scissors and tape


Download Student Worksheet & Exercises


  1. Sandwich the twine between the two rare earth magnets. These are the stronger magnets.
  2. Use a tiny dab of glue on one of the magnets and attach a mirror to the magnet. Do this on the other side for the second magnet and mirror.
  3. Lower the mirror-magnets into the container, leaving it hanging an inch above the bottom of the jar. Cut the twine at the mouth level of the container.
  4. Glue the top of the twine to the bottom of the lid, right in the center.
  5. When the glue has dried, place your mirror-magnets inside the jar and close the lid. Make sure that the mirror-magnets don’t touch the side of the jar, and are free to rotate and move.
  6. You’ve just built a compass! The small magnets will align with the Earth’s magnetic field. Slowly rotate the jar, and watch to see that the mirror-magnets inside always stay in the same configuration, just like the needle of a standard compass.
  7. Set your new compass aside and don’t touch it. You want the mirror-magnets to settle down and get very still.
  8. You are going to build the magnet array now. Stack your four doughnut magnets together in a tall stack.
  9. Fold your index card in half, and then open it back up. On one side of the crease you’re going to glue your magnets. When the magnets are attached, you’ll fold the card over so that it sits on the table like a greeting card with the magnets facing your glass jar.
  10. Tape your index card down to the table as you build your magnet array. (Otherwise the paper will jump up mid-way through and ruin your gluing while you are working.)
  11. Place a strip of glue on the bottom magnet of your stack and press it down onto the paper, gluing it into place.
  12. Lift the stack off (the bottom magnet should stay put on the paper) and place glue on the bottom magnet. Glue this one next to the first.
  13. Continue with the array so you have a rectangle (or square) arrangements of magnets with their poles oriented the same way. Don’t flip the magnets as you glue them, or you’ll have to start over to make sure they are lined up right.
    Since we live in a gigantic magnetic field that is 10,000 times more powerful than what the instrument is designed to measure, we have to “zero out” the instrument. It’s like using the “tare” or “zero” function on a scale. When you put a box on a scale and push “zero”, then the scale reads zero so it only measures what you put in the box, not including the weight of the box, because it’s subtracting the weight of the box out of the measurement. That’s what we’re going to do with our instrument: we need to subtract out the Earth’s magnetic field so we just get the tiny fluctuations in the field.
  14. Place your instrument away from anything that might affect it, like magnets or anything made from metal.
  15. Fold the card back in half and stand it on the table. We’re normally going to keep the array away from the jar, or the magnet array will influence the mirror-magnets just like bringing a magnet close to a compass does. But to zero out our instrument, we need to figure out how far away the array needs to be in order to cancel out the Earth’s field.
  16. Bring the array close to your jar. You should see the mirror-magnets align with the array.
  17. Slowly pull the magnet array away from the compass to a point where if it were any closer, the mirror-magnets would start to follow it, but any further away and nothing happens. It’s about 12 inches away. Measure this for your experiment and write it on your array or jar so you can quickly realign if needed in the future.
  18. Insert your laser pointer into the clothespin so that the jaws push the button and keep the laser on. Place it at least the same distance away as the array. You might have to prop the laser up on something to get the height just right so you can aim the laser so that it hits the mirror inside. (Note that you’ll have a reflection from the glass as well, but it won’t be nearly as bright.)
  19. Find where the laser beam is reflected off the mirror and hits the wall in your room. Walk over and tape a sheet of paper so that the dot is in the middle of the paper. Use a pen and draw right on top of the dot, and mark it with today’s date.
  20. Do you notice if it moves or it is stays put? Sometimes the dot will move over time, and other times the dot will wiggle and move back and forth. The wiggles will last a couple of seconds to a couple of minutes, and those are the oscillations and fluctuations you are looking for!
  21. Tape a ruler next to the dot so you can measure the amount of motion that the dot makes. Does it move a lot or a little when it wiggles? Two inches or six?

What’s Going On?

The reason this project works is because of tiny magnetic disturbances caused by the ripples in the ionosphere. Although these disturbances happen all the time and on a very small scale (usually only 1/10,000th of the Earth’s magnetism strength), we’ll be able to pick them up using this incredibly simple project. Your reflected laser beam acts like an amplifier and picks up the movement from the magnet in the glass.


Construction tip for experiment:
You need to use a filament that doesn’t care how hot or humid it is outside, so using one of the hairs from your head definitely won’t work. Cotton tends to be too stretchy as well. Professionals use fine quartz fibers (which are amazingly strong and really don’t care about temperature or humidity). Try extracting a single filament from a multi-stranded nylon twine length about 30″ long. If you happen to have a fine selection of nylon twine handy, grab the one that is about 25 microns (0.01″) thick. Otherwise, just get the thinnest one you can find.


You can tape a wooden clothespin down to the table and insert your laser pointer inside – the jaws will push the button of the laser down so you can watch your instrument and take your measurements. When you’re ready, tape a sheet of paper to the wall where your reflected beam (reflected from the mirror, not the glass… there will be two reflected beams!) hits the wall and mark where it hits. Over periods of seconds to minutes, you’ll see deflections and oscillations (wiggles back and forth) – you are taking the Earth’s magnetic pulse!


In order for this experiment to work properly, ALL magnets (including the penny described below) need to be in the same plane. That is, they all need to be the same height from the ground. You can, of course, rotate the entire setup 90 degrees to investigate the magnetic ripples in the other planes as well!


To make this instrument even more sensitive, glue a copper penny (make sure it’s minted before 1982, or you’ll get an alloy, not copper, penny) to the glass jar just behind the magnets (opposite the laser). When your magnets move now, they will induce eddy currents in the penny that will induce a (small) magnetic field opposite of the rotation of the magnets to dampen out “noise” oscillation. In short, add a penny to the glass to make your instrument easier to read.


Also, note that big, powerful magnets will not respond quickly, so you need a lightweight, powerful magnet.


You can walk around with your new instrument and you’ll find that it’s as accurate as a compass and will indicate north. You probably won’t see much oscillation as you do this. Because the Earth has a large magnetic field, you have to “tare” the instrument (set it to “zero”) so it can show you the smaller stuff. Use the doughnut magnets about 30 centimeters away as shown in the video.


Exercises


  1. Does the instrument work without the magnet array?
  2. Why did we use the stronger magnets inside the instrument?
  3. Which planet would this instrument probably not work on?

Advanced Students: Download your Electromagnetic Field Lab here.


[/am4show]


Detecting the Electric Field

Friday August 14, 2009

You are actually fairly familiar with electric fields too, but you may not know it. Have you ever rubbed your feet against the floor and then shocked your brother or sister? Have you ever zipped down a plastic slide and noticed that your hair is sticking straight up when you get to the bottom? Both phenomena are caused by electric fields and they are everywhere!


[am4show have=’p8;p9;p11;p38;p72;p76;p92;’ guest_error=’Guest error message’ user_error=’User error message’ ]
An electric field exists when at least one body is electrically charged. Atoms are filled with positively charged protons and negatively charged electrons. If an object has more electrons than protons, it will be negatively charged and if it has fewer electrons than protons, it will be positively charged. Electric fields, like magnetic fields, can attract and repel. If two bodies have the same kind of charge, that is either both are negative or both are positive, they will push themselves away from each other. If one body has a positive charge and the other has a negative charge, they will attract each other. Charged bodies can also attract bodies that are neither positive nor negative but are just neutral.


Electric fields are extremely common. If you comb your hair with a plastic comb, you cause that comb to have a small electric field. When you take off a fleece jacket or a polyester sweat shirt, you create an electric field that may be thousands of volts! Don’t worry, you can’t get hurt. There may be lots of voltage but there will be very little amperage. It’s the amperage that actually hurts you.


Here’s a simple experiment you can do that only needs four simple items:
– head of hair
– balloon
– yardstick or meterstick
– large spoon


Here’s what you do:



 
Download Student Worksheet & Exercises


Make sure you’ve tried out these Static Electricity experiments and learn how to light a bulb without plugging it into the wall!


Exercises 


  1. What happens if you rub the balloon on other things, like a wool sweater?
  2. If you position other people with charged balloons around the table, can you keep the yardstick going?
  3.  Can we see electrons?
  4.  How do you get rid of extra electrons?
  5.  Does the shape of the balloon matter?
  6.  Does hair color matter?
  7.  Rub a balloon on your head, and then lift it up about 6”. Why is the hair attracted to the balloon?
  8. Why does the hair continue to stand on end after the balloon is taken away?
  9. What other things does the balloon stick to besides the wall?
  10. Why do you think the yardstick moved?
  11. What other things are attracted or repelled the same way by the balloon? (Hint: try a ping pong ball.)

[/am4show]


Flying Paper Clip

Friday August 14, 2009

Have you ever been close to something that smells bad? Have you noticed that the farther you get from that something, the less it smells, and the closer you get, the more it smells? Well forces sort of work in the same way.


Forces behave according to a fancy law called the inverse-square law. To be technical, an inverse-square law is any physical law stating that some physical quantity or strength is inversely proportional to the square of the distance from the source of that physical quantity.


The inverse-square law applies to quite a few phenomena in physics. When it comes to forces, it basically means that the closer an object comes to the source of a force, the stronger that force will be on that object. The farther that same object gets from the force’s source, the weaker the effect of the force.


Mathematically we can say that doubling the distance between the object and the source of the force makes the force 1/4th as strong. Tripling the distance makes the force 1/9th as strong. Let’s play with this idea a bit.


Here’s what you need:


[am4show have=’p8;p9;p11;p38;p72;p77;p92;’ guest_error=’Guest error message’ user_error=’User error message’ ]


  • magnet
  • paper clip
  • string
  • tape

Using a magnet (the stronger the better), paper clip, string (or yarn or even dental floss!), and tape, you can make a flying paper clip.


OPTIONAL: If you happen to have a spring scale and ruler, get those out, too…otherwise, just skip these items – they are not essential to understanding the concept here.



 
Download Student Worksheet & Exercises


1. Tie about 4 inches of string to a paper clip.
2. Tape the magnet to the table.
3. Hold the end of the string that is not tied to the paper clip and let the paper clip dangle.
4. Slowly bring the paper clip closer and closer to the magnet.
5. Notice that the closer you get to the magnet, the stronger the force of the magnetic field is on the paper clip.


If you have a spring scale:
6. Attach the paper clip to the spring scale.
7. Move the paper clip closer to the magnet until the magnetic field affects the paper clip.
8. Measure how far the paper clip is from the magnet with a ruler.
9. Measure how much pull there is on the paper clip. Use newtons if your spring scale shows that measurement, but grams are OK if it doesn’t.
10. Bring the paper clip a half inch closer and measure the force of the pull again either in grams or Newtons.
11. Continue to get closer to the magnet half an inch at a time, measuring the force until you can’t get any closer.


What you may have noticed here was that the closer you got the paper clip to the magnet (the object causing the force field) the stronger the force was on the paper clip. You have just seen the inverse-square law in action!


Exercises: 


  1. Circle one: The closer you get to the magnet, the (stronger | weaker) the force of the magnetic field is on the paper clip.
  2. Why does it matter which way you orient the magnet in this experiment?
  3. Which magnet has the strongest magnetic field?
  4. Is the north or south pole stronger on a magnet?

[/am4show]


Force-full Cereal

Friday August 14, 2009

cerealDid you know that your cereal may be magnetic? Depending on the brand of cereal you enjoy in the morning, you’ll be able to see the magnetic effects right in your bowl. You don’t have to eat this experiment when you’re done, but you may if you want to (this is one of the ONLY times I’m going to allow you do eat what you experiment with!) For a variation, pull out all the different boxes of cereal in your cupboard and see which has the greatest magnetic attraction.


[am4show have=’p8;p9;p11;p38;p72;p77;p92;’ guest_error=’Guest error message’ user_error=’User error message’ ]


Materials:


  • a bowl of cereal with milk
  • spoon


 
Download Student Worksheet & Exercises


  1. Fill the bowl with milk.
  2. Put about 20 pieces of cereal (not the whole box!) into the bowl.
  3. Stir up the bowl a little and watch what happens.

If you watched carefully, you saw that as the cereal “O’s” got close to one another, they attracted each other. The closer they got, the stronger was their attraction to each other and the faster they moved towards each other. If you wait and watch long enough, you get a nice tight batch of cereal all clustered together in one or two big blobs. This activity is a great illustration of what is meant by the inverse square law because the attraction between “O’s” was stronger the closer they got to each other.


I discovered this activity one morning as I was eating cereal. The same thing happens with bubbles when you’re doing the dishes. Science is everywhere! Feel free to eat the cereal!


Exercises 


  1. Why do the pieces of cereal stick to each other?
  2.  Does the cereal move slower or faster the closer the pieces come in contact with each other?
  3.  What other cereals does it work for?

[/am4show]


Building Bridges

Friday August 14, 2009

What keeps building from toppling over in the wind? Why are some earthquake-proof and others not? We’re going to look at how engineers design buildings and bridges while making our own.


Here’s what you need:
[am4show have=’p8;p9;p11;p38;p72;p92;’ guest_error=’Guest error message’ user_error=’User error message’ ]


  • Index cards
  • Blocks
  • Straws
  • Clay
  • Disposable cups

Watch the video:




Download Student Worksheet Exercises


Exercises 


  1. What are three different kinds of forces?
  2.  Using only blocks, what kind of wall design is the weakest?
  3.  Why does the bridge seem stronger when a card is arched underneath?

[/am4show]


Net Forces

Friday August 14, 2009

It is very rare, especially on Earth, to have an object that is experiencing force from only one direction. A bicycle rider has the force of air friction pushing against him. He has to fight against the friction between the gears and the wheels. He has gravity pulling down on him. His muscles are pushing and pulling inside him and so on and so on.


Even as you sit there, you have at least two forces pushing and pulling on you. The force of gravity is pulling you to the center of the Earth. The chair is pushing up on you so you don’t go to the center of the Earth. So with all these forces pushing and pulling, how do you keep track of them all? That’s where net force comes in.


The net force is when you add up all of the forces on something and see what direction the overall force pushes in. The word “net”, in this case, is like net worth or net income. It’s a mathematical concept of what is left after everything that applies is added and subtracted. The next activity will make this clearer.


Here’s what you need:


[am4show have=’p8;p9;p11;p38;p92;’ guest_error=’Guest error message’ user_error=’User error message’ ]


  • a rope (at least 3 feet long is good)
  • a friend
  • a sense of caution

(Be careful with this. Don’t pull too hard and please don’t let go of the rope. This is fun but you can get hurt if you get silly.)



Download Student Worksheet Exercises


1. You and your friend each grab an opposite end of the rope.
2. Both of you pull just a bit on the rope.
3. Have your friend pull a bit harder than you. Notice the direction that you both move.
4. Now you pull harder than your friend. Now which way do you go?
5. Lastly, both of you pull with the same strength on the rope. Even though you are both pulling, neither one of you should move.


In this experiment, there were always at least three forces pulling on the rope. Can you think of the three? They are you, your friend and gravity. You were pulling in one direction. Your friend was pulling in another direction and gravity was pulling down.


When one of you pulled harder (put more force on the rope) than the other person, there was a net force in the direction of the stronger pull. The rope and you guys went in that direction.


When both of you were pulling the same amount, there was an equal force pulling the rope one way and another equal force pulling the rope the other way. Since there were two equal forces acting in two opposite directions, the net force equaled zero, so there was no movement in either direction. No net force in this case means no movement.


As you’ll see when you learn about Newton’s second law, no net force means no acceleration.


Let’s take another look at the bicycle rider we talked about earlier. To make things easier, we’ll call him Billy. For Billy to speed up, he needs to win the tug of war between all of the forces involved in riding a bicycle. In other words, his muscles need to put more force on the forward motion of the bike than all of the forces of friction that are pushing against him.


If he wants to slow down, he needs to allow the forces of friction to win the tug of war so that they will cancel out his forward motion and slow down the bike. If he wants to ride at a steady speed, he wants the tug of war to be tied. His muscles need to exert the same amount of force pushing forward as the friction forces pulling in other directions.


Advanced Students: Download your Net Forces Lab here.


Exercises:


  1. For scenario 1, in which direction did you both move?  Draw the free body diagram below
  2. For scenario 2, in which direction did you both move?  Draw the free body diagram below
  3.  For scenario 3, in which direction did you both move?  Draw the free body diagram below
  4. For scenario 4, in which direction did the rope move?  Draw the free body diagram below
  5. What was the same about question 1 and question 4?  What was different?
  6.  Even though the forces were less in question 1 than question 4, what was the net force for both?
  7.  There were always at least 3 forces acting on the rope, what were they?  Did you include the third force in your free body diagram?
  8. If the rope was slightly off the ground, with gravity pulling it down, what does that tell you about the force you and your friend exerted?

[/am4show]


Barrel Roof

Friday August 14, 2009

This roof can support over 400 times its own weight, and you don’t need tape! One of the great things about net forces is that although the objects can be under tremendous force, nothing moves! For every push, there’s an equal and opposite pull (or set of pulls) that cancel each other out.


This barrel roof is an excellent example of how to the forces all cancel out and the roof stands strong (hopefully!) If you have trouble with this experiment, just use cardstock or other heavy weight paper instead of regular copy paper.


Here’s what you need:
[am4show have=’p8;p9;p11;p38;p72;p92;’ guest_error=’Guest error message’ user_error=’User error message’ ]


Watch the video:



 
Download Student Worksheet & Exercises


Exercises 


  1. What is Newton’s Third Law?
  2. What kind of groups do forces come in?
  3. What is another name for Newton’s Third Law?

[/am4show]


Exercises for Force

Friday August 14, 2009

Let’s see how much you’ve picked up with these experiments and the reading – answer as best as you can. (No peeking at the answers until you’re done!)  Just relax and see what jumps to mind when  you read the question.  You can also print these out and jot down your answers in your science notebook.
[am4show have=’p8;p9;p11;p38;’ guest_error=’Guest error message’ user_error=’User error message’ ]
Here’s printer-friendly versions of the exercises and answers for you to print out: Simply click here for K-8 and here for K-12.


1. Name at least one force that is acting on you right now.


2. Name at least two invisible force fields that are surrounding you right now.


3. What kind of an object can be affected by a gravitational force field?


4. What kind of an object can be affected by an electrical force field?


5. What kind of an object can be affected by a magnetic force field?


6. What happens to the force on an object as it gets closer and closer to a magnet?


7. How does the force of the Sun’s gravitational pull on Neptune (the farthest planet from the Sun if you don’t count Pluto) compare to the force of the Sun’s gravitational pull on Mercury (the closest planet to the Sun).


Need answers?
[/am4show][am4show have=’p9;p38;’ guest_error=’Guest error message’ user_error=’User error message’ ]


For advanced students…

…we have a more advanced set of exercises at the back of your textbook download.


[/am4show]


Answers to Force Exercises

Friday August 14, 2009

Let’s see how you did! If you didn’t get a few of these, don’t let it stress you out – it just means you need to play with more experiments in this area. We’re all works in progress, and we have our entire lifetime to puzzle together the mysteries of the universe!


Answers:
[am4show have=’p8;p9;p11;p38;’ guest_error=’Guest error message’ user_error=’User error message’ ]
1. Gravity is pulling on you. If you’re sitting your chair is pushing up on you as well.


2. Gravity and magnetic fields. To be honest, you are probably also sitting in an electromagnetic field as well. Can you get a radio or a cell phone to work where you are? If so, you’re in an electromagnetic field.


3. Any object can be pulled by a gravitational force field.


4. Any object. An electrically charged object or a neutral object can be pushed or pulled by an electric field.


5. Another magnet or something with a metal in it that can be magnetic.


6. The force the magnet exerts on the object becomes greater and greater as the object gets closer. The inverse-square rule is a way of describing how force increases as objects get closer together.


7. Since Neptune is farther away, the inverse-square rule says that the Sun’s gravitation pull on it is much smaller.


[/am4show]


Unit 1: Mechanics (Gravity) Video

Wednesday August 12, 2009

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.  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). Let’s learn more about gravity right now:


[am4show have=’p8;p9;p11;p38;’ guest_error=’Guest error message’ user_error=’User error message’ ]



[/am4show]


Unit 1: Mechanics (Gravity) Galileo, Orbits, and Throwing Things

Wednesday August 12, 2009

How did gravity experiments start?

Italian scientist Galileo Galilei, a brilliant astronomer who made many contributions to the world of science.
Italian scientist Galileo Galilei, a brilliant astronomer who made many contributions to the world of science.

Galileo was actually one of the first people to do science experiments on gravity using the tallest tower he could find – the Tower of Pisa in Italy. At least, that’s what we’ve been told. But Galileo, who always wrote everything down, never mentioned in his notes of dropping things off the tower. What he did document, however, was rolling things down ramps (also called ‘inclines’). Remember that at this time in history, most people were still answering questions by simply arguing about them instead of doing any scientific studies!


[am4show have=’p8;p9;p11;p38;’ guest_error=’Guest error message’ user_error=’User error message’ ]
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). He quickly learned how to find the acceleration (which older kids are going to do during a few experiments later) and that the higher you dropped the ball, the bigger the impact. But we still don’t know why.


The rest of this article is for advanced students…

[/am4show][am4show have=’p9;p38;’ guest_error=’Guest error message’ user_error=’User error message’ ]
Newton used Galileo’s work along with lots of cool mathematics to figure out that the Moon constantly falling around the Earth. Think of it this way – if you throw a ball toward the sunset, it will fall 16 feet during the first second. If you throw the ball faster and faster, it will still fall 16 feet during the first second. If you shot a bullet horizontally, it will also fall 16 feet during the first second. It doesn’t matter how fast you get that bullet to travel – it will always fall 16 feet during that first second.


What if you could shoot the bullet fast enough so that the rate the Earth curves away from the bullet at the same rate that it falls? If you shoot the bullet at 5 miles per second, it will fall at the same rate that the Earth is curving away from it… and that’s how we get objects into orbit. And that’s also what scientists mean when they say that ‘the Moon is falling around the Earth’.


Problems with Gravity

Things with gravity were going along just fine, until we started looking at the planets. It turns out that Mercury’s orbit doesn’t follow the math the way that it should – meaning that scientists can describe the orbits of the planets using complicated math equations… all except for Mercury.


And for a while, scientists actually thought there was another planet between the sun and Mercury that accounted for the trouble they were having (they even named this undiscovered planet Vulcan!). It turned out to be a problem with the math itself, and it took years before Einstein came along and tweaked the equations around so it made more mathematical sense.


Einstein changed the way we see the universe by viewing it from a totally different point of view that gravity is nothing more than geometry (more on that when you get to college!). There are still problems with gravity, though. The math that scientists use to describe gravity breaks down when they try to describe black holes – things go to infinity and zero at the same time, and in math, that’s usually bad news.


One of the things that scientist do know about gravity is that it is not instantaneous. It appears that it does take time for gravity to travel, the same way that it takes time for sound waves to travel to your ears. (Ever notice how you see the lightening before you hear the thunder?) Meaning that if you swapped our sun for a tiny star (with much less gravitational pull), the Earth would not know about it for about 8 minutes. The best guess we have is that gravity propagates at the speed of light.


Is gravity strong?

Another thing that we know is that gravity is the weakest of the four fundamental forces on small scales (we’re talking about the size of atoms here). On large scales, however, it’s the force that keeps the planets in orbit, galaxies in orbit, and everything slinging around each other as they should. Think about when you stick a magnet to the fridge: the magnetic forces are keeping the magnet up (well, most of the time anyway!) and overcoming the gravitational field effects from the planet.


Here’s an interesting thought experiment I came across not too long ago: if you could construct a human body that held together by only gravity, it would take a single breath to shatter the entire body. That’s how weak gravity is on a small scale. On a large scale, though the strong forces that keep the atoms together get so weak that gravity can take over. When you get to scales larger than an atom, the forces that bind the nucleus together get suddenly weaker.


Scientists are really bothered that they cannot figure out the how and why gravity works. They suspect that there are these little particles (gravitons) that shuttle information back and forth about gravity, but they are still invisible to us.


For advanced students, we have a more advanced version of the topic in the form of a textbook download. These files are also in PDF form and include exercises in addition to solid physics content. Note that these downloads are written for upper level science students, so if math isn’t your focus right now, just skip over it and read on. (You’ll find these also posted in the main reading section for each lesson.) Here’s the textbook download for gravity: Gravity Lesson textbook download.


Click here to get started with the experiments!

[/am4show]


A Weighty Issue

Wednesday August 12, 2009

This lesson may give you a sinking sensation but don’t worry about it. It’s only because we’re talking about gravity. You can’t go anywhere without gravity. Even though we deal with gravity on a constant basis, there are several misconceptions about it. Let’s get to an experiment right away and I’ll show you what I mean.

If I drop a ping pong ball and a golf ball from the same height, which one hits the ground first? How about a bowling ball and a marble?

Here's what you need:

[am4show have='p8;p9;p11;p38;p72;p92;' guest_error='Guest error message' user_error='User error message' ]

  • ping pong ball
  • golf ball
  • you

Download Student Worksheet & Exercises

For this experiment, you'll need:

Two objects of different weights. A marble and a golf ball, or a tennis ball and a penny for example.
A sharp eye
A partner

1. Take a careful look at both objects and make a prediction about which object will hit the ground first if they are dropped from the same height.

2. Test your prediction. Hold both objects at the same height. Make sure the bottom of both objects are the same distance from the floor.

3. Let them go as close to the same time as possible. Sometimes it's helpful to roll them off a book.

4. Watch carefully. Which hits the ground first, the heavier one or the lighter one? Try it a couple of times and watch carefully. It will be a little easier for the person who isn’t dropping them to see what happens.

What you should see is that both objects hit the ground at the same time! Gravity accelerates both items equally and they hit the ground at the same time. Any two objects will do this, a brick and a Buick, a flower and a fish, a kumquat and a cow!

“But,” I hear you saying, “if I drop a feather and a flounder, the flounder will hit first every time!” Ok, you got me there. There is one thing that will change the results and that is air resistance.

The bigger, lighter and fluffier something is, the more air resistance can effect it and so it will fall more slowly. Air resistance is a type of friction which we will be talking about later. In fact, if you removed air resistance, a feather and a flounder would hit the ground at the same time!!!

Where can you remove air resistance? The moon!!! One of the Apollo missions actually did this (well, they didn’t use a flounder they used a hammer). An astronaut dropped a feather and a hammer at the same time and indeed, both fell at the same rate of speed and hit the surface of the moon at the same time.


Ask someone this question: Which will hit the ground first, if dropped from the same height, a bowling ball or a tennis ball? Most will say the bowling ball. In fact, if you asked yourself that question 5 minutes ago, would you have gotten it right? It’s conventional wisdom to think that the heavier object falls faster. Unfortunately, conventional wisdom isn’t always right. Gravity accelerates all things equally. In other words, gravity makes all things speed up or slow down at the same rate. We will be discussing acceleration more in a later lesson. If you would like more details on the math of this, it will be at the end of this lesson in the Deeper Lesson section.

This photo shows a statue of Aristotle, a famous Greek philosopher who contributed many ideas to science.

This is a great example of why the scientific method is such a cool thing. Many, many years ago, there was a man of great knowledge and wisdom named Aristotle. Whatever he said, most people believed to be true. The trouble was he didn’t test everything that he said. One of his statements was that objects with greater weight fall faster than objects with less weight. Everyone believed that this was true.

Hundreds of years later Galileo came along and said “Ya know...that doesn’t seem to work that way. I’m going to test it” The story goes that Galileo grabbed a melon and an orange and went to the top of the Leaning Tower of Pisa. He said, “Look out below!” and dropped them! By doing that, he showed that objects fall at the same rate of speed no matter what their size.

It is true that it was Galileo who “proved” that gravity accelerates all things equally no matter what their weight, but there is no real evidence that he actually used the Leaning Tower of Pisa to do it.

Advanced Students: Download your Gravity Lab here.

Exercises

  1. What did you notice from your data? Did heavier or lighter objects fall faster? Did more massive objects or smaller objects fall faster? What characteristic seemed to matter the most?
  2. Is gravity a two-way force, like the attractive-repulsive forces of a magnet?
  3. If I were to drop a bowling ball and a balloon filled with a gas six times heavier than air (sulfur hexafluoride SF6) and inflated to the exact size of the bowling ball from my roof, which will strike the ground first?

[/am4show]


Forever Falling

Wednesday August 12, 2009

If I toss a ball horizontally at the exact same instant that I drop another one from my other hand, which one reaches the ground first? For this experiment, you need: [am4show have=’p8;p9;p11;p38;p72;p92;’ guest_error=’Guest error message’ user_error=’User error message’ ]


  • 2 rulers or paint sticks. Any thing wide and flat
  • 2 coins or poker chips
  • A sharp eye and ear
  • A partner is good for this one too


 


Download Student Worksheet & Exercises
1. Place one of the rulers flat so that it is diagonal across the edge of a table with half the ruler on the table and half sticking off.


2. Place one coin on the table, just in front of the ruler and just behind the edge of the table. Place the other coin on the ruler on the side where it’s off the table.


3. Put your finger right in the middle of the ruler on the table so that you are holding it in such a way that it can spin a bit under your finger. Now with the other ruler you are going to smack the end of the first ruler so that the first ruler pushes the coin off the desk and the coin that’s resting on the ruler falls to the ground.


4. Now, before you smack the ruler, make a prediction. Will the coin that falls straight down or the coin that is flying forward hit the ground first?


5. Try it. Do the test and look and listen carefully to what happens. It’s almost better to use your ears here than your eyes. Do it a couple of times.


Are you surprised by what you see and/or hear? Most people are. It’s not what you would expect.


The coins hit the ground at the SAME time. Is that odd or what?


bullet


Did you read the first sentence at the top of this lab? What do you think will happen?


The balls will hit the ground at the exact SAME time.


Gravity doesn’t care if something is moving horizontally or not. Everything falls toward the center of the Earth at the same rate.


Let me give you a better example: A bullet fired parallel to the ground from a gun and a bullet dropped from the same height at the same time will both hit the ground at the same time. Even though the one fired lands a mile away! It seems incredible, but it’s true.


Gravity doesn’t care what size something is or whether or not it is moving, Gravity treats all things equally and accelerates them the same.


Notice, that I say gravity accelerates all things equally, not gravity pulls on all things equally. Gravity does pull harder on some things than on other things. This is why I weigh more than a dog. I am made of more stuff (I have more atoms) than the average dog, so gravity pulls on me more.


Weight is nothing more than a measure of how much gravity is pulling on you. This is why you can be “weightless” on a scale in space. You are still made of stuff, but there’s a balance of the gravity that is pulling on you and the outward force due to the acceleration since you’re moving in a circle (which you do in order to remain in orbit), so it feels like you have no weight.


The larger a body is, the more gravitational pull or 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, both the dog and I both have gravitational fields! Since we are both bodies of mass we have a gravitational field which will pull things towards us. All bodies have a gravitational field. However, my mass is sooooo 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 10’s 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!)


Exercises


  1. True or false? Gravity pulls on all things equally.
  2. True or false? Gravity accelerates all things equally.
  3. In your own words, why do the coins hit the ground at the same time? Is this what you’d expect to happen on Mars?

The rest of this experiment is for advanced students…[/am4show]


[am4show have=’p9;p38;’ guest_error=’Guest error message’ user_error=’User error message’ ]


For advanced students:

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, lets 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. Soooo the bowling ball should accelerate faster since there’s more force on it. However, the bowling ball is heavier soooo 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 many years, it still remains one of the most elusive mysteries of science. At this point, nobody knows what makes things move towards a body of mass.


Why did the rock drop towards 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!


Advanced students: Download your Forever Falling Lab here.


[/am4show]


Fast Ball

Wednesday August 12, 2009

You have just taken in a nice bunch of information about the wild world of gravity. This next section is for advanced students, who want to go even deeper. There’s a lot of great stuff here but there’s a lot of math as well. If you’re not a math person, feel free to pass this up. You’ll still have a nice understanding of the concept. However, I’d recommend giving it a try. There are some fun things to do and if you’re not careful, you might just end up enjoying it!


Here’s what you need:


[am4show have=’p9;p38;p92;’ guest_error=’Guest error message’ user_error=’User error message’ ]


  • ball
  • pencil, paper
  • stopwatch
  • yardstick or tape measure


 
Download Student Worksheet & Exercises


Okay, let’s see where we can go here. Gravity accelerates all things equally…what does that mean? All things accelerate at 32 feet per second squared due to gravity. In metric, it accelerates 9.8 meters per second squared.


What that means is, every second something falls, its speed increases by 32 feet/second or 9.8 meters/second. Believe it or not, that’s about 22 miles per hour!! Gravity will accelerate something from 0 to 60 mph in about 3 seconds. Faster then all but the fastest sports cars!


So what is acceleration anyway? Well speed is the amount of distance something travels in a certain amount of time. Five miles per hour, for example, tells you that something can travel five miles in an hour. Acceleration is how much the speed changes over time. So acceleration would be miles per hour per hour or feet per second per second.


Acceleration is a rate of change of speed or, in other words, how fast is the speed is changing. Feet per second per second is the same as ft/s/s which is the same as ft/s². (I told you we were going deeper!) Let’s say you’re riding your bicycle at a positive acceleration (your getting faster) of 5 ft/s².


That means in 1 second you’re moving at a speed of 5 ft/s.


After 2 seconds you’re moving at a speed of 10 ft/s.


After 3 seconds you’re now clipping along at 15 ft/s (about 10 mph).


gravity1So you can see that as long as you accelerate, you will be getting faster and faster. The formula for this is v=at where v is velocity, a is acceleration and t is time. (We will be doing more with acceleration in a future lesson.)


If we want to find out how fast something is going after it has been dropped, we use the formula v=gt. The letter “v” stands for velocity (which basically means speed.) “g” stands for the gravitational constant and “t” stands for time.


If we want to find out how fast a golf ball is dropping after it falls for 3 seconds we multiply 3 seconds by 32 feet/second squared and that equals 96 feet/second. So, if I dropped a golf ball off a building, it would be going 96 feet per second after 3 seconds of dropping.


The formula looks like this when we fill in the numbers:


v=3s x 32 ft/s²


If we do more math, we’ll see that after one second something will be item7going 32 ft/s, after 2 seconds it will be going 64 ft/s, after 3 seconds 96 ft/s after 4 seconds 128 ft/s. Get it? Anything dropped will be going that speed after that many seconds because gravity accelerates all things equally (air resistance will effect these numbers so you won’t get exactly the numbers in practice that you will mathematically).


All right, lets go even deeper. We now know how to calculate how fast something will be going if it is dropped, but what happens if we throw it up? Well, which way does gravity go? Down right? Gravity accelerates all things equally so, gravity will slow things down as they travel up by 32 ft/s². If a ball is thrown up at 64 ft/s how long will it travel upwards? Well, since it is negatively accelerating (in physics there’s no such thing as deceleration) after the first second the ball will be traveling at 32 ft/s and after 2 seconds the ball will come to a stop, turn around in midair, and begin to accelerate downwards at 32 ft/s². Using this, you can tell how fast you can throw by using nothing more then a timer. Let’s try it.


For this experiment, you will need:


– A ball (a tennis ball or baseball would be perfect)


– A stopwatch


– Pencil and paper


– A friend


– A calculator


1. Go outside and pick one person to be the thrower and another to be the timer.
2. Have the timer say “Ready, Set, Go!” and at go he or she should start the stopwatch.
3. When the timer says go, the thrower should toss the ball as high as he or she can.
4. The timer should stop the stopwatch when the ball hits the ground.
5. Write down the time that the ball was in the air.
6. Let each person take a couple of turns as timer and thrower.
7. Now, come back inside and do a bit of math.


Ok, let’s see how you did. Let’s say you threw the ball into the air and it took 3 seconds to hit the ground. The first thing you have to do is divide 3 in half. Why? Because your ball traveled 1.5 seconds up and 1.5 seconds down! (By the way, this isn’t completely accurate because of two things. One, air resistance and two, the ball falls a little father then it rises because of the height of the thrower.) Now, take your formula and figure out the speed of the throw.


v=gt,


so v=32 ft/s² x 1.5 sec or


v = 48 ft/s.


So, if that’s how fast it left your hand…how fast was it going when it hit the ground? Yup, 48 ft/s. It has to be going the same speed because it had just as much time to speed up as it had to slow down, 1.5 seconds. Try that with your time and see how fast your throw was.


Ok, hold your breath, just a little deeper now. Let’s talk about distance. If something starts from rest you can tell how far it drops by how long it has dropped. This formula is d=1/2gt² or distance equals one half the gravitational constant multiplied by time squared. Let’s try it. If I drop a ball and it drops 3 seconds how far has it dropped?


d=1/2 32ft/s² x (3s)² or


d = 16 ft/s x 9s² or


d=144 ft So it has dropped 144 ft.


Now try this with your time. What’s the first thing you have to do? Divide your time in half again, right. It took your ball half the time to go up and half the time to come down. Now plug your numbers into 1/2gt² and find out how high you threw your ball! Is Major League Baseball in your future?!


Advanced students: Download your Fast Ball Lab here.


Exercises 


  1. Is gravity a speed, velocity, or acceleration?
  2. Does gravity pull equally on all things?
  3. Does gravity accelerate all objects equally?
  4. How is acceleration different from speed and velocity?

[/am4show]


Unit 1: Mechanics (Gravity) Exercises

Wednesday August 12, 2009

Let’s see how much you’ve picked up with these experiments and the reading – answer as best as you can. (No peeking at the answers until you’re done!) Just relax and see what jumps to mind when you read the question. You can also print these out and jot down your answers in your science notebook.
[am4show have=’p8;p9;p11;p38;’ guest_error=’Guest error message’ user_error=’User error message’ ]


Here’s printer-friendly versions of the exercises and answers for you to print out: Simply click here for K-8 and here for K-12.


1. 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


2. Gravity accelerates all things differently…True or False??


3. Gravity pulls on all things differently…True or False??


4. If I drop a golf ball and a golf cart at the same time from the same height, which hits the ground first?


5. 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.)


6. Why don’t a feather and a brick hit the ground at the same time?


[/am4show]


[am4show have=’p9;p38;’ guest_error=’Guest error message’ user_error=’User error message’ ]


For advanced students:

Would you like to try out your new formulas? All right, give these problems a try. Don’t worry about air resistance for these.


Since you are finding velocity use this formula for these problems, v=gt. v is velocity, g is the gravitational constant (32 ft/sec²), t is time.


1. You dropped a ball off a building 3 seconds ago. How fast is it going now?


2. 6 seconds have passed since your meat ball rolled off the roof. How fast is it going?


3. If you shoot a model rocket into the air and it takes 8 seconds before it hits the ground how fast was it going when it left the launch pad?


Now for these you’re looking for distance, so use the formula d=1/2gt². d is distance, g is the gravitational constant, and t is time.

4. If you dropped a ball off the edge of the roof of your house to your buddy on the ground and it took 5 seconds to get to your friend, how tall is your house?


5. If you’re in the outfield and a fly ball takes 3 seconds to go from the highest point of the hit to your mitt, how high was the ball hit?
[/am4show]


Unit 1: Mechanics (Gravity) Answers to Exercises

Wednesday August 12, 2009

Let’s see how you did! If you didn’t get a few of these, don’t let it stress you out – it just means you need to play with more experiments in this area. We’re all works in progress, and we have our entire lifetime to puzzle together the mysteries of the universe!


Answers:
[am4show have=’p8;p9;p11;p38;’ guest_error=’Guest error message’ user_error=’User error message’ ]


1. D. All bodies are attracted to other bodies by gravity. But a body has to be really stinkin’ big before it’s noticeable.


2. FALSE!!! Gravity accelerates all things at the same rate. All things fall at the same rate of speed no matter what (ignoring air resistance, that is).


3. True. That’s why some things weigh more then other things. Gravity pulls more on the big stinky guy sitting next to me on the bus, then it does on me.


4. They hit the ground at the same time. Gravity accelerates all things equally.


5. The monkey and the dart fall downward at the same rate of speed. So the dart would hit exactly where the biologist aimed! In fact, if the monkey didn’t let go, the dart would have hit lower then the biologist aimed.


6. They do…if you’re on the moon! On Earth, the friction between the air and the feather causes the feather to slow down and the brick to win the race.


[/am4show]
[am4show have=’p9;p38;’ guest_error=’Guest error message’ user_error=’User error message’ ]


For advanced students:

I’ve converted feet/second to miles/hour for you so that you can get more of a feel for the speed.


1. 96 ft/s which is 64 mph


2. 192 ft/s or 131 mph (thatsa fasta meata balla!)


3. 128 ft/s or 87 mph (remember that you have to half the time. It took 4 seconds to go up and 4 seconds to fall down.


4. 400 ft. Ok, so, you have a big house!


5. 144 ft Nice catch!
[/am4show]


Unit 1: Mechanics (Friction) Video

Monday August 10, 2009

Friction is a very complicated interaction that uses ideas from both the electromagnetic field as well as the chemistry field to fully explain exactly what it is and how it works. From ice skates to moving furniture, you encounter friction everyday. We’re going to use rubber bands, shoes, ramps, and more to experiment with these ideas on our own.


[am4show have=’p8;p9;p11;p38;’ guest_error=’Guest error message’ user_error=’User error message’ ]



[/am4show]


Unit 1: Mechanics (Friction) Static, Kinetic, Dry, Fluid, & Skin Friction

Monday August 10, 2009

[am4show have=’p8;p9;p11;p38;’ guest_error=’Guest error message’ user_error=’User error message’ ]


Different Kinds of Friction

Dry friction is just like it sounds – if you’ve ever tried to shove a heavy box across the pavement, you know that it’s harder to get it started than keep it going. That’s because when you first start to shove the box, you’ve got to overcome the (stronger) static friction, but once you’re moving you are dealing with only with the (weaker) kinetic friction. Sometimes kinetic friction is called ‘sliding’ or ‘dynamic’ friction.


Engineers and scientists reduce friction by adding lubrication, like oil. Your car engine uses oil to slick up the metal-to-metal surfaces and keep the pistons moving smoothly.


One of the troubles with liquids is that they tend to heat up quickly (there’s still friction between the liquid and the solid surface!), so you have to devise a system to cool the liquid. You can also use gas as a lubrication. Hovercraft ride on a cushion of air, using air as the ‘lubrication’ between the skirt and the ground.


Fluid friction is when you have two fluids flowing up against each other, like a stream of water and a stream of oil together. Skin friction happens when an airplane flies through the air – the air rushing by the airplane body heats up the outside of the aircraft.


Contrary to earlier explanations found in textbooks, we now know that kinetic friction happens not because of surface roughness, but rather because of the chemical bonding that happens between the two surfaces.


Click here to get started with the experiments!

[/am4show]


Tracking your Treads

Monday August 10, 2009

Now let’s talk about the other ever present force on this Earth, and that’s friction. Friction is the force between one object rubbing against another object. Friction is what makes things slow down.


Without friction things would just keep moving unless they hit something else. Without friction, you would not be able to walk. Your feet would have nothing to push against and they would just slide backward all the time like you’re doing the moon walk.


Friction is a very complicated interaction between pressure and the type of materials that are touching one another. Let’s do a couple of experiments to get the hang of what friction is.
Here’s what you need:


[am4show have=’p8;p9;p11;p38;p92;’ guest_error=’Guest error message’ user_error=’User error message’ ]


  • About 5 different shoes
  • A board, or a tray, or a large book at least 15 inches long and no more then 2 feet long.
  • A ruler
  • Paper
  • Pencil
  • A partner


 
Download Student Worksheet & Exercises


1. Put the board (or whatever you’re using) on the table.


2. Put the shoe on the board with the back of the shoe touching the back of the board.


3. Have a partner hold the ruler upright (so that the12 inches end is up and the 1 inch end is on the table) at the back of the board.


4. Slowly lift the back of the board leaving the front of the board on the table. (You’re making a ramp with the board). Eventually the shoe will begin to slide.


5. Stop moving the board when the shoe slides and measure the height that the back of the board was lifted to.


6. Look at the 5 shoes you chose and test them. Before you do, make a hypothesis for which shoe will have the most friction. Make a hypothesis. On a scale from 1 to 5 (or however many shoes you’re using) rate the shoes you picked. 1 is low friction and 5 would be high friction. Write the hypothesis next to a description of the shoes on a piece of paper. The greater the friction the higher the ramp has to be lifted. Test all of the shoes.


7. Analyze the shoes. Do the shoes with the most friction show any similarities? Are the bottoms made out of the same type of material? What about the shoes with very little friction?


Any surprises with which shoe had the most or least friction? Compare the shoe with the most friction and the shoe with the least friction. Do you notice anything? Usually, the shoe that has the most friction has more shoe surface touching the board then most of the other shoes.


Also, often the shoe with the least friction, has the least amount of shoe touching the board. Since friction is all about two things rubbing together, the more surface that’s rubbing, the more friction you get. A tire on you car should have treads but a race car tire will be absolutely flat with no treads at all. Why?


The race car doesn’t have to worry about rain or wetness so it wants every single bit of the tire to be touching the surface of the track. That way, there is as much friction as possible between the tire and the track. The tire on your car has treads to cut through mud and water to get to the nice firm road underneath. The treads actually give you less friction on a flat dry road!


Some of you might have used a skateboard shoe for your experiment. Notice, that the skateboard shoe has quite a flat bottom compared to most other shoes. This is because a skateboarder wants as much of his or her shoe to touch the board at all times.


Exercises 


  1. What is friction?
  2. What is static friction?
  3. What is kinetic friction?

[/am4show]


What a Drag!

Monday August 10, 2009
There’s a couple of misconceptions that I’d like to make sure get cleared up here a bit. I don’t want to go into too much detail but I want to make sure to mention these as they may be important as you go deeper into your physics education.

First, friction is not a fundamental force. Friction is actually caused by the elemental force of electro-magnetism between two objects.

Secondly, friction isn’t “caused” by the roughness or smoothness of an object. Friction is caused by two objects, believe it or not, chemically bonding to one another. Scientists call it “stick and slip”.

Think about it this way. When you pull the wood in this experiment, notice that the force needed to get the board moving was more then the force was to keep it moving. The surface you were pulling the board on never got any rougher or smoother, it stayed pretty much the same.

So why was it harder to get the board moving?

When the board is just sitting there, the chemical bonds between the board and the surface can be quite strong. When the board is moving however, the bonds are much weaker. Here's what you need:

[am4show have='p8;p9;p11;p38;p92;' guest_error='Guest error message' user_error='User error message' ]

  • A 6 inch long piece of 2 x 4 wood, or a heavy book
  • A string
  • A spring scale or a rubber band and a ruler.
  • Paper
  • Pen
  • 5 or so different surfaces, table tops, carpet, chairs, etc.




Download your worksheet here!

1. Write the different surfaces that you chose on a piece of paper.

2. Make a hypothesis. On a scale from 1 to 5 (or however many surfaces you chose) rate the surfaces you picked. 1 is low friction and 5 would be high friction. Write your ranking next to the surfaces on the paper.

3. Take your block or your book and attach a string to it.

4. Place your block on the surface to be tested.

5. If you have a spring scale, attach it to the string and carefully pull on your block until it just moves. What you will probably see, is that you will keep pulling and pulling until suddenly your block moves. Try to record the number that the scale said just before the block moved. It takes a little bit of practice to read that number so keep trying.

6. If you don’t have a spring scale, tie a rubber band to the string. Now put a ruler with the first inch at the end of the rubber band farthest from the block. Now pull on the rubber band holding it next to the ruler. When the block moves, record the number on the ruler where the end of the rubber band was. In other words, you are measuring how far the rubber band stretches before the board moves.

7. Remember, with the scale or the rubber band, this takes some getting used to so try not to get frustrated.

8. Write down your results next to your hypothesis.

9. The higher the number, the more friction there is between your board and the surface the board is on. In other words, the harder you had to pull to get the board moving, the more friction there is between the board and the surface.

10. Now analyze your data and see how the data matches your hypothesis. Which surface really had the most friction and which had the least. Write numbers 1 to 5 (or however many surfaces you chose) next to the results.

11. How did the data correlate with your hypothesis? Any surprises?

You’ve probably noticed with this experiment that the kind of surfaces rubbing together make a huge difference.

Flat, hard, smooth surfaces will have much less friction than a rubber, soft, or rough surface. Muddy, wet or icy surfaces will often have even less friction. So, if you remember what we talked about with shoes and tires, the job of the tread on a shoe and a tire is to cut through the lower friction water or mud and get down to the higher friction road or dry ground.

Something else I’d like you to notice is that friction acts differently depending on what something is doing. If you have ever had to push something heavy like a refrigerator you may have noticed that it was harder to get it to move than it was to keep it moving. This is because there are two types of friction; static friction and kinetic friction.

Static friction happens when something is resting on something else and not moving. Kinetic friction is when one thing is moving on something else. Static friction is usually greater than kinetic friction. This means that it is harder to get your fridge moving than to keep it moving. You may have noticed this during “What a Drag” (if not, go ahead and play with it some more). When you first got the board to move, your scale had measurements much higher than when it was actually moving. It was harder to get it moving than to keep it moving.

For the advanced students, here's a way to calculate the amount of force you're pulling with by figuring out how 'spring-y' your rubber band is...

[/am4show][am4show have='p9;p38;' guest_error='Guest error message' user_error='User error message' ]
Advanced Students: Download your Friction Lab here. [/am4show]

Stick & Slip

Monday August 10, 2009

Friction is everywhere! Imagine what the world would be like without friction! Everything you do, from catching baseballs to eating hamburgers, to putting on shoes, friction is a part of it. If you take a quick look at friction, it is quite a simple concept of two things rubbing together.


However, when you take a closer look at it, it’s really quite complex. What kind of surfaces are rubbing together? How much of the surfaces are touching? And what’s the deal with this stick and slip thing anyway? Friction is a concept that’s many scientists are spending a lot of time on. Understanding friction is very important in making engines and machines run more efficiently and safely.


Here’s what you need:


[am4show have=’p8;p9;p11;p38;p92;’ guest_error=’Guest error message’ user_error=’User error message’ ]


  • 2 Business card magnets (those thin flat magnets that are the size of business cards)
  • Fingers


 
Download Student Worksheet & Exercises


1. Take two business card magnets and stick them together black side to black side. They should be together so that the pictures (or whatever’s on the magnets) are on the outside like two pieces of bread on a sandwich.


2. Now grab the sides of the magnets and drag one to the right and the other to the left so that they still are magnetically stuck together as they slide over one another.


Did you notice what happened as they slid across one another? They stuck and slipped didn’t they? This is a bit like friction. As two surfaces slide across one another, they chemically bond and then break apart. Bond and break, bond and break as they slide. The magnets magnetically “bonded” together and then broke apart as you slide them across on another. (The chemical bonds don’t work quite like the magnetic “bonds” but it gives a decent model of what’s happening.) There are many mysteries and discoveries to be uncovered with this concept. Go out and make some!


Exercises 


  1. What is the difference between static and kinetic friction? Which one is always greater?
  2. Design an experiment where you can observe and/or measure the difference between static and kinetic friction.

[/am4show]


Exponential Friction

Monday August 10, 2009
expfrictionFind a smooth, cylindrical support column, such as those used to support open-air roofs for breezeways and outdoor hallways (check your local public school or local church). Wind a length of rope one time around the column, and pull on one end while three friends pull on the other in a tug-of-war fashion.

Experiment with the number of friends and the number of winds around the column. Can you hold your end with just two fingers against an entire team of football players? You bet!

[am4show have='p8;p9;p11;p38;p92;' guest_error='Guest error message' user_error='User error message' ] Here's what you need:

  • nylon long rope (10 feet or longer)
  • column or pillar (as talked about in the video)
  • at least two people, but more is better
What’s going on? This is a great example of what “exponential growth” truly means. There is friction between the rope and the support column – you can feel it as you tug on the rope. With every additional turn around the pole, the amount of friction increases (exponentially grows), until it skyrockets so much the rope feels as if it’s welded to the pole.

Download Student Worksheet & Exercises

Einstein himself stated that “exponential growth” was the eighth wonder of the world! Exercises 
  1. How much money would you earn on Day 20 if I gave you one penny on Day 1, and doubled it every day after so Day 2 you received 2 pennies, and Day 3 you got 4 pennies?
  2.  Why do you think this experiment with friction works? Does it work with a flat surface the same way as a curved surface?
[/am4show]

Bearings

Monday August 10, 2009

Stand on a cookie sheet or cutting board which is placed on the floor (find a smooth floor with no carpet). Ask someone to gently push you across the floor. Notice how much friction they feel as they try to push you.


Want to make this job a bit easier?


Here’s what you need:


[am4show have=’p8;p9;p11;p38;p92;’ guest_error=’Guest error message’ user_error=’User error message’ ]


  • two boards (about 12″ x 12″, or whatever you have handy)
  • 4-10 dowels (or round, not hexagonal, pencils)
  • handful of marbles

Now place three or four dowels parallel about six inches apart between the board and the floor. Smooth wooden pencils can work in a pinch, as can the hard cardboard tubes from coat-hangers. Ask someone to push you. Is there a direction you still can’t travel easily? Now let’s add another direction to your motion…


Replace the dowels with marbles. What happens? Why do the marbles make you do in all directions? What direction(s) did the dowels roll you in?



 
Download Student Worksheet & Exercises


BONUS EXPERIMENT IDEA! To really drive this point home, you can make your own low-friction ball bearings: Get two cans (with a deep groove in the rim, such as paint cans) and stack them. Turn one (still on top of the other) and notice the resistance (friction) you feel. Now sandwich marbles all along the rim between the cans. Place a heavy book on top and note how easily it turns around. Oil the marbles (you can spray with cooking spray, but it is a bit messy) and it turns more easily yet.


Exercises 


  1. Why do the marbles make you go in all directions?
  2. What direction(s) did the dowels roll you in?

[/am4show]


Hovercraft

Monday August 10, 2009

hovercraftHovercraft transport people and their stuff across ice, grass, swamp, water, and land. Also known as the Air Cushioned Vehicle (ACV), these machines use air to greatly reduce the sliding friction between the bottom of the vehicle (the skirt) and the ground. This is a great example of how lubrication works – most people think of oil as the only way to reduce sliding friction, but gases work well if done right.


In this case, the readily-available air is shoved downward by the hover motor and the skirt traps the air and keeps it inside, thus lifting the vehicle slightly. The thruster motor’s job is to propel the craft forward. Most hovercraft use either two motors (one on each side) for steering, or just one with a rudder that can deflect the flow (as your project does).


The first hovercraft were thought about in the 1800s, but it wasn’t until the 1950s that real ones were first tested. Today, the military use them for patrolling hard-to-drive areas, scientists use them for swamp research studies, and businesses use them to transport toys and food across rough and icy areas. Scientists are already planning future ACVs to use magnetic levitation in addition to the air power… but it’s still on the drawing board.


Are you ready to make your own? We have TWO different models to choose from. Click this link for the Easy Balloon-Powered Model, or keep reading below for the advanced version.
[am4show have=’p8;p9;p11;p38;p72;p76;p92;’ guest_error=’Guest error message’ user_error=’User error message’ ]
You will need:


  • 1 wood skewer
  • 1 wood popsicle stick
  • 1 straw
  • 16 oz. styrofoam cup (the kind used for sodas). Note that waxed paper cups will not work!
  • 1 foam hamburger container (the one in the video is 5.5″ square and 3″ high when closed)
  • 1 foam meat tray (the one in the video is approx. 10″x12″x1″ – it does not need to be these exact dimensions – try a few different sizes out to see what happens! You can get them for free if you ask for a clean one from your butcher.
  • Two 3VDC motors (use this motor for the thruster and this motor for the hover motor)
  • 2  propellers (the ones in the video are 3″ diameter, so check your local hobby store and get a variety to test out) – read comments below for ideas on where to get props!
  • 9V battery clip with wires
  • 9V battery (get a good kind, like Duracell or Energizer)
  • 9V battery holder (looks like a “C”) OR use tape to attach the battery to your hovercraft
  • a couple of extra wires (speaker wire, alligator clips, etc.)
  • 1 SPST switch


Download Student Worksheet & Exercises


  1. First, we’ll work to make the hovercraft hover. Start by finding the center of the Styrofoam meat tray. This will be your base.
  2. Use the ruler to measure the diameter of your cup to make sure it’s 3.5 inches. If it measures correctly, use the cup and pen to draw a circle in the middle of the tray
  3. Carefully cut out the circle, supporting the bottom of the foam.
  4. Cut your skewer into three pieces, making sure they are longer than the cut-out circle is wide.
  5. Use the hot glue gun to attach the lip of the round motor onto the skewer pieces, keeping them as parallel as possible.
  6. Gently attach the skewers onto the foam.
  7. Attach a propeller onto the shaft of the motor which is now attached to the skewers and foam tray.
  8. Now we will work with the takeout container. Open it and cut it in half and place one half to the side.
  9. Check the diameter of the bottom of the foam cup to ensure it’s about 2 ¼ inches. Then you can trace it with a pen on the top of the hamburger container half.
  10. Cut out the circle and discard it.
  11. Using the slide switch as a guide, cut out a small rectangle in the front for the switch. Reinforce it with hot glue, being careful to NOT get hot glue in the switch. Make sure it still slides back and forth.
  12. Rest the hamburger half on top – we aren’t going to attach it just yet.
  13. Find the small motor and look for the small contacts (they are very small and fragile – they are copper and look a little like foil). Gently bend them up a little in the back.
  14. Hot glue the motor onto the end of the popsicle stick with the shaft pointing away from the stick and the contacts pointing up.
  15. Use hot glue to secure the stick across the top of the hole in the hamburger box.
  16. Attach a propeller and give it a spin to make sure it will spin.
  17. Find the 9-volt battery clip and hot glue the bottom of it onto the middle of the popsicle stick.
  18. Cut your wire into two equal length pieces. Remove the insulation from the ends (about ¾ of an inch to an inch – get adult help if you need it). Twist the exposed wires together. Do this for both wires.
  19. If you aren’t going to solder the project, you’ll need to cut off the metal ends of the 9 volt battery clip’s wires and strip the wire insulation. Twist these wires together as well.
  20. Now we’ll work on wiring the inside motor. Take the end of one wire and put it halfway through one of the posts. Bend it up and twist it around itself very well to ensure it’s connected well. Do this with the other wire and connection.
  21. One of these wires will go to the switch. Thread the wire through a tab and twist it around itself.
  22. Attach the black wire from your 9-volt battery clip to the other tab on the switch.
  23. Thread your remaining wires (the red one from the battery clip and the remaining red wire from the first, hovering motor) up through the hamburger tray to attach them to the second motor. This is the thruster motor.
  24. Now that everything is wired, glue the hamburger tray to the bottom tray by placing hot glue at each of the four corners and pressing down gently.
  25. To test, grab your 9-volt battery. Check to make sure everything is wired correctly – the hovercraft should hover, not be sucked down to the table, and you should feel air blowing if you hold your hand in front of the thruster motor. Switch the appropriate wires if you note any issues during testing.
  26. Now we’ll build a shroud around the thruster motor. You’ll need the cup, the last piece of wooden skewer, the straw, and the remaining big piece of foam. Measure about halfway down the cup and cut it all the way around – essentially cutting it in half. You’ll be using the top of the cup – the cuff-like portion. It should fit around the propeller.
  27. Starting on the cut side of the foam, cut out a rectangle to use as a shim. Hot glue the rectangle down to the hovercraft. Then hot glue the cup cuff down to the rectangle.
  28. If the propeller is hitting the Styrofoam, you can move the cup around and hot glue as needed to make sure there is room for movement.
  29. Make a vein from a rectangular piece of Styrofoam that fits inside the cup cuff.
  30. Glue the straw onto the long end of this piece and trim the straw down. The wooden skewer should fit right through the straw.
  31. Push the wooden skewer down through the top of the cup. Pierce the bottom of the cup but DO NOT pierce the bottom of the hovercraft.
  32. Put the straw and Styrofoam piece in, and then thread the skewer back down through the straw.
  33. Troubleshooting: make sure the bottom of the hovercraft – the tray’s lip – is as smooth as possible. You can sand it down lightly if you need to. You’ll need a clean, smooth, flat surface to hover on as well! You might also double check the motor directions. If necessary, you can lightly weigh down the front of the hovercraft to balance out the weight from the back.
  34. Modification: Once the hovercraft is operational, you can hot glue foam tubing to the bottom to make a water hovercraft. However, it will no longer work on land!

Exercises:


1.  What happens if you use a larger meat tray?


2.  Add another 9V battery?


3.  se a 12VDC motor for the 3VDC motor?


4.  Remove the battery pack entirely and add longer wires so you can hold the battery in your hand as the hovercraft zooms down the hallway?


[/am4show]


Unit 1: Mechanics (Friction) Exercises

Monday August 10, 2009

Let’s see how much you’ve picked up with these experiments and the reading – answer as best as you can. (No peeking at the answers until you’re done!) Just relax and see what jumps to mind when you read the question. You can also print these out and jot down your answers in your science notebook.


[am4show have=’p8;p9;p11;p38;’ guest_error=’Guest error message’ user_error=’User error message’ ]
Here’s printer-friendly versions of the exercises and answers for you to print out: Simply click here for K-8 and here for K-12.


1. What is friction?


2. Walking would be easier without friction….True or False.


3. Why does a feather fall slower then a brick?


4. Put a coin on a piece of paper. Then quickly pull the paper out from under the coin. What does static friction and kinetic friction have to do with this?


5. What was the experiment with the magnets showing? [/am4show]


Unit 1: Mechanics (Friction) Answers to Exercises

Monday August 10, 2009

Let’s see how you did! If you didn’t get a few of these, don’t let it stress you out – it just means you need to play with more experiments in this area. We’re all works in progress, and we have our entire lifetime to puzzle together the mysteries of the universe!


Answers:
[am4show have=’p8;p9;p11;p38;’ guest_error=’Guest error message’ user_error=’User error message’ ]


1. Friction is the force between one object rubbing against another object. Air resistance, by the way, is the friction of one object rubbing against millions and billions of air molecules.


2. FALSE!!! Walking would be impossible without friction. Your feet couldn’t push back against the floor to move you forward.


3. Air friction slows the feather down. The feather rubs against many, many, many air molecules as it falls through the air. The feather is light and large enough that the air molecules actually slow it down.


4. If you pull the paper slowly, the static friction between the penny and the paper isn’t broken. So the penny rides along with the paper. If you pull it quickly, you can overcome that static friction and the paper will slide along under the penny without moving it. As long as the paper is moving fast enough the kinetic friction between the paper and the penny isn’t enough to move the penny.


5. That objects “stick and slip” as they rub against one another. (Don’t forget, that the magnet thing is a good model but it doen’t work quite like that in the real world.)


[/am4show]