We’ve already studied the different types of forces and learned how to draw free body diagrams. We’re going to use those concepts to put forces into two different categories: internal and external forces. Internal forces include forces due to gravity, magnetism, electricity, and springs. External forces include applied, normal, tension, friction, drag and air resistance forces.
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Click here to go to next lesson on Mechanical Energy Relationship .
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The reason why we put the forces into two different categories will be obvious when we start solving physics problems, but for now, you can think of it like this: when total amount of work is done on an object is done by only internal forces, energy will change forms (like going from kinetic to potential energy), and the total amount of mechanical energy is conserved, and the forces are also conserved. When the total amount of work done is done by an external force, the forces are not conserved and the object with either gain or lose energy.
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As Phillip holds the ball at the top of the building, the ball has 100 Joules of potential energy (the number is just an example). When he drops it, the potential energy of the ball drops since the height of the ball gets less and less. At the same time, however, kinetic energy increases because the speed of the ball increases. All the way down, the sum of the two energies equals 100. No energy gets lost, it only gets transferred. Energy is conserved.
Now here’s a question you may be asking yourself, “If energy is neither created or destroyed in a closed system then why doesn’t a pendulum swing forever?”
That’s a very good question. Energy is neither created or destroyed, but it can be transferred into non- useful energy.
In the case of a pendulum, every swing loses a little bit of energy,which is why each swing goes slightly less high (achieves slightly less PE) than the swing before it. Where does that energy go? To heat. The second law of thermodynamics states basically that eventually all energy ends up as heat. If you could measure it, you’d find that the string, and the weight have a slightly higher temperature then they did when they started due to friction. The energy of your pendulum is lost to heat!
If you could prevent the loss of energy to useless energy, you could create a perpetual motion machine. A machine that works forever! There have been a lot of folks who have spent a lot of time trying to make a perpetual motion machine. So far, they have all failed. A perpetual motion machine is one that is said to be 100% energy efficient. In other words all the energy that goes into it goes to useful energy.
Your pendulum could be said to be about 90% efficient.Very little energy is converted into useless energy. In most systems, energy is converted to useless heat and sound energy.
As Phillip holds the ball at the top of the building, the ball has 100 Joules of potential energy (the number is just an example). When he drops it, the potential energy of the ball drops since the height of the ball gets less and less. At the same time, however, kinetic energy increases because the speed of the ball increases. All the way down, the sum of the two energies equals 100. No energy gets lost, it only gets transferred. Energy is conserved.
Now here’s a question you may be asking yourself, “If energy is neither created or destroyed in a closed system then why doesn’t a pendulum swing forever?”
That’s a very good question. Energy is neither created or destroyed, but it can be transferred into non- useful energy.
In the case of a pendulum, every swing loses a little bit of energy,which is why each swing goes slightly less high (achieves slightly less PE) than the swing before it. Where does that energy go? To heat. The second law of thermodynamics states basically that eventually all energy ends up as heat. If you could measure it, you’d find that the string, and the weight have a slightly higher temperature then they did when they started due to friction. The energy of your pendulum is lost to heat!
If you could prevent the loss of energy to useless energy, you could create a perpetual motion machine. A machine that works forever! There have been a lot of folks who have spent a lot of time trying to make a perpetual motion machine. So far, they have all failed. A perpetual motion machine is one that is said to be 100% energy efficient. In other words all the energy that goes into it goes to useful energy.
Your pendulum could be said to be about 90% efficient.Very little energy is converted into useless energy. In most systems, energy is converted to useless heat and sound energy.
Click here to go to next lesson on Energy Exchange between Kinetic and Potential Energy.
[/am4show]This is a nit-picky experiment that focuses on the energy transfer of rolling cars. You’ll be placing objects and moving them about to gather information about the potential and kinetic energy.
We’ll also be taking data and recording the results as well as doing a few math calculations, so if math isn’t your thing, feel free to skip it.
Here’s what you need:
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- a few toy cars (or anything that rolls like a skate)
- a board, book or car track
- measuring tape
The setup is simple. Here’s what you do:
1. Set up the track (board or book so that there’s a nice slant to the floor).
2. Put a car on the track.
3. Let the car go.
4. Mark or measure how far it went.
Download Student Worksheet & Exercises
As you lifted the car onto the track you gave the car potential energy. As the car went down the track and reached the floor the car lost potential energy and gained kinetic energy. When the car hit the floor it no longer had any potential energy only kinetic.
If the car was 100% energy efficient, the car would keep going forever. It would never have any energy transferred to useless energy. Your cars didn’t go forever did they? Nope, they stopped and some stopped before others. The ones that went farther were more energy efficient. Less of their energy was transferred to useless energy than the cars that went less far.
Where did the energy go? To heat energy, created by the friction of the wheels, and to sound energy. Was energy lost? NOOOO, it was only changed. If you could capture the heat energy and the sound energy and add it to the the kinetic energy, the sum would be equal to the original amount of energy the car had when it was sitting on top of the ramp.
For K-8 grades, click here to download a data sheet.
For Advanced Students, click here for the data log sheet. You’ll need Microsoft Excel to use this file.
Exercises
- Where is the potential energy greatest?
- Where is the kinetic energy greatest?
- Where is potential energy lowest?
- Where is kinetic energy lowest?
- Where is KE increasing, and PE is decreasing?
- Where is PE increasing and KE decreasing?
Click here to go to next lesson on Inclined Plane.
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What’s an inclined plane? Jar lids, spiral staircases, light bulbs, and key rings. These are all examples of inclined planes that wind around themselves. Some inclined planes are used to lower and raise things (like a jack or ramp), but they can also used to hold objects together (like jar lids or light bulb threads).
Here’s a quick experiment you can do to show yourself how something straight, like a ramp, is really the same as a spiral staircase.
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Here’s what you need to find:
- sheet of paper
- short dowel or cardboard tube from a coat hanger
- tape
- ruler
Cut a right triangle out of paper so that the two sides of the right angle are 11” and 5 ½” (the hypotenuse – the side opposite the right angle – will be longer than either of these). Find a short dowel or use a cardboard tube from a coat-hanger. Roll the triangular paper around the tube beginning at the short side and roll toward the triangle point, keeping the base even as it rolls.
Notice that the inclined plane (hypotenuse) spirals up as a tread as you roll. Remind you of screw threads? Those are inclined planes. If you have trouble figuring out how to do this experiment, just watch the video clip below:
Download Student Worksheet & Exercises
Inclined planes are simple machines. It’s how people used to lift heavy things (like the top stones for a pyramid).
Here’s another twist on the inclined plane: a wedge is a double inclined plane (top and bottom surfaces are inclined planes). You have lots of wedges at home: forks, knives, and nails just name a few.
When you stick a fork in food, it splits the food apart. You can make a simple wedge from a block of wood and drive it under a heavy block (like a tree stump or large book) with a kid on top.
Exercises
- What is one way to describe energy?
- The amount of atoms moving around at any given moment
- Electrons flowing from one area to another
- The ability to do work
- The square root of the speed of an electron
- Work is when something moves when:
- Force is applied
- Energy is used
- Electrons are lost or gained
- A group of atoms vibrate
- Name two simple machines:
- Name one example of a simple machine:
Click here to go to next lesson on Energy Exchange.
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When you toss down a ball, gravity pulls on the ball as it falls (creating kinetic energy) until it smacks the pavement, converting it back to potential energy as it bounces up again. This cycles between kinetic and potential energy as long as the ball continues to bounce.
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But note that when you drop the ball, it doesn’t rise up to the same height again. If the ball did return to the same height, this means you recovered all the kinetic energy into potential energy and you have a 100% efficient machine at work. But that’s not what happens, is it? Where did the rest of the energy go? Some of the energy was lost as heat and sound. (Did you hear something when the ball hit the floor?)
Click here to go to next lesson on Elastic Potential Energy.
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Note: Do the pendulum experiment first, and when you’re done with the heavy nut from that activity, just use it in this experiment.
You can easily create one of these mystery toys out of an old baking powder can, a heavy rock, two paper clips, and a rubber band (at least 3″ x 1/4″). It will keep small kids and cats busy for hours.
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Here’s what you get:
- can with a lid
- heavy rock or large nut
- two paper clips
- rubber band
You’ll need two holds punched through your container – one in the lid and the bottom. Thread your rubber band through the heavy washer and tie it off (this is important!). Poke the ends of the rubber band through one of the holes and catch it on the other side with a paper clip. (Just push a paper clip partway through so the rubber band doesn’t slip back through the hole.) Do this for both sides, and make sure that your rubber band is a pulled mildly-tight inside the can. You want the hexnut to dangle in the center of the can without touching the sides of the container.
Download Student Worksheet & Exercises
Now for the fun part… gently roll the can on a smooth floor away from you. The can should roll, slow down, stop, and return to you! If it doesn’t, check the rubber band tightness inside the can.
The hexnut is a weight that twists up the rubber band as the can rolls around it. The kinetic energy (the rolling motion of the can) transforms into potential (elastic) energy stored in the rubber band the free side twists around. The can stops (this is the point of highest potential energy) and returns to you (potential energy is being transformed into kinetic). The farther the toy is rolled the more elastic potential energy it stores.
Exercises
- Explain in your own words two types of energy transfer:
- True or false: All energy in a system is lost to heat.
- True
- False
Click here to go to next lesson on Pendulums and Energy Transfer.
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This is a very simple yet powerful demonstration that shows how potential energy and kinetic energy transfer from one to the other and back again, over and over. Once you wrap your head around this concept, you’ll be well on your way to designing world-class roller coasters.
For these experiments, find your materials:
- some string
- a bit of tape
- a washer or a weight of some kind
- set of magnets (at least 6, but more is better)
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Here’s what you do:
1. Make the string into a 2 foot or so length.
2. Tie the string to the washer, or weight.
3. Tape the other end of the string to a table.
4. Lift the weight and let go, causing the weight to swing back and forth at the end of the pendulum.
Download Student Worksheet & Exercises
Watch the pendulum for a bit and describe what it’s doing as far as energy goes. Some questions to think about include:
- Where is the potential energy greatest?
- Where is the kinetic energy greatest?
- Where is potential energy lowest?
- Where is kinetic energy lowest?
- Where is KE increasing, and PE is decreasing?
- Where is PE increasing and KE decreasing?
- Where did the energy come from in the first place?
Remember, potential energy is highest where the weight is the highest.
Kinetic energy is highest were the weight is moving the fastest. So potential energy is highest at the ends of the swings. Here’s a coincidence, that’s also where kinetic energy is the lowest since the weight is moving the least.
Where’s potential energy the lowest? At the middle or lowest part of the swing. Another coincidence, this is where kinetic energy is the highest! Now, wait a minute…coincidence or physics? It’s physics right?
In fact, it’s conservation of energy. No energy is created or destroyed, so as PE gets lower KE must get higher. As KE gets higher PE must get lower. It’s the law…the law of conservation of energy! Lastly, where did the energy come from in the first place? It came from you. You added energy (increased PE) when you lifted the weight.
(By the way, you did work on the weight by lifting it the distance you lifted it. You put a certain amount of Joules of energy into the pendulum system. Where did you get that energy? From your morning Wheaties!)
Chaos Pendulum
For this next experiment, we’ll be using magnets to add energy into the system by having a magnetic pendulum interact with magnets carefully spaced around the pendulum. Watch the video to learn how to set this one up. You’ll need a set of magnets (at least one of them is a ring magnet so you can easily thread a string through it), tape, string, and a table or chair. Are you ready?
Exercises
- Why can we never make a machine that powers itself over and over again?
- Energy is mostly lost to heat.
- Energy is completely used up.
- Energy is unlimited, but is absorbed by neighboring air molecules.
- None of these
- In the pendulum, as kinetic energy increases, potential energy ______________.
- Increases
- Decreases
- As potential energy decreases, kinetic energy _________________.
- Increases
- Decreases
Click here to go to next lesson on Potato Cannon.
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This experiment is for Advanced Students.There are several different ways of throwing objects. This is the only potato cannon we’ve found that does NOT use explosives, so you can be assured your kid will still have their face attached at the end of the day. (We’ll do more when we get to chemistry, so don’t worry!)
These nifty devices give off a satisfying *POP!!* when they fire and your backyard will look like an invasion of aliens from the French Fry planet when you’re done. Have your kids use a set of goggles and do all your experimenting outside.
Here’s what you need:
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- potatoes
- an acrylic tube (clear is best so you can see what’s happening inside!)
- wooden dowel
- washer (this is your ‘hand-saver’)
Where is the potential energy the greatest? How much energy did your spud have at this point? Hmmm… let’s see if we can get a few actual numbers with this experiment. In order to calculate potential energy at the highest point of travel, you’ll need to figure out how high it went.
Here are instructions for making your own height-gauge:
Once you get your height gauge working right, you’ll need to track your data. Start a log sheet in your journal and jot down the height for each launch. Let’s practice a sample calculation:
If you measured an angle of 30 degrees, and your spud landed 20 feet away, we can assume that the spud when highest right in the middle of its flight, which is halfway (10 feet). Use basic trigonometry to find the height 45 degrees up at a horizontal distance ten feet away to get:
height = h = (10′) * (tan 30) = 5.8 feet
(Convert this to meters by: (5.8 feet) * (12 inches/foot) / (39.97 inches/meter) = 1.8 meters)
I measured the mass of my spud to be 25 grams (which is 0.025 kg).
Now, let’s calculate the potential energy:
PE = mgh = (0.025 kg) * (1.8 meters) * (10 m/s2) = 0.44 Joules
How fast was the spud going before it smacked into the ground? Set PE = KE to solve for velocity:
mgh = 0.5 mv2 gives v = (2gh)1/2
Plug in your numbers to get:
v = [(2) * (10) * (1.8)]2 = 6 m/s (or about 20 feet per second). Cool!
Click here to go to next lesson on Bobsleds.
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Bobsleds use the low-friction surface of ice to coast downhill at ridiculous speeds. You start at the top of a high hill (with loads of potential energy) then slide down a icy hill til you transform all that potential energy into kinetic energy. It’s one of the most efficient ways of energy transformation on planet Earth. Ready to give it a try?
This is one of those quick-yet-highly-satisfying activities which utilizes ordinary materials and turns it into something highly unusual… for example, taking aluminum foil and marbles and making it into a racecar.
While you can make a tube out of gift wrap tubes, it’s much more fun to use clear plastic tubes (such as the ones that protect the long overhead fluorescent lights). Find the longest ones you can at your local hardware store. In a pinch, you can slit the gift wrap tubes in half lengthwise and tape either the lengths together for a longer run or side-by-side for multiple tracks for races. (Poke a skewer through the rolls horizontally to make a quick-release gate.)
Here’s what you need:
- aluminum foil
- marbles (at least four the same size)
- long tube (gift wrapping tube or the clear protective tube that covers fluorescent lighting is great)
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If you’re finding that the marbles fall out before the bobsled reaches the bottom of the slide, you need to either crimp the foil more closely around the marbles or decrease your hill height.
Check to be sure the marbles are free to turn in their “slots” before launching into the tube – if you’ve crimped them in too tightly, they won’t move at all. If you oil the bearings with a little olive oil or machine oil, your tube will also get covered with oil and later become sticky and grimy… but they sure go faster those first few times!
Download Student Worksheet & Exercises
Exercises Answer the questions below:
- Potential energy is energy that is related to:
- Equilibrium
- Kinetic energy
- Its system
- Its elevation
- If an object’s energy is mostly being used to keep that object in motion, we can say it has what type of energy?
- Kinetic energy
- Potential energy
- Heat energy
- Radiation energy
- True or False: Energy is able to remain in one form that is usable over and over again.
- True
- False
Click here to go to next lesson on Roller Coasters.
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We’re going to build monster roller coasters in your house using just a couple of simple materials. You might have heard how energy cannot be created or destroyed, but it can be transferred or transformed (if you haven’t that’s okay – you’ll pick it up while doing this activity).
Roller coasters are a prime example of energy transfer: You start at the top of a big hill at low speeds (high gravitational potential energy), then race down a slope at break-neck speed (potential transforming into kinetic) until you bottom out and enter a loop (highest kinetic energy, lowest potential energy). At the top of the loop, your speed slows (increasing your potential energy), but then you speed up again and you zoom near the bottom exit of the loop (increasing your kinetic energy), and you’re off again!
Here’s what you need:
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- marbles
- masking tape
- 3/4″ pipe foam insulation (NOT neoprene and NOT the kind with built-in adhesive tape)
To make the roller coasters, you’ll need foam pipe insulation, which is sold by the six-foot increments at the hardware store. You’ll be slicing them in half lengthwise, so each piece makes twelve feet of track. It comes in all sizes, so bring your marbles when you select the size. The ¾” size fits most marbles, but if you’re using ball bearings or shooter marbles, try those out at the store. (At the very least you’ll get smiles and interest from the hardware store sales people.) Cut most of the track lengthwise (the hard way) with scissors. You’ll find it is already sliced on one side, so this makes your task easier. Leave a few pieces uncut to become “tunnels” for later roller coasters.
Read for some ‘vintage Aurora’ video? This is one of the very first videos ever made by Supercharged Science:
Download Student Worksheet & Exercises
Tips & Tricks
Loops Swing the track around in a complete circle and attach the outside of the track to chairs, table legs, and hard floors with tape to secure in place. Loops take a bit of speed to make it through, so have your partner hold it while you test it out before taping. Start with smaller loops and increase in size to match your entrance velocity into the loop. Loops can be used to slow a marble down if speed is a problem.
Camel-Backs Make a hill out of track in an upside-down U-shape. Good for show, especially if you get the hill height just right so the marble comes off the track slightly, then back on without missing a beat.
Whirly-Birds Take a loop and make it horizontal. Great around poles and posts, but just keep the bank angle steep enough and the marble speed fast enough so it doesn’t fly off track.
Corkscrew Start with a basic loop, then spread apart the entrance and exit points. The further apart they get, the more fun it becomes. Corkscrews usually require more speed than loops of the same size.
Jump Track A major show-off feature that requires very rigid entrance and exit points on the track. Use a lot of tape and incline the entrance (end of the track) slightly while declining the exit (beginning of new track piece).
Pretzel The cream of the crop in maneuvers. Make a very loose knot that resembles a pretzel. Bank angles and speed are the most critical, with rigid track positioning a close second. If you’re having trouble, make the pretzel smaller and try again. You can bank the track at any angle because the foam is so soft. Use lots of tape and a firm surface (bookcases, chairs, etc).
Troubleshooting Marbles will fly everywhere, so make sure you have a lot of extras! If your marble is not following your track, look very carefully for the point of departure – where it flies off.
-Does the track change position with the weight of the marble, making it fly off course? Make the track more rigid by taping it to a surface.
-Is the marble jumping over the track wall? Increase your bank angle (the amount of twist the track makes along its length).
-Does your marble just fall out of the loop? Increase your marble speed by starting at a higher position. When all else fails and your marble still won’t stay on the track, make it a tunnel section by taping another piece on top the main track. Spiral-wrap the tape along the length of both pieces to secure them together.
HOT TIPS for ULTRA-COOL PARENTS: This lab is an excellent opportunity for kids to practice their resilience, because we guarantee this experiment will not work the first several times they try it. While you can certainly help the kids out, it’s important that you help them figure it out on their own. You can do this by asking questions instead of rushing in to solve their problems. For instance, when the marble flies off the track, you can step back and say:
“Hmmm… did the marble go to fast or too slow?”
“Where did it fly off?”
“Wow – I’ll bet you didn’t expect that to happen. Now what are you going to try?”
Become their biggest fan by cheering them on, encouraging them to make mistakes, and try something new (even if they aren’t sure if it will work out).
Check out this cool roller coaster from one of our students!
Exercises
- What type of energy does a marble have while flying down the track of a roller coaster?
- What type of energy does the marble have when you are holding it at the top of the track?
- At the top of a camel back hill, which is higher for the marble, kinetic or potential energy?
- At the top of an inverted loop, which energy is higher, kinetic or potential energy?
Click here to go to next lesson on Including Friction in your Calculations.
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Most real life situations do involve friction. But how does it fit in with kinetic and potential energy equations? Here’s how:
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Click here to go to next lesson on Springs.
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Springs are used a lot in physics problems, because you can model things like car suspension systems, springs on door hinges, or even how flexible (or elastic) a material is by modeling it as a spring on paper for your analysis. Here's how:
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Springs in Physics Problems Now that you understand how to set up spring type problems, here's how to solve one using the conservation of energy equation:
Click here to go to next lesson on Springs and the Conservation of Energy.
[/am4show]Springs can launch projectiles huge distances, and they’re really easy to model on paper using the conservation of energy:
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Click here to go to next lesson on Car Suspension Problem.
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Here’s how you can model a car suspension system using a simple spring model and a couple of energy equations:
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Click here to go to next lesson on Elevator Nightmare.
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A lot of people’s worst nightmare is an elevator cable breaking while they are in the elevator. Let’s find out exactly how bad this type of accident can be from a physics perspective:
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Click here to go to next lesson on Speedy Waterslide.
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Do you like water slides? Did you know that you can find your speed that you hit the water without even knowing the shape of the slide? Here’s how…
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Click here to go to next lesson on Gravity Go-Karts.
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Nothing says summer time fun than a home-built go-kart that can race down the driveway with just as much thrill as two story roller coasters.
A go-karts (also called “go-cart”) can be gravity powered (without a motor) or include electric or gas powered motors. The gravity powered kind are also known as Soap Box Derby racers, and are the simplest kind to make since all you need is wheels, a frame, and a good hill (and a helmet!).
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Materials:
Hardware Bits and Pieces:
- 4-6″ wheels that do NOT swivel
- 3-5′ rope
- two door hinges
- One 4-1/2″ threaded hex head bolt with 4 washers and 2 nuts
- 2 heavy duty eye hooks
- Box of 1-1/2″ long coarse threaded wood or drywall screws
- Six 3″ long coarse threaded wood or drywall screws
Wood:
- one piece that is 4″ x 1″ x 24″
- one piece that is 6″ x 2″ x 8 feet cut into three pieces (one 4′ long piece and two pieces that are 2′ long each)
Tools:
- Crescent wrench, open end wrenches, or socket wrench
- Saw for cutting wood to size
- Drill and drill bits (also a 1/2″ bit)
- Measuring tape
- Pencil
Make sure you wear your HELMET and get someone to help you with the power tools!
The go-kart we’re going to make is long enough to hold two passengers, so feel free to shorten it up a bit if you’re only needing it for one passenger. You’ll need only a couple of tools like a drill and a saw, and also some experienced adult help and you’ll be off and riding this vehicle in under two hours, from start to finish.
After you’ve got this working, you’re probably going to be more than a little popular, especially with younger kids that might be too small to ride safely. Here’s a smaller version you can build them with only a few parts. You’ll not only get points for making something really cool, but it’ll keep them busy so you can ride your new go-kart!
And yes, you must INSIST that everyone wears helmets, or you’ll take the wheels off. Helmet hair is way more fashionable than squashed brain cells.
Click here to go to next lesson on Shooting the Sand.
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If you’ve ever thrown a ball down into the sand, you know it can bury itself below the surface. Here’s how you figure out the non-conservative forces into the equation of the sand exerting a force on the ball as it slows down and stops deep in the sand.
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Click here to go to next lesson on Friction Energy.
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Friction can be tricky to deal with, especially since it’s a non-conservative force (meaning that you can’t recover the energy from it for a useful purpose the way you can with potential and kinetic energy).
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Click here to go to next lesson on Driving with Physics in Mind.
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Have you learned how to drive yet, or are you excited to learn? Here’s a question on the driver’s test that is really kind of scary from a physics point of view, but it will make a lot of sense once you see how it works. And might even keep you from speeding, now that you understand what can happen if you lock up your brakes while going too fast.
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Click here to go to next lesson on Light Speed Particles.
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How do you calculate the energies of particles going near the speed of light? It’s a little tricky, but you can do it if you have the right equation. Since the kinetic energy equation comes from Newton’s Laws of Motion, which don’t apply to particles moving near the speed of light, we have to add a correction factor from Einstein’s Theory of Relativity in order to compensate and make the equations accurate. Here’s the equation for particles going close to light speed:
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Yay! You’ve completed this set of lessons! Now it’s time for you to work your own physics problems.
Download your Work, Energy and Power Problem Set here.
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Energy is the ability to do work. Work happens when something moves a distance against a force. Although it seems a little hard to comprehend, this is truly one of the most useful concepts in physics.
[am4show have='p9;p58;' guest_error='Guest error message' user_error='User error message' ] I’m willing to bet you spend a lot of your time moving things a distance against a force. Do you ever climb stairs, walk, ride a bicycle, or lift a fork to your mouth to eat? Of course you do. Each one of those things requires you to move something a distance against a force. You’re using energy and you’re doing work.
[am4show have='p9;p58;' guest_error='Guest error message' user_error='User error message' ] I’m willing to bet you spend a lot of your time moving things a distance against a force. Do you ever climb stairs, walk, ride a bicycle, or lift a fork to your mouth to eat? Of course you do. Each one of those things requires you to move something a distance against a force. You’re using energy and you’re doing work.
Click here to go to next lesson on What is Work?
[/am4show]Work is not that hard… it’s force that can be difficult. Imagine getting up a 10-step flight of stairs without a set of stairs. Your legs don’t have the strength or force for you to jump up… you’d have to climb up or find a ladder or a rope. The stairs allow you to, slowly but surely, lift yourself from the bottom to the top. Now imagine you are riding your bike and a friend of yours is running beside you.
Who’s got the tougher job? Your friend, right? You could go for many miles on your bike but your friend will tire out after only a few miles. The bike is easier (requires less force) to do as much work as the runner has to do. Now here’s an important point, you and your friend do about the same amount of work.
You also do the same amount of work when you go up the stairs versus climbing up the rope. The work is the same, but the force needed to make it happen is much different. Don’t worry if that doesn’t make sense now. As we move forward, it will become clearer. Before we start solving physics problems, we first have to accurately define a couple of terms we’re going to be using a lot that you might already have a different definition for.
Here are three concepts we’re going to be working with in this section:
- Work
- Energy
- Power
Energy is the ability to do work. Work is done on an object when a force acts on it so the object moves somewhere. It can be a large or small displacement, but as long as it’s not in its original position when it’s done, work is said to be done on the object. An example of work is when an apple falls off the tree and hits the ground. The apple falls because the gravitational force is acting on it, and it went from the tree to the ground. If you carry a heavy box up a flight of stairs, you are doing work on the box.
An example of what is not work is if you push really hard against a brick wall. The wall didn’t go anywhere, so you didn’t do any work at all (even though your muscles may not agree!). Mathematically, work is a vector, and is defined as the force multiplied by the distance like this: W = F d
If there’s an angle between the force and displacement vectors, then you’ll need to also multiply by the cosine of the angle between the two vectors. This is an important concept: Notice that the force has to cause the displacement. If you’re carrying a heavy box across the room (no stairs) at a constant speed, then you are not doing work on the box.
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The box is traveling in the horizontal direction at a constant speed. You are holding the heavy box up in the vertical direction. The force you are applying to the box is not causing it to be displaced in the same direction. There has to be a component of the force in the horizontal direction if you’re doing work on the box ((Remember F=ma? Constant speed means no acceleration!) Mathematically, the work equation would have angle between the force and the displacement vectors at 90 degrees, and the cosine of 90 degrees is zero, thus cancelling the work out to zero.
Click here to go to next lesson on Units for Energy.
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We’ll cover power in a little bit, but first we need to have a unit of measurement for work. The units for work and energy are the same, but note that energy and work are not the same. (Remember, energy is the ability to do work.)
For energy, a couple of units are the Joule (J) and the calorie (cal or Cal). A Joule is the energy needed to lift one Newton one meter. A Newton is a unit of force. One Newton is about the amount of force it takes to lift 100 grams or 4 ounces or an apple.
It takes about 66 Newtons to lift a 15-pound bowling ball and it would take a 250-pound linebacker about 1000 Newtons to lift himself up the stairs! So, if you lifted an apple one meter (about 3 feet) into the air you would have exerted one Joule of energy to do it.
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The calorie is generally used to talk about heat energy, and you may be a bit more familiar with it due to food and exercise. A calorie is the amount of energy it takes to heat one gram of water one degree Celsius. Four Joules are about one calorie. A 100-gram object takes about one Newton of force to lift. Since it took one Newton of force to lift that object, how much work did we do? Remember work = force x distance so in this case work = 1 Newton x 20 meters or work = 20 Joules.
Click here to go to next lesson on Moving Against a Force.
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This experiment is for Advanced Students. We’re going to really get a good feel for energy and power as it shows up in real life. For this experiment, you need:
- Something that weighs about 100 grams or 4 ounces, or just grab an apple.
- A meter or yard stick
This might seem sort of silly but it’s a good way to get the feeling for what a Joule is and what work is.
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Download Student Worksheet & Exercises
1. Grab your 100 gram object, put it on a table.
2. Now lift it off the table straight up until you lift it one meter (one yard).
3. Lift it up and down 20 times.
A 100 gram object takes about one Newton of force to lift. Since it took one Newton of force to lift that object, how much work did we do? Remember work = force x distance so in this case work = 1 Newton x 20 meters or work = 20 Joules.
You may ask “but didn’t we move it 40 meters, 20 meters up and 20 down?” That’s true, but work is moving something against a force. When you moved the object down you were moving the object with a force, the force of gravity. Only in lifting it up, are you actually moving it against a force and doing work. Four Joules are about 1 calories so we did 5 calories of work.
“Wow, I can lift an apple 20 times and burn 5 calories! Helloooo weight loss!” Well…not so fast there Richard Simmons. When we talk about calories in nutrition we are really talking about kilo calories. In other words, every calorie in that potato chip is really 1000 calories in physics. So as far as diet and exercise goes, lifting that apple actually only burned .005 calories of energy,…rats.
It is interesting to think of calories as the unit of energy for humans or as the fuel we use. The average human uses about 2000 calories (food calories that is, 2,000,000 actual calories) a day of energy. Running, jumping, sleeping, eating all uses calories/energy. Running 15 minutes uses 225 calories. Playing soccer for 15 minutes uses 140 calories. (Remember those are food calories, multiply by 1000 to get physics calories). This web site has a nice chart for more information: Calories used in exercise.
Everything we eat refuels that energy tank. All food has calories in it and our body takes those calories and converts them to calories/energy for us to use. How did the food get the energy in it? From the sun! The sun’s energy gives energy to the plants and when the animals eat the plants they get the energy from the sun as well.
So, if you eat a carrot or a burger you are getting energy from the sun! Eating broccoli gives you about 50 calories. Eating a hamburger gives you about 450 calories! We use energy to do things and we get energy from food. The problem comes when we eat more energy than we can use. When we do that, our body converts the energy to fat, our body’s reserve fuel tank. If you use more energy then you’ve taken in, then your body converts fat to energy. That’s why exercise and diet can help reduce your weight.
Let’s take the concept of work a little bit farther. If Bruno carries a 15 pound bowling ball up a 2 meter (6 foot) flight of stairs, how much work does he do on the bowling ball? It takes 66 Newtons of force to lift a 15 pound bowling ball 1 meter. Remember work = force x distance.
So, work = 66 Newtons x 2 meters. In this case, Bruno does 132 Joules of work on that bowling ball. That’s interesting, but what if we wanted to know how hard poor Bruno works? If he took a half hour to go up those stairs he didn’t work very hard, but if he did it in 1 second, well then Bruno’s sweating!
That’s the concept of power. Power is to energy like miles per hour is to driving. It is a measure of how much energy is used in a given span of time. Mathematically it’s Power = work/time. Power is commonly measured in Watts or Horsepower. Let’s do a little math and see how hard Bruno works.
In both cases mentioned above Bruno, does 132 Joules of work, but in the first case he does the work in 30 minutes (1800 seconds) and in the last case he does it in 1 second. Let’s first figure out Bruno’s power in Watts. A Watt is 1 Joule/second so:
For the half hour Bruno’s Power = 132 Joules/1800 seconds = .07 Watts
For the second Bruno’s Power = 132 joules/1 second = 132 Watts
You can see that the faster you exert energy the more power you use. Another term for power is horsepower. You may have heard the term horsepower in car ads. The more powerful car can exert more energy faster, getting the car moving faster. A Dodge Viper has 450 horsepower which can accelerate a 3,300 pound car from 0 to 60 mph in 4.1 seconds…WOW!
One horsepower is 745 Watts or one Watt is .001 horsepower. So converting Watts to horsepower poor Bruno exerts:
.07 x .001 = .00007 horsepower over the half hour
132 x .001 = .132 horsepower over the second (not exactly a Dodge Viper!)
Exercises
- If something has a weight of 2 Newtons and is moved half a meter, how many Joules of energy are used? Show your work.
- What is the source of all this energy we’re working with here?
- It doesn’t count as work when you move the apple back down. Why not?
Click here to go to next lesson on Energy of Food.
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A peanut is not a nut, but actually a seed. In addition to containing protein, a peanut is rich in fats and carbohydrates. Fats and carbohydrates are the major sources of energy for plants and animals.
The energy contained in the peanut actually came from the sun. Green plants absorb solar energy and use it in photosynthesis. During photosynthesis, carbon dioxide and water are combined to make glucose. Glucose is a simple sugar that is a type of carbohydrate. Oxygen gas is also made during photosynthesis.
The glucose made during photosynthesis is used by plants to make other important chemical substances needed for living and growing. Some of the chemical substances made from glucose include fats, carbohydrates (such as various sugars, starch, and cellulose), and proteins.
Photosynthesis is the way in which green plants make their food, and ultimately, all the food available on earth. All animals and nongreen plants (such as fungi and bacteria) depend on the stored energy of green plants to live. Photosynthesis is the most important way animals obtain energy from the sun.
Oil squeezed from nuts and seeds is a potential source of fuel. In some parts of the world, oil squeezed from seeds-particularly sunflower seeds-is burned as a motor fuel in some farm equipment. In the United States, some people have modified diesel cars and trucks to run on vegetable oils.
Fuels from vegetable oils are particularly attractive because, unlike fossil fuels, these fuels are renewable. They come from plants that can be grown in a reasonable amount of time.
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Materials
- Shelled peanut
- Small pair of pliers
- Match or lighter
- Sink
Download Student Worksheet & Exercises
Procedure
ASK AN ADULT TO HELP YOU WITH THIS EXPERIMENT. DO NOT DO THIS EXPERIMENT BY YOURSELF. The fuel from the peanut can flare up and burn for a longer time than expected.
Close the drain in the kitchen sink. Fill the sink with water until the bottom of the sink is just covered.
Using a small pair of pliers, hold the peanut over the sink containing water. Ask an adult to hold the flame of a lit match or lighter directly under the peanut. When the peanut starts to burn, the lit match or lighter can be removed.
Allow the peanut to burn for one minute. MAKE SURE AN ADULT REMAINS PRESENT AND MAKE SURE TO HOLD THE PEANUT OVER THE SINK. To extinguish the burning peanut, drop it into the water. After you have extinguished the peanut, allow it to cool and then examine it carefully.
Observations
How long does it take for the peanut to start to burn? Does the peanut burn with a clean flame or a sooty flame? What color is the flame? What color does the peanut turn when it burns? Did the size of the peanut change after it has burned for several minutes?
Discussion
You should find that the peanut ignites and burns after a lit match or lighter is held under it for a few seconds. Although you only let the peanut burn for one minute as a safety measure, the peanut would burn for many minutes.
In this experiment, when the peanut burns, the stored energy in the fats and carbohydrates of the peanut is released as heat and light. When you eat peanuts, the stored energy in the fats and carbohydrates of the peanut is used to fuel your body.
Other Things to Try
Hold one end of a piece of uncooked spaghetti in a pair of pliers. Ask an adult to hold the flame of a lit match or lighter under the other end of the spaghetti. When the spaghetti starts to burn, place it in an aluminum pie pan that is in the sink. Make sure to extinguish the burning spaghetti with water when you are finished with the experiment. How does the burning of the spaghetti compare with the burning of the peanut?
Exercises
- What is the process called where plants get food from the sun?
- Osteoporosis
- Photosynthesis
- Chlorophyll
- Metamorphosis
- Where does all life on the planet get its food?
- List two ways that we could use the energy in a peanut:
Click here to go to next lesson on Bomb Calorimeter.
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This experiment is for advanced students. Did you know that eating a single peanut will power your brain for 30 minutes? The energy in a peanut also produces a large amount of energy when burned in a flame, which can be used to boil water and measure energy.
Peanuts are part of the bean family, and actually grows underground (not from trees like almonds or walnuts). In addition to your lunchtime sandwich, peanuts are also used in woman’s cosmetics, certain plastics, paint dyes, and also when making nitroglycerin.
What makes up a peanut? Inside you’ll find a lot of fats (most of them unsaturated) and antioxidants (as much as found in berries). And more than half of all the peanuts Americans eat are produced in Alabama. We’re going to learn how to release the energy inside a peanut and how to measure it.
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Materials:
- raw peanuts
- chemistry stand with glass test tube and holder (watch video)
- flameproof surface (large ceramic tile or cookie sheet)
- paper clip
- alcohol burner or candle with adult help
- fire extinguisher
Download Student Worksheet & Exercises
What’s Going On? There’s chemical energy stored inside a peanut, which gets transformed into heat energy when you ignite it. This heat flows to raise the water temperature, which you can measure with a thermometer. You should find that your peanut contains 1500-2100 calories of energy! Now don’t panic… this isn’t the same as the number of calories you’re allowed to eat in a day. The average person aims to eat around 2,000 Calories (with a capital “C”). 1 Calorie = 1,000 calories. So each peanut contains 1.5-2.1 Calories of energy (the kind you eat in a day). Do you see the difference?
But wait… did all the energy from the peanut go straight to the water, or did it leak somewhere else, too? The heat actually warmed up the nearby air, too, but we weren’t able to measure that. If you were a food scientist, you’d use a nifty little device known as a bomb calorimeter to measure calorie content. It’s basically a well-insulated, well-sealed device that catches nearly all the energy and flows it to the water, so you get a much more accurate temperature reading. (Using a bomb calorimeter, you’d get 6.1-6.8 Calories of energy from one peanut!)
How do you calculate the calories from a peanut?
Let’s take an example measurement. Suppose you measured a temperature increase from 20 °C to 100 °C for 10 grams of water, and boiled off 2 grams. We need to break this problem down into two parts – the first part deals with the temperature increase, and the second deals with the water escaping as vapor.
The first basic heat equation is this:
Q = m c T
Q is the heat flow (in calories)
m is the mass of the water (in grams)
c is the specific heat of water (which is 1 degree per calorie per gram)
and T is the temperature change (in degrees)
So our equation becomes: Q = 10 * 1 * 80 = 800 calories.
If you measured that we boiled off 2 grams of water, your equation would look like this for heat energy:
Q = L m
L is the latent heat of vaporization of water (L= 540 calories per gram)
m is the mass of the water (in grams)
So our equation becomes: Q = 540 * 2 = 1080 calories.
The total energy needed is the sum of these two:
Q = 800 calories + 1080 calories = 1880 calories.
Click here to go to next lesson on Back to work!
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We’re going to learn how to calculate the amount of work done by forces by looking at how the force acts on the object, and if it causes a displacement. Have you spotted the three things you need to know in order to calculate the work done?
- Force
- Displacement
- Angle between the force and displacement vector (called theta)
The easiest way to do this is to show you by working a set of physics problems. So take out your notebook and a pencil, and do these problems right along with me. Here we go!
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Click here to go to next lesson on Work done by Friction.
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How do we calculate the work done by friction? Here’s a classic problem that shows you how to handle friction forces in your physics problems.
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Click here to go to next lesson on How much work in climbing stairs?.
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Ever get out of breath while climbing stairs? How much work do you think you did? Let’s find out…
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Click here to go to next lesson on Non-conservative Forces.
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Work done by friction is never conserved, since it’s turned into heat or sound, and we can’t get that back. It’s a non-conservative force. Other forces like gravity and speed are said to be conservative, since we can transfer that energy to a different form for a useful purpose. When you pull back a swing and then let go, you’re using the energy created by the gravitational force on the swing and transforming it into the forward motion of the swing as it moves through its arc. Energy from friction forces cannot be recovered, so we say that it’s an external energy, or work done by an external force.
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Click here to go to next lesson on Kinetic Energy.
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All the different forms of energy (heat, electrical, nuclear, sound, and so forth) can be broken down into two main categories: potential and kinetic energy. Kinetic energy is the energy of motion. Kinetic energy is an expression of the fact that a moving object can do work on anything it hits; it describes the amount of work the object could do as a result of its motion. Whether something is zooming, racing, spinning, rotating, speeding, flying, or diving… if it’s moving, it has kinetic energy.
How much energy it has depends on two important things: how fast it’s going and how much it weighs. A bowling ball cruising at 100 mph has a lot more kinetic energy than a cotton ball moving at the same speed.
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Click here to go to next lesson on Bow and Arrow Problem.
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Imagine an arrow is shot from a bow and by the time it hits an apple it is traveling with 10 Joules of kinetic energy (kinetic energy is the energy of motion). What’s meant by kinetic energy is that when it hits something, it can do that much work on whatever is hit.
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Soooo, back to the arrow... if the arrow hits that apple it can exert 10 Joules of energy on that apple. It takes about 1 Newton of force to move that apple so the arrow can move the apple 10 meters. One Joule equals one Newton x one meter so 10 Joules would equal one Newton x 10 meters.
It could also exert a force of 10 Newtons over one meter or any other mathematical calculation you’d like to play with there. (This, by the way, is completely hypothetical. With the apple example we are conveniently ignoring a bunch of stuff like the fact that the arrow would actually pierce the apple, and that there’s friction, heat, sound, and a variety of other forces and energies that would take place here.)
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Click here to go to next lesson on Freefall Pennies.
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Here’s a fun experiment that uses a penny in free fall to practice calculating kinetic energy.
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Energy changes to other forms of energy all the time. The electrical energy coming out of a wall socket transfers to light energy in the lamp. The chemical energy in a battery transfers to electrical energy which transfers to sound energy in your personal stereo. In the case of the ball falling, gravitational potential energy transfers to kinetic energy, the energy of motion.
Click here to go to next lesson on Potential Energy.
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Think of potential energy as the “could” energy. The battery “could” power the flashlight. The light “could” turn on. I “could” make a sound. That ball “could” fall off the wall. That candy bar “could” give me energy. Potential energy is the energy that something has that can be released. Objects can store energy as a result of their position.
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Click here to go to next lesson on Elastic Energy.
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There are many different kinds of potential energy. We’ve already worked with gravitational potential energy, so let’s take a look at elastic potential energy.
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Materials: a rubber band
A simple way to demonstrate elastic energy is to stretch a rubber band without releasing it. The stretch in the rubber band is your potential energy. When you let go of the rubber band, you are releasing the potential energy, and when you aim it toward a wall, it’s converted into motion (kinetic energy).
Here’s another fun example: the rubber band can also show how every is converted from one form to another. If you place the rubber band against a part of you that is sensitive to temperature changes (like a cheek or upper lip), you can sense when the band heats up. Simply stretch and release the rubber band over and over, testing the temperature as you go. Does it feel warmer in certain spots, or in just one location?
Click here to go to next lesson on Energy Transforms.
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The rubber band can also show how energy is converted from one form to another. If you place the rubber band against a part of you that is sensitive to temperature changes (like a cheek or upper lip), you can sense when the band heats up. Simply stretch and release the rubber band over and over, testing the temperature as you go. Does it feel warmer in certain spots, or in just one location?
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There are other ways to store potential energy. For example, the battery has the potential energy to light the bulb of the flashlight if the flashlight is turned on and the energy is released from the battery. Your legs have the potential energy to make you hop up and down if you want to release that energy (like you do whenever it’s time to do science!). The fuel in a gas tank has the potential energy to make the car move. Those are all ways to store potential energy.
Click here to go to next lesson on WackaPOW!
[/am4show]In this experiment, you’re looking for two different things: first you’ll be dropping objects and making craters in a bowl of flour to see how energy is transformed from potential to kinetic, but you’ll also note that no matter how carefully you do the experiment, you’ll never get the same exact impact location twice.
To get started, you’ll need to gather your materials for this experiment. Here’s what you need:
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- several balls of different weights no bigger then the size of a baseball (golf ball, racket ball, ping pong ball, marble etc. are good choices)
- fill a good size container or mixing bowl with flour or corn starch (or any kind of light powder)
- If you’re measuring your results, you’ll also need a tape measure (or yard stick) and a spring scale (or kitchen scale).
Are you ready?
1. Fill the container about 2 inches or so deep with the flour.
2. Weigh one of the balls (If you can, weigh it in grams).
3. Hold the ball about 3 feet (one meter) above the container with the flour.
4. Drop the ball.
5. Whackapow! Now take a look at how deep the ball went and how far the flour spread. (If all your balls are the same size but different weights it’s worth it to measure the size of the splash and the depth the ball went. If they are not, don’t worry about it. The different sizes will effect the splash and depth erratically.
6. Try it with different balls. Be sure to record the mass of each ball and calculate the potential energy for each ball.
Each one of the balls you dropped had a certain amount of potential energy that depended on the mass of the ball and the height it was dropped from. As the ball dropped the potential energy changed to kinetic energy until, “whackapow”, the kinetic energy of the ball collided with and scattered the flour. The kinetic energy of the ball transferred kinetic energy and heat energy to the flour.
For Advanced Students:
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Calculate the gravitational potential energy of the ball. Take the mass of the ball, multiply it by 10 m/s2 and multiply that by 1 meter. For example, if your ball had a mass of 70 grams (you need to convert that to kilograms so divide it by 1000 so that would be .07 grams) your calculation would be
PE=.07 x 10 x 1 = .7 Joules of potential energy.
So, how much kinetic energy did the ball in the example have the moment it impacted the flour? Well, if all the potential energy of the ball transfers to kinetic energy, the ball has .7 Joules of kinetic energy.
Create a table in your science journal or use ours. (You’ll need Microsoft Excel to use this file.)
Click here to go to next lesson on Power.
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We didn’t finish with our three concepts of energy, work, and power yet! The important concept of Power is work done over time, and is measured in watts (W), which is a Joule per second (J/s).
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Work doesn’t have anything to do with time, but power does. Sometimes work is done slow, and other times faster. Someone hiking a mountain can reach the peak way before a rock climber, even though they are both traveling the same vertical distance. A hiker in our example has a higher power rating than a rock climber. Power is the rate that work is done.
Click here to go to next lesson on Power is Scalar.
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Is power a vector or a scalar quantity? Power is a scalar, but it’s made up of two vector quantities of force and velocity like this:
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Click here to go to next lesson on What size engine do you need?.
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What if you’re wanting to get a motor for a winch on the front of your jeep? What size motor do you need? Here’s how to calculate the minimum power so you don’t spend more cash than you need to for a motor that will still do the job. (Near the end of the video below, I’ll show you how to convert watts to horsepower.)
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I love to water ski (no kidding!). Here’s a neat problem about how to determine some things about the boat and deal with weird units like knots in your calculations.
Click here to go to next lesson on Work-Energy Relationship.
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