Kinetic and Potential Energy
Where energy hides, and what happens when it gets loose.
Where we left off
Last week you learned that when car #12 slammed into car #9, the two cars pushed on each other with equal and opposite forces — and that energy moved from the speeding car into the stalled one. This week we go after the part we skipped: what energy actually is, where it was hiding before the crash, and where it goes afterward.
We will not repeat Newton's Third Law here. You already own that one. Today is about energy.
Energy of motion
Energy is the ability to make something happen — to move an object, heat it up, light it, or bend it. Anything that is moving carries kinetic energy (KE), the energy of motion.
Two things decide how much kinetic energy something has
- Mass. A loaded semi and a bicycle rolling at the same speed do not carry the same energy. Double the mass, double the KE.
- Speed. Speed matters much more. Double the speed and the KE becomes four times bigger.
You are not required to use the formula this year — but it is worth seeing why speed wins: the speed gets multiplied in twice.
This is the physics behind every speed limit sign. A car at 40 mph does not hit twice as hard as one at 20 mph — it hits about four times as hard.
Quick check
Tap an answer and read the reason.
Energy in storage
Potential energy (PE) is stored energy — energy an object has because of its position or its shape. Nothing is moving yet. The energy is just waiting.
Two kinds you will use all year
- Gravitational PE — stored by height. Lift something higher and you store more. It also depends on mass: a bowling ball on the top shelf stores far more than a pencil up there.
- Elastic PE — stored by stretching or squeezing: a drawn bow, a wound rubber band, a compressed spring, a stretched trampoline mat.
Students hear "potential" as "possible" and decide the energy is not real. It is completely real — it is just not moving anything yet. Ask what a 40-pound rock on a roof could do to a car below.
Quick check
Adding them together
Most objects carry both kinds at once. Add them and you get Total Mechanical Energy (TME).
Now the big rule of this entire unit:
The Law of Conservation of Energy: energy can never be created and never be destroyed. It can only be changed from one form into another, or moved from one object to another.
A roller coaster is the clearest example on Earth. After the chain lift drops you over the first hill, there are no motors left. The ride has exactly one tank of energy — the height it gave you at the top — and it spends the whole ride trading that energy back and forth:
- Going down: potential energy turns into kinetic energy. You speed up.
- Going up: kinetic energy turns back into potential energy. You slow down.
Nothing is ever lost. It is the same energy wearing different clothes.
"The coaster runs out of energy at the end" sounds right but is not. Friction between wheels and track gradually turns the mechanical energy into thermal energy (heat) and sound. The energy still exists — it just left the ride. That is the bridge into next week's thermodynamics lesson.
Reading a roller coaster
Three points on one track. Watch how the two energies trade places while the total stays the same.
Add the purple bar and the orange bar at any point on the track. You always get the same total. That total is the Total Mechanical Energy.
Notice the second hill is shorter than the first. It has to be. The coaster can never climb higher than the hill it started on, because it can never have more energy than it started with — and friction quietly takes a little on the way.
Experiment 1: Ramp Height Test
20 minTurn height into speed, and speed into a shoved cup.
Materials
- A stiff piece of cardboard or a binder
- A marble or small ball
- A stack of books
- A paper or plastic cup
- Tape measure or ruler
- Masking tape
Steps
- Prop one end of the cardboard on one book to make a ramp. Tape the bottom edge down so it cannot slide.
- Lay the cup on its side at the bottom of the ramp, about 4 inches past the end. Mark its starting spot with tape.
- Release the marble from the very top of the ramp. Do not push it — just let go.
- Measure how far the cup moved. Write it down.
- Raise the ramp to three books, then to six books. Release from the top each time and measure again.
- Do each height three times and use the middle result.
The marble is the same marble every time and you never push it. So why should the ramp height change anything? Predict what happens to the cup at six books compared to one book — and say where the extra energy came from.
What actually happened
The higher ramp shoves the cup much farther. Lifting the marble higher stored more gravitational potential energy in it. Rolling down converts that stored energy into kinetic energy, so the marble reaches the bottom moving faster.
When the marble hits the cup it transfers energy to the cup — the same transfer you saw between the two derby cars last week. More energy in means more energy delivered, so the cup slides farther.
Push it further: keep the ramp at six books but tape a coin to the marble's path, or swap in a heavier ball. More mass at the same height means more stored energy too.
Experiment 2: Rubber Band Launcher
15 minThis time the energy is stored in a shape, not a height.
Materials
- 1 rubber band
- A ruler
- A cotton ball or crumpled paper ball
- Tape measure
- Open floor space
Steps
- Hook the rubber band over the end of the ruler and hold the ruler flat, pointing across the room.
- Put the cotton ball against the rubber band and pull it back exactly 2 cm, using the ruler markings.
- Release. Measure how far the cotton ball flew.
- Repeat at 4 cm, then 6 cm. Same ball, same ruler, same height above the floor every time.
- Record all three distances. Aim away from faces and windows.
Pulling back 6 cm is three times as far as 2 cm. Will the ball fly three times as far — more than that, or less? And at the exact moment you are holding the band stretched, where is the energy?
What actually happened
Farther pull, farther flight — and usually more than three times as far, because stretching stores energy faster and faster the more you pull.
While you hold the band stretched, the energy is elastic potential energy, stored in the stretched rubber. It is not doing anything yet. Let go and it converts almost instantly into kinetic energy in the cotton ball.
Feel the rubber band right after a launch: it is slightly warm. A small piece of that stored energy became thermal energy instead of motion. Nothing was destroyed — it just took a different form.
Stored or moving?
Decide whether each one is mainly kinetic energy (something is moving right now) or potential energy (something is stored and waiting). Tap to find out why.
"It could move, so it is kinetic" is the classic error. Kinetic energy requires motion happening now. If it is only waiting to move, it is potential.
Coaster Energy Lab
Set how tall the first hill is, then start the ride. The bar on the right shows how the total energy is split between stored (purple) and moving (orange) at every instant.
Two things to try
- Set the first hill to 10 m and start the ride. The car cannot climb the 12 m hill — it rolls partway up, stops, and comes back. It never had enough energy.
- Turn friction ON and let it run. The car settles lower and lower in the valley. That missing energy is not gone; it became heat in the wheels and rails.
Quick check
Word bank
Tap a card to flip it, then tap again and try to say the meaning yourself.
Four traps to avoid
✗ "The coaster used up its energy."
Energy is never used up. Friction turned it into heat and sound, which spread out into the track and the air. The total amount in the universe did not change by even a little.
✗ "Potential energy is not real energy."
Stand under the object and see how you feel about that. Stored energy is real — it is simply not doing work yet.
✗ "Doubling the speed doubles the kinetic energy."
It multiplies it by four, because the speed gets multiplied in twice. This is why crashes at highway speed are so much worse than parking-lot crashes.
✗ "Heavy things always have more kinetic energy."
Only if they are moving at the same speed. A parked dump truck has zero kinetic energy; a thrown baseball has plenty.
Ask: "A ball is at the very top of its bounce, in mid-air, not moving for one instant. How much kinetic energy does it have?" Correct answer: zero — and its potential energy is at maximum. This exposes students who think "in the air" automatically means "kinetic."
Energy trading in real life
Every one of these is potential energy turning into kinetic energy, or the reverse.
Kings Dominion
The chain lift spends two minutes storing potential energy in you. The first drop spends it all in about four seconds.
Hoover Dam
Water sitting high behind the dam is stored energy. Let it fall through turbines and it becomes electricity for millions of homes.
Free throw
Your legs and arms give the ball kinetic energy. At the top of its arc, most of it has become potential energy — then it falls back down.
Pogo stick
Your weight squeezes the spring (elastic PE), the spring throws you up (KE), gravity stores it again (gravitational PE). Repeat.
Skate park
Dropping into a halfpipe converts height into speed. That speed is exactly what carries you up the far wall.
Sledding
The walk up the hill is you storing potential energy in yourself. The ride down spends it.
Archery
Pulling the bowstring stores elastic energy in the limbs of the bow; releasing hands it all to the arrow.
Bike brakes
Squeezing the brakes turns your kinetic energy into heat in the brake pads. Touch them after a long hill — carefully.
Pick one and trace the energy out loud: "It started as ____ energy, then it became ____ energy, and it ended up as ____."
Final quiz
8 questions. Every answer comes with its reason.
What you now know
- Kinetic energy is the energy of motion; it grows with mass and grows much faster with speed.
- Potential energy is stored energy — from height (gravitational) or from stretch and squeeze (elastic).
- PE + KE = Total Mechanical Energy, and on a coaster the two trade back and forth all ride long.
- Energy is never created or destroyed. It only changes form or moves to another object.
Next up → Topic 3: Energy and Thermodynamics
We kept saying that friction turns motion into heat, and then walked away from it. Next lesson goes straight there: how heat energy moves from hot things to cold things, what equilibrium means, and why the Law of Conservation of Energy is also called the First Law of Thermodynamics. Same law, new job.