Elastic potential energy
Pull back a bowstring, squash a mattress, bend a diving board — and you've stored energy just by changing a shape. Let go, and it all comes rushing back.
Energy you can hide in a shape
Think about pulling back the string of a bow. Your muscles do work, the bow bends, and now something feels loaded — ready to go. You haven't fired anything yet, but the energy is in there, waiting. The instant you let go, it leaps out and flings the arrow.
That stored, ready-to-go energy is called elastic potential energy — EPE for short. Another way to say it is energy in an elastic store. You build it up whenever you change the shape of something springy: pulling a rubber band, squashing the springs in a mattress when you lie down, bending a ruler over the edge of a desk.
Here's the key idea: changing the shape of a springy object stores energy, and letting it snap back gives that energy out again. People have used this for thousands of years — a bow carved more than 2000 years ago worked on exactly the same trick as a modern trampoline.
Elastic, or just squashed?
When you stretch or squash something so its shape changes, we say it has deformed. But not everything that deforms stores useful energy. Press your thumb into modelling clay and it just stays dented — the clay keeps the new shape and gives nothing back. Press a spring and it pushes right back at you.
A material that springs back to its original shape once you stop pushing or pulling is called elastic. Only elastic objects store EPE and hand it back. Clay isn't elastic; a rubber band is.
| What you do | What happens | Springs back? | Stores EPE? |
|---|---|---|---|
| Stretch a rubber band | It gets longer and thinner | Yes | Yes |
| Lie on a mattress | The springs squash down | Yes | Yes |
| Bend a diving board | It curves downward | Yes | Yes |
| Squash modelling clay | It stays dented | No | No |
Load the slingshot, then let go
Energy never vanishes — it moves
Energy is never made or destroyed. It just gets passed between stores — like money moving between pockets. Watching where it goes is the whole game.
A bungee jump, store by store
Grace stands on a high bridge with an elastic cord tied to her legs. Up there she has lots of gravitational energy (energy because she is high up). She jumps. As she falls she speeds up: gravitational energy turns into kinetic energy — the energy of moving. Near the bottom the cord stretches tight and slows her down, soaking up that movement and storing it as elastic potential energy. For one instant she stops dead. Then the cord snaps back, pushing her up: EPE becomes kinetic energy again, then gravitational energy as she rises. The energy keeps getting handed around the same little circle.
A bouncing ball loses the game slowly
When a ball hits the ground it squashes — for a moment it deforms and stores EPE, exactly like the bungee cord. Then it springs back to its round shape and pushes off the floor: the EPE becomes kinetic energy and up it goes. So one bounce is the chain movement → elastic → movement → height.
So why does each bounce get lower? Misconception alert: the missing energy hasn't been used up or destroyed — that can't happen. Every time the ball squashes, a little energy is dissipated: it spreads out into the surroundings as heat (the ball and floor warm up the tiniest bit) and sound (that's the bounce you hear). That energy is too spread out to bounce the ball, so each bounce is smaller, until it finally rests and all of it has drifted off into the warmth of the room.
| Moment | Where the energy is |
|---|---|
| On the bridge | Gravitational (she is high up, not moving) |
| Falling | Gravitational turning into kinetic (movement) |
| Cord stretched, stopped at the bottom | Elastic potential — stored in the stretched cord |
| Bouncing back up | Elastic → kinetic → gravitational again |
| Finally at rest | Dissipated — spread out as heat and sound |
The rubber-band ruler launcher
- Hook a rubber band over your thumb and first finger to make a little slingshot. Scrunch up a small ball of paper to fire.
- Predict first: pull the band back just a little and guess where the paper ball will land. Mark the spot in your head.
- Fire it. Then pull the band back twice as far and predict again — a bit further, or a lot further?
- Fire it and compare. Try three pull-back distances and notice how the landing spot races away as you stretch more.
Use a soft paper ball, never anything hard, and never aim at anyone's face or eyes — point it at a wall or across an empty floor. A loose rubber band can sting, so keep fingers clear of the snap-back.
Four springy surprises
Elastic energy is hiding all over the living world and the sports field — sometimes doing things no rubber band could.
Stretch it and it warms up
Almost everything cools when you pull it, but a rubber band does the opposite. Stretch one quickly against your lip and you'll feel it go warm; let it relax and it turns cool. It's one of the few materials that breaks this rule.
Fleas are living catapults
A flea is too small for its muscles to jump fast — so instead it slowly squeezes a tiny rubbery spring, then trips a latch to release it all at once. It launches at about 100 times the force of gravity, far more than any pilot could survive.
Kangaroos hop on pogo sticks
A kangaroo's long leg tendons stretch and store elastic energy each time it lands, then snap back to power the next hop. By recycling the energy this way, it can bound faster while barely burning any extra fuel.
A pole that flings you skyward
A pole vaulter sprints, jams a bendy pole into the ground, and their speed bends it into a giant spring. As it un-bends it flings them over 6 metres up — about as high as a two-storey house.
Feel the warmth in a rubber band
- Find a fairly thick rubber band and rest it gently against your top lip — lips notice tiny temperature changes really well.
- Predict: when you stretch it, will it feel warmer, cooler, or the same?
- Now stretch it out quickly and hold it tight against your lip. Notice the warmth.
- Keep it on your lip and let it slowly relax — it cools back down again.
Completely safe. Just don't let the band snap — keep a firm grip on both ends and stretch gently.
Key points
- Changing the shape of something springy — stretching, squashing or bending it — stores elastic potential energy (EPE).
- A material that springs back to its original shape when the force is removed is called elastic; only elastic objects give the stored energy back.
- The more you stretch or squash an elastic object, the more EPE it stores.
- Energy is never destroyed — it is passed between stores (gravitational ↔ kinetic ↔ elastic) as things fall, fly and bounce.
- Some energy is dissipated as heat and sound every time a material deforms, so a bouncing ball never returns to its starting height.