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Stretched and squashed

Energy and power · level 2, lesson 3 of 6

A question to start

Pull back an elastic band and let go, and it flies across the room. Wind up a toy and it scurries along the floor. Where is the energy hiding before you let go?

The idea

When you stretch, squash, bend or twist something springy, you give it energy, and it stores that energy for as long as it stays stretched. This is called elastic energy (or strain energy).

Let go, and the stored energy comes out, often as movement:

  • a bow stores energy when you pull the string back, and gives it to the arrow;
  • a trampoline stores your energy as it stretches, then throws you up;
  • a wind-up toy has a coiled spring inside that you tighten with the key;
  • a ball squashes a little when it hits the floor, then springs back up.
elastic energypulled back

The more you stretch or squash something, the more energy it stores. But stretch it too far and it may snap, or stay bent out of shape, and then it gives no energy back.

Elastic energy works just the same on Mars, because it has nothing to do with gravity. A spring wound up on Mars stores the same energy as on Earth.

Worked examples

1. A catapult. Pull the elastic back farther, and the stone flies faster, because more energy was stored.

2. A jack-in-the-box. Pushing the lid shut squashes the spring. Opening it lets the spring push the toy up.

3. Landing legs. Many space landers have legs with springs or squashy parts, which take in the energy of landing so the lander does not bounce or break.

Try it

  1. What do we call the energy stored in a stretched spring?
  2. Give two toys that store elastic energy.
  3. Does a stretched spring store less energy on Mars than on Earth?
  4. What happens if you stretch an elastic band too far?
Answers
  1. Elastic energy.
  2. For example: a bow, a catapult, a wind-up toy, a trampoline.
  3. No: elastic energy does not depend on gravity.
  4. It snaps, or stays stretched and stops springing back.

Common mistake

"A squashed ball just loses its energy." For a moment, the energy is stored in the squashed ball. A bouncy ball gives most of it back; a soft one turns more of it into a little heat.

Where it leads