Energy stored in a stretch
Pull back a rubber band and hold it still. Does it have energy?
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- A spring and a block, at rest.
- Pull the block out. Your force does work on the spring.
- Hold it still. The work is stored in the spring.
- Let go. The spring pushes the block away.
Rule
Yes. A pulled-back band stores energy, even though it does not move.
Rule: A stretched or squeezed springy object stores potential energy. Held still, it has no kinetic energy.
- Potential energy — energy stored by a springy shape change, or by position.
Here, position means where a thing is. Height above the ground is one example.
Kinetic energy needs motion, so a band held still has none.
A force does work to stretch a band. That work is stored in the band. Let go, and the band pushes a small stone. It does work on the stone. The stone gains kinetic energy.
A bigger stretch needs more work. So it stores more energy.
This holds while the object can spring back. Wet clay stays squashed. It does not spring back, so the rule does not fit it.
Two magnets pull on each other, north pole to south pole. Pulled apart, they store energy too. Release them, and they rush together.
| Case | Moving? | Potential energy stored? | Energy it has |
|---|---|---|---|
| A drawn bow, held still | no | yes | potential |
| A cycle on a flat road | yes | no | kinetic |
| A ball falling from a roof | yes | yes | both |
| A brick on the ground | no | no | neither |
Work done against friction is not stored as potential energy. It becomes heat. A box pushed along a rough floor stores no potential energy.