Work and energy
Work is done when a force moves an object through a distance, and it transfers energy from one form or object to another.
Practise this conceptWork has a precise meaning in physics that's stricter than its everyday one: it's done only when a force moves an object through a distance in the direction of that force. Push as hard as you like against a wall that doesn't budge, and you've done zero work on it, no matter how tired you get.
That last point trips a lot of people up, because effort and work aren't the same thing. Holding a heavy box still, arms shaking, does no work on the box either — there's a force, but no displacement.
When work is done, it transfers energy: pushing a box across a floor transfers energy from you (or your muscles' chemical stores) into the box's kinetic energy and, thanks to friction, into heat. Work is the bridge between force and energy — it's how one gets converted into changes in the other.
Key ideas
Requires: Newton's second law
Unlocks: Conservation of energy, Specific heat capacity
Formulas
| Symbol | Name | Unit |
|---|---|---|
| kinetic energy | J (joule) | |
| mass | kg (kilogram) | |
| velocity | m/s (metre per second) |
| Symbol | Name | Unit |
|---|---|---|
| work | J (joule) | |
| force | N (newton) | |
| distance | m (metre) |
Common misconceptions
- Work is done whenever a force is applied, even if the object does not move.
- Doubling an object's speed doubles its kinetic energy.
- Energy is used up and disappears when work is done against friction.