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, Electric potential and potential energy, Lagrangian mechanics, Rotational kinetic 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.
Resources
Part of these UT courses
This concept is mapped as covering part of the following course(s) in the University of Tartu curriculum.
- The physical world viewLOFY.01.002
- Materials physics practical I — mechanics and heatLOFY.02.055
- Physics practical I — mechanics and heatLTFY.01.004
- Mechanics and heatLTFY.01.006