Work done and energy transfer
Work done
When a force makes an object move, the force . Doing work transfers energy. You must recall this equation:
work done = force × distance, or W = Fs
| Quantity | Symbol | Unit |
|---|---|---|
| Work done | W | joules (J) |
| Force | F | newtons (N) |
| Distance | s | metres (m), moved along the line of the force |
of work is done when a force of moves an object one metre. So 1 J = 1 N m.
Example 1: you push a box with a force of 50 N for 4 m.
- W = Fs = 50 × 4
- W = 200 J
Example 2: 1500 J of work lifts a 300 N load. How high does it go?
- Rearrange: s = W ÷ F
- s = 1500 ÷ 300 = 5 m
Key terms
Work against friction
When work is done , energy is transferred to the stores of the surfaces, so their .
- Rub your hands together and they get warm.
- A car's brakes get hot when it stops: friction does work on the brake discs.
- Sliding down a rope too fast can burn your hands.
Exam tip
The distance must be in the . Carrying a bag across a flat floor does no work against gravity, because the bag doesn't move up or down.