Work done
- When a force causes an object to move through a distance, work is done on the object. Work done is energy transferred.
- W = F s: work done (J) = force (N) × distance moved along the line of action of the force (m).
- One joule of work is done when a force of one newton causes a displacement of one metre: 1 J = 1 N m.
Work and energy transfers
- When you push a box across a floor, energy is transferred from your chemical store to the kinetic store of the box.
- Work done against frictional forces causes a rise in the temperature of the object and its surroundings.
- Lifting an object does work against gravity, increasing its gravitational potential energy (W = m g h).
Using the equation
- Make sure the distance is in the direction of the force.
- Rearranging: F = W ÷ s, and s = W ÷ F.
- Convert kilojoules to joules (× 1000) and centimetres to metres (÷ 100) first.
Key terms
- Work done
- The energy transferred when a force moves an object: W = Fs.
- Joule
- The unit of work and energy; 1 J = 1 N m.
- Newton metre
- A unit equal to the joule when used for work done.
- Friction
- A force that opposes motion when surfaces rub together.
- Displacement
- Distance moved in a particular direction.