# Work done

Work done is a measure of what is required to move an object over a distance. The work done is also equal to the energy transferred to the object in order to move it. Work done is calculated with the following formula:

As we already know force = mass x acceleration (F=ma). You can also say:

Work done = mass x acceleration x distance moved

NOTE

Work done is measured in Newton metres (Nm). However, 1Nm is equal to 1 Joule (J) so you may see some questions which state Joules rather than Newton metres. Always read the question carefully to see if the answer is required in Joules or Newton metres.

Example

If a car has a mass of 800kg and is travelling at 25m/s, what force is required to stop the car in 50 metres?

To answer this question we must first calculate the kinetic energy of the car that has to be transferred to heat, as this is the work done through the braking of the car.

Kinetic energy = mass x velocity2

Therefore the car's kinetic energy = 800 x 252 = 800 x 625 = 500000 Joules

Energy transferred = work done = force x distance

So 500000 = force x 50

\frac{500000}{50\ }=\frac{force\ \times\strike{50}}{\strike{50}}

Therefore the braking force required to stop in 50 metres is:

Braking force =\frac{500000}{50}=10000N