work done – a measure of what is required to move an object over a distance (measured in joules (J))
Note: Work done is equal to the energy transferred to the object in order to move it and is calculated with the following formula (see physics work done formula):
Work done = force x distance moved
As force is equal to mass x acceleration you can also say (see physics mechanics formula):
Work done = mass x acceleration x distance moved
work done = MAD
To remember the meaning of work done, use the following mnemonic:
Trying to get all this work done made me MAD (mass x acceleration x distance moved).
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?
Answer
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}}
$\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
