Energy is a key principle in physics. Forces cause changes and energy calculations allow us to measure how much change can happen.

When a force causes a body to move, work is being done on the object by the force. Work is the measure of how much energy is transferred when a force ( ) moves an object through a distance ( ).

So when work is done, energy has been transferred from one energy store to another. Therefore:

energy transferred = work done

Energy transferred and work done are both measured in joules (J).

The amount of work done when a force acts on a body depends on two things:

- the size of the force acting on the object
- the distance through which the force causes the body to move in the direction of the force

The equation used to calculate the work done is:

work done = force × distance

This is when:

- work done ( ) is measured in joules (J)
- force ( ) is measured in newtons (N)
- distance ( ) is in the same direction as the force and is measured in metres (m)

In this example, a force of 10 N causes the box to move a horizontal distance of 2 m.

= 10 × 2

= 20 J

One joule of work is done (or one joule of energy is transferred) when a force of one newton causes a body to move through a distance of one metre. Work could have the unit Nm (Newton metre) but the unit J, Joules, is used.

- Question
A horizontal force of 50 N causes a trolley to move a horizontal distance of 30 m. How much work is done on the trolley by the force?

= 50 × 30

= 1,500 J

- Question
12,000 J of energy is supplied to move a small truck a distance of 80 m. What is the size of the force applied?

= 150 N