Energy in a system can be changed, in particular by mechanical work. It cannot be created or destroyed but it can be transferred, dissipated or stored in different ways.

When a force causes a body to move, work is being done on the object by the force. Work is the measure of energy transfer when a force ‘*F*’ moves an object through a distance ‘*d*’. So when work is done, energy has been transferred from one energy store to another, and so:

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 (
*E*) is measured in joules (J) - force (
*F*) is measured in newtons (N) - distance (
*d*) 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, so:

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 in the direction of the force.

- 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?

- 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?