Energy is a key principle in physics, as it allows work to be done. The rate at which energy is transferred is called power and the amount of energy that is usefully transferred is called efficiency.

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

\[W=F\times d\]

This is when:

- work done (
*W*) 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:

\[W=F \times d\]

\[W=10 \times 2\]

\[W=20~J\]

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.

- 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?

\[W=F \times d\]

\[W=50 \times 30\]

\[W=1,500~J\]

- 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?

\[W=F \times d\]

\[F= \frac{W}{d}\]

\[F = \frac{12,000}{80}\]

\[F = 150~N\]