Jonny Nelson introduces an animated explanation of energy stores

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 ( \text{F}) moves an object through a distance ( \text{d}).

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).

Calculating work done

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

\text{W} = \text{F} \times \text{s}

This is when:

  • work done ( \text{W}) is measured in joules (J)
  • force ( \text{F}) is measured in newtons (N)
  • distance ( \text{s}) is in the same direction as the force and is measured in metres (m)


A man pushes a box with a force of 10 newtons to move it a distance of 2 metres

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

\text{W} = \text{F} \times \text{s}

\text{W} = 10 × 2

\text{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. Work could have the unit Nm (Newton metre) but the unit J, Joules, is used.

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?

\text{W} = \text{F} \times \text{s}

\text{W} = 50 × 30

\text{W} = 1,500 J


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?

\text{W} = \text{F} \times \text{s}

\text{F} = \frac{\text{W}}{\text{s}}

\text{F} = \frac{12,000}{80}

\text{F} = 150 N