Momentum can be thought of as the product of mass and velocity. Momentum helps explain some of the most important interactions in nature.

Work is done when energy is transferred from one store to another. Work is also done when a force causes an object to move. When work is done against frictional forces acting on an object, the object’s temperature increases. For example, a person’s hands warm up when they rub them together repeatedly.

To calculate the work done on an object when a force moves it, use the equation:

work done = force × distance

This is when:

- work done is measured in joules (J)
- force is measured in newtons (N)
- distance moved along the line of action of the force is measured in metres (m)

Note that one joule of work is done when a force of one newton causes a movement of one metre. This means that work done can also be measured in newton-metres (Nm).

1 J = 1 Nm

It is important not to confuse newton-metres (a unit of work done) with Newton meters (calibrated spring balances used to measure weights).

A doctor weighs 600 N. A lift moves her 40 m to the top floor of a hospital. Calculate the work done on the doctor by the lift.

work done = force × distance

work done = 600 N × 40 m

work done = 24,000 J (or 24 kJ)

- Question
In a scrum, a rugby team pushes the other team backwards 5 m using a force of 1,000 N. Calculate the work done moving the other team.

work done = force × distance

work done = 1,000 N × 5 m

work done = 5,000 J (or 5 kJ)