Unbalanced forces on an object will change its motion. Ideas about force and momentum can be used to explain road safety measures.

When a force acts on an object that is moving, or able to move, there is a change in momentum:

- in equations, change in momentum is shown as
- is the change in velocity (∆ is the Greek letter delta, representing 'change in')

The two equations can be combined to show how to calculate the force involved when a change in momentum happens:

force = mass × acceleration

Acceleration (*a*) appears in both equations, giving:

This is when:

- force (
*F*) is measured in newtons (N) - change in momentum (
*m∆v*) is measured in kilogram metres per second (kg m/s) - time taken (∆
*t*) is measured in seconds (s)

The equation shows that the force involved is equal to the rate of change of momentum.

A 1,500 kg car accelerates from rest to a velocity of 30 m/s. This takes 20 seconds. Calculate the force acting on the car.