Car safety, momentum & force – GCSE Physics – The force on a person in a car crash can be very large and the force can be worked out from the deceleration in a crash. You can also work out the force in a crash using the equation for momentum. Car safety is designed using the concept of momentum and force related to a car crash. The objective is to keep the force on a person as small as possible. Let’s take an example of a car travelling a steep road.