Car crashes dont kill people - FORCES KILL PEOPLE. People die in car crashes because impact is too often instant, and the force of impact too large. If the time of impact can be prolonged, then the acceleration and the force of impact can be reduced. This is because of Newton's 2 nd Law of Motion,(1).

We can use the equationin (1) to getIf we increase the time of impact, we can reduce the average force of impact and this will result in injuries that are less severe. There are three main ways to do this:

Crumple Zones: These are semi rigid structures a the front and back of the vehicle, which are designed to absorb energy and collapse slowly on impact.

Seat Belts are the secondary line of defence. They are designed to stop passengers becoming projectiles inside the car by restraining them in their seats. If rear passengers were not restrained they might kill passengers in the front seat by impacting them from behind, as well as seriously injuring themselves. If front seat passengers were not restrained they could impact the dashboard/steering wheel/windscreen, which are all rigid and will cause serious injury.

The final line of defence is air bags. They are designed to inflate if subject to a force above a certain level. They then absorb as much of the kinetic energy of impact as possible when they are hit by a person. Gas is very compressible and very well suited to absorbing the energy of impact.