Momentum and Car safety

During a crash, the momentum of the driver and passenger falls from a large value to zero very quickly. This places large forces on them, which can be lethal. So, car manufacturers use crash tests to design safety features for cars.

By reducing the rate of change of momentum, we can make cars safer. Typical safety features of cars include:

  • Airbags
  • Seat belts
  • Crumple zones


Airbags inflate quickly during a car crash, from hitting hard objects inside the car, such as the steering wheel. They also increase the time it takes for the momentum of the person’s head to reach zero. This reduces the rate of change in the person’s momentum.

Seat belts

There are two ways that seat belts protect people during a car crash.

1. They prevent people from being thrown out of cars during a crash.

2. Seat belts can stretch slightly during a crash, which increases the time taken for the person to stop moving. This results in a reduced rate of change in the person’s momentum.

Crumple zones

Crumple zones are areas of a vehicle which are designed to give way (by denting, bending and folding) during a collision. By doing so, they increase the time taken for the vehicle to come to a stop, therefore reducing the rate of deceleration for the occupants inside.

This extended time decreases the force of impact on the passengers, making the crash less severe for them.