If your car hits a barrier or another car, it will slow or even stop. Modern cars are designed so that the front end collapses, extending the time that the forces of the barrier or other car act, thus reducing the force needed to stop the car.

Cars also have airbags that don’t act on the car, but on the passenger. When a sudden very large acceleration of the car is detected the airbags are deployed. A chemical reaction within the bag rapidly fills the bag with gas. The front surface of the airbag speeds toward the passenger at speeds up to 180 mph! But the momentum of the passenger is decreased slowly because he or she compresses the airbag. In addition, because the airbag has a large area, the force isn’t concentrated, but spread out. This reduces the pressure on the body, reducing the chance of injury.

Most recent cars also have side airbags to protect passengers from side collisions. But, airbags have caused injuries to smaller persons. Safer airbags inflate less, reducing the force on the passengers, who, because they have less mass, require less impulse to be stopped.