Physics is something that is directly applicable to car safety. Cars go fast, but they also sometimes collide with obstacles, which brings them to a sudden stop, subjecting the car and occupants to very high accelerations, which can cause injury or death. So the major idea to improve car safety is to reduce these accelerations and there are a number of ways to do this:
Passive safety: Seat belts: keep passengers in the vehicle, preventing them from continuing with constant velocity, flying through the windshield and suffering a worse deceleration when they make contact with the road.
Crumple zones: increase the distance over which deceleration occurs, thereby reducing peak magnitude of deceleration. Air bags: increase the distance over which the head decelerates, again reducing peak magnitude of deceleration of the head.