Fig. 4-1 Hitting a golf ball with the head of a club is an example of collision.

Collisions are very common in daily life. Hitting a golf ball with the head of a club, a table tennis ball rebounding from the ground, two cars colliding in a traffic accident are all examples of collision. Collision is a kind of fun in sports and games, but a huge collision in an accident may result in serious injury or even fatality. What factors determine the force in a collision? How to minimize the impact force on car passengers so as to protect their lives in a traffic accident? To answer these questions, we have to study the physics of impulse and impact force.