Tennis

When hitting a tennis ball, have you ever wondered about momentum? The whole point of swinging as hard as you can is because of the law of conservation of momentum. In the inelastic collision of a tennis ball and the racket, momentum from the racket is transferred to the ball. The harder you swing, the more the ball can change in momentum. And because momentum is mass multiplied by velocity (and the mass only changes when you hit it so hard that you flatten the ball), velocity not only changes direction, but usually increases in magnitude. Not only that, but projectile motion, impulse after a bounce and much more all apply to tennis. Awesome!