Why Do Balls Bounce? Real balls bounce fast, too fast for your eye to see. Harold "Doc" Edgerton used the strobe lights he invented to take the first clear photos of balls in the process of bouncing. These photos show that when a bat hits a ball, for instance, the ball becomes greatly deformed--just like the water balloon. If the ball is made from an elastic material, such as rubber, it springs back to its initial shape. As the ball pushes on the bat, the bat pushes back on the ball. (As Newton pointed out: for every action there is an equal and opposite reaction.) The ball bounces off the bat and into the air. Strange as it may seem, a ball bounces off the floor because the floor pushes it up!

The Edgerton photos reveal that when a bat strikes a ball, the ball remains in contact with the bat for only a few thousandths of a second. To reverse the ninety-mile-per-
hour speed of a 5 1/8 ounce baseball in one millisecond, the bat must push on the ball with eight thousand pounds of force. Imagine a baseball squashed under four tons of iron and you will begin to understand why the baseballs in Edgerton's photos are deformed.

From Edgerton's photos and your observations of water balloons, you can see that balls bounce when they spring back into their original shape. But why do some balls bounce better than others? The widely varying results of your experiments suggest that the reasons depend on a ball's materials and construction.

 Footballs, basketballs, volleyballs, and tennis balls take advantage of the springiness of trapped air.