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.

When you drop a ball, gravity pulls it toward the floor. The ball gains energy of motion, known as kinetic energy . When the ball hits the floor and stops, that energy has to go somewhere. The energy goes into deforming the ball--from its original round shape to a squashed shape. When the ball deforms, its molecules are stretched apart in some places and squeezed together in others. As they are pushed about, the molecules in the ball collide with and rub across each other.

Exactly what happens to these molecules as they stretch and squeeze depends on what the ball is made of. Suppose you drop a ball of putty. Rather than bouncing, it hits the floor and flattens. All of the organized motion of the falling ball becomes the random motion of jiggling molecules. The random motion of jiggling molecules is a measure of thermal energy. The putty gets warmer, but it doesn't bounce. Putty is inelastic --it doesn't return to its original shape.

Now suppose you drop a rubber ball. Rubber is made from long-chain polymer molecules. When you hold the ball in your hand, these long molecules are tangled together like a ball of molecular spaghetti. During a collision, these molecules stretch--but only for a moment. Atomic motions within the rubber molecules then return them toward their original, tangled shape. Much of the energy of the ball's downward motion becomes upward motion as the ball returns to its original shape and bounces into the air. The energy in the ball that isn't converted into motion becomes warmth. (You can verify this the next time you play a game of racquetball. At the end of the game, the ball will be warmer than when you started.)

Rubber balls are elastic because they return to their original shape. But rubber polymers can be formulated in different ways: if the polymers are tightly linked, they do not rub against each other much. The organized motion of the falling ball becomes an organized deformation of the rubber of the ball, which then becomes an organized motion of the bounced ball. Very little of the organized motion is lost by warming the ball; most of it goes into bouncing the ball back into the air. Balls made from this type of rubber are called "superballs." On the other hand, rubber polymers can be made in which the molecules move more freely, rub together more, and turn organized motion into disorganized vibration. The ball will hardly bounce. Instead, it gets warm.