Bouncing balls

Summer is high sports season, and most of the games being played involve balls: baseballs, tennis balls, volleyballs, soccer balls.

At first glance, every ball appears much the same as every other ball: round and bouncy. The only thing that’s different is the size.

But in fact, each ball is designed specifically for the sport in which is it used, and trying to use, say, a tennis ball in a baseball game will produce strange results. The ball will take off from the bat like crazy, but will lose speed in a hurry and, if it isn’t fielded correctly, will bounce right over the fielders’ head.

To understand why different balls work in different ways, you first have to understand the one thing they all have in common: the bounce.

When a ball bounces against the floor, a bat, a foot or your head, the side that’s in contact with the object stops moving and flattens out, while the far side of the ball continues to move forward. This deforms the ball. The material of which the ball is made doesn’t like being deformed, though, and snaps back into shape. When it does so, it pushes against the floor, bat or foot, and since, as Newton pointed out, for every action there is an equal and opposite reaction, the floor, bat or foot pushes back, hurling the ball away.

This is easy to believe when you’re dealing with a very soft ball, like a tennis ball; it may be harder for you to believe when the ball in question is as hard as a baseball. But in order for the bat to reverse the direction of a ball weighing 5 1/8 ounce and travelling 90 miles per hour in the few thousandths of a second it’s in contact with it, the bat has to push on the ball with eight thousand pounds of force. With four tons of force on it, it’s not surprising at all that the ball deforms.

Although all balls bounce for the same reason, they don’t all bounce the same amount. That’s because they’re all made differently.

Bouncing a ball takes energy. Kinetic energy is first imparted to the ball when it’s set in motion by throwing, kicking or dropping. When the ball hits an object and stops moving, the kinetic energy goes into deforming the ball. During this deformation, the ball’s molecules are stretched apart in some place and squeezed together in others.

In rubber, the molecules are very long and stringy and all tangled together. When the ball is deformed, these molecules stretch, but only for a moment. The atomic connections within the molecules then snap them back into their original tangled shape. As a result, the ball bounces back almost as fast as it was moving when it hit. (Some energy is turned into heat as the molecules rub against each other; that’s why a racquetball is warmer when you finish playing a game than it was when you started. The loss of energy to heat is also why even the bounciest balls don’t bounce quite as high as the height they were dropped from.)

Rubber comes in many different forms, however. The long molecules can be made so they don’t rub against each very much, which means little energy is lost as heat, which means the ball bounces very, very well. (Remember the superball you may have played with as a kid?) Rubber can also be made so that the molecules rub against each other a lot, so they produce much heat, but not much bounce. There’s even a “no-bounce” ball which looks like a superball but doesn’t bounce at all; the molecules in it don’t regain their original shape quickly enough.

Some balls, such as volleyballs and basketballs, get their springiness not from rubber, but from the air inside them. Air can be compressed, but as soon as the compression is removed, it expands again. If you put it in a flexible container, the container will spring back into its original shape after it has been compressed, which produces bounce. Most large balls are air-filled for practical reasons: just imagine trying to set a volleyball made of solid rubber!

Every ball is a little bit the same as every other ball, but every ball is also unique. One thing you can say for certain, though, is that wherever there’s a ball, somebody is sure to start playing a game.

Permanent link to this article: https://edwardwillett.com/1999/08/bouncing-balls/

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