I still remember my first encounters with balls made from these two different kinds of rubber. My father gave me a superball. It looked ordinary, black and hard, but when I dropped it I was startled as it bounced nearly back into my hand. I spent hours bouncing the superball into corners and off the kitchen floor into the bottom of a table.

Recently one of my students handed me a ball that looked like a superball, and stood by, quietly smiling. I tossed the ball onto the floor, and it stopped dead. I was stunned. I had just been handed an anti-superball, a "no-bounce ball." After hours of experimenting, I found that the no-bounce ball does return to its original shape, but too slowly to bounce off the floor. The no-bounce ball is made from a polymer called norbonene, which converts the organized energy of motion into thermal energy.

Balls are specifically designed for a particular sport. They are suited to the games that we play with them. A change in the bounciness of a ball can drastically affect a game.

Hot Bounce

Baseball legend tells of managers preparing for the arrival of a visiting team full of home-run hitters by placing baseballs in the freezer for several hours to alter their bounce. You can try this yourself. Take two identical baseballs, freeze one, and then compare the bounciness of the two balls. Experiment with other kinds of balls, comparing a cold ball to a warm one.

We found that frozen baseballs did not bounce as high as warm ones. We continued our research with golf balls and found that frozen golf balls are also less bouncy. (If you play golf in the winter, you'll hit the ball farther if you keep it warm in your pocket.) A cold superball bounces less than a warm one. Generally, cold balls are less bouncy than warm ones. That's because cold rubber is generally not as flexible as warm rubber. When a cold ball hits the floor, the deformation that follows the collision is concentrated at the bottom of the ball. This concentrated deformation causes the rubber molecules to collide with each other, producing warmth rather than rebound.

The one exception is the no-bounce ball. A cold no-bounce ball actually bounces better than a room-temperature one. The stiff, cold, norbonene polymer does not deform as much to dissipate the energy of the ball.


A Bored Tennis Ball

Basketballs, tennis balls, footballs, and many other balls take advantage of the springiness of air. If you compress a closed container of air--a balloon, for example--it will spring back into shape as soon as you release it. An air-filled ball is lighter than a solid rubber one, and that condition

has certain advantages. If you made a solid rubber tennis ball with the same bounciness as an airfilled one, the mass of the ball would exert damagingly large forces on the forearms of tennis players. The very thought of a solid rubber volleyball makes my fingers ache.

Air-filled tennis balls bounce well. However, you can take the bounce out of a tennis ball just by boring a few holes into the ball. Use a soldering iron to melt a few holes into a tennis ball, then compare the bounce of the bored ball to that of a normal tennis ball. When the bored ball bounces, air is compressed and forced out through the holes. Only a portion of the air remains inside the ventilated ball. When this air expands, only a fraction of the original energy is returned to the ball, so the ball does not bounce very well.

If I had known about this back when I was a kid, I could have made our baseball game more interesting by boring some holes in the tennis ball. Then maybe the ball wouldn't have been lost in the woods after the first hit.
"That's The Way the Ball Bounces" originally appeared in the Sports Issue of Exploring Magazine in 1991.  

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