Why Do Balls Bounce?
It may seem like a rather stupid question at first since we all know the answer. Balls bounce because once you throw it and the ball comes in contact with something, like the floor, the ball compresses, and when there is no more kinetic energy pressing the ball further into the floor, it goes back and bounces up. Right?
Photo by Alexandre Lion on Unsplash
You’ve got one part right. Take a standard eight ball from one of those pool tables and get an ordinary-looking rubber ball, then bounce them from the same height, and the same strength. You would think the rubber ball would bounce higher because the rubber would compress more than the eight balls, but it comes up as roughly the same height, why is this?
I started to do some curious research the day when school was out and we were ready to play ball. Only one problem, we had bats and gloves, but no one had remembered to bring a baseball. All we could turn up was a tennis ball. We decided to play anyway.
I took the long jog out to the right-field as Eddie positioned himself next to home plate, which was a cardboard dinner plate. The first pitch came in high. Andrew, the pitcher, had to get used to the light ball. The next pitch came right down the middle. Eddie swung and the ball vanished into the sky. It was headed in my direction. I ran to make the catch but misjudged the ball’s trajectory — the tennis ball lost speed faster than a baseball. I positioned myself to play the ball on the bounce. Boy, did it bounce! High over my head and into the woods.
I had just learned that balls are specifically designed for a particular sport. Using a ball designed for tennis in a baseball game produced strange results.
To understand what happened to me back in my grade school days I suggest that you take a tennis ball and a baseball, hold them side by side at waist level and drop them onto a hard, bare floor. The baseball will bounce back only about a third as high as its starting height, well below knee level. But the tennis ball will bounce higher, rising to half its initial height.
Balls are suited to the games that we play with them. A change in the bounciness of a ball can drastically affect a game — as the history of baseball demonstrates. When baseball was invented, there were no polymer chemists around to brew up rubber with the appropriate bounciness. As the game and the ball evolved, the game’s character and dimensions changed. Prior to 1911, baseball was a lifeless version of its modern counterpart. During this “Deadball” era, long base hits were rare, and “hitting it where they ain’t” was the strategy of the day. In 1911, the use of a cork centered, springier, “lively ball” made the home run king, and paved the way for Babe Ruth, Hank Aaron, and generations of fence-busters.
Even a slight change in the bounciness of a baseball can have a large effect on the game. A few years ago there was an unprecedented rash of home runs in the major leagues. That year’s crop of baseballs bounced off the bats with unusual liveliness, turning what should have been long outs into home runs. Everyone suspected that these baseballs had been particularly tightly wound, producing so-called “rabbit” balls. (One disgruntled pitcher said that you could hold one up to your ear and hear its little heart beating a mile a minute.) If such a seemingly minor change could have such a dramatic effect, it’s not surprising that the bounciness of the tennis ball completely transformed my childhood baseball game.
So now the question, 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.
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 norbornene, which converts the organized energy of motion into thermal energy.
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, norbornene polymer does not deform as much to dissipate the energy of the 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 air-filled 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, the 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.
