What Makes a Ball Bouncy?
Understanding what makes a ball bouncy involves a few simple scientific concepts that have major applications in the world of competitive sports. In essence, the bounciness of balls is caused by the action of their molecules, and changes to that action can dramatically affect the way a game is played.
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Experiment
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The best way to understand how a ball bounces is to watch a ball bounce in slow motion, explains Paul Doherty in the Exploratorium.edu article "That's the Way the Ball Bounces." Take a balloon and lubricate the outside with the cooking oil. Push the lubricated balloon into a second balloon, then fill the inner balloon with water (not too full). Drop the double water balloon onto a smooth surface. The bottom of the balloon will come to rest when the balloon hits the surface, but the top will keep moving down. This is because the rubber stretches as the balloon hits the floor, which deforms the initial shape of the balloon. When the elastic rubber goes back to its original shape, it pushes the balloon into the air.
Significance
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Special photography reveals that a baseball, for instance, also becomes deformed when it's hit by a bat, Doherty writes. A ball made from an elastic material, like rubber, springs back to its initial shape. In the impact, the ball pushes on the bat and the bat pushes on the ball, in accordance with Newton's law: for every action there is an equal and opposite reaction. Likewise, a ball bounces off the floor because the floor pushes it up, as strange as that sounds.
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Application
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Some balls are bouncier than others, which is of major significance in the world of sports. Rubber balls used in the beginning of baseball had not been improved by polymer chemists, so the home run only became central to the game after a new, springier ball was invented in 1911. After that it was a whole new ballgame, permitting the advent of legends like Babe Ruth, Doherty explains.
Action
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When a ball falls, gravity pulls it towards the floor and the ball gains kinetic energy, the energy of motion. The energy the ball gains goes into deforming the ball, which causes the molecules in the ball to rub across and collide with each other. If a ball made of an inelastic material hits the floor, the organized motion of the falling ball becomes the random jiggling of molecules, and the ball warms up because of thermal energy, but does not bounce. With a rubber ball, in contrast, the tangled long-chain molecules (polymers) stretch for a moment during collision, then return to their tangled shape, writes Doherty.
Other Factors
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Many balls, such as basketballs, tennis balls and footballs, also use air to help them bounce, Doherty explains. Air is springy, returning to its original shape after it has been compressed--for instance, in a balloon. Air-filled balls are lighter than solid rubber ones, which would damage the forearms of tennis players or the fingers of volleyball players.
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References
- Photo Credit baseball lying on the us flag, great background image by sumos from Fotolia.com
