Why does a Ball Bounce?

Have you ever wondered why a ball will bounce at least slightly regardless of whether it is made of rubber or plastic or whatever? And why will a ball have some bounce whether it is hollow or solid? Of course, some balls bounce higher than others, but even bowling balls rise off the floor a little bit. Any idea what’s up with this weird fact?

The scientific fact at play here is elasticity and the law that energy taken must be returned. The effort of dropping a ball or throwing a ball creates pressure against the object which it is thrown against. If you take a rubber ball and press it gently with your finger you will see the effect of elasticity in action. Of course, if you try that with a bowling ball you may wind up with a bent finger. Nonetheless, even when a bowling ball is tossed or dropped, the same principle applies. It is not just the rubber of your average ball that contains elasticity, but also the air inside any ball. That’s why a hollow plastic wiffle ball bounces, but not as much as a solid plastic ball.

When the ball with holes in it is bounced, the energy gets expelled through those holes and the resulting elasticity is not as great as a solid plastic ball. Likewise, a solid plastic ball doesn’t have as great an amount of elasticity as the solid rubber ball.

So why doesn’t a ball, regardless of its elasticity, ever bounce as far as it was thrown or dropped? The ball on its bounce is still going on the original energy of the toss that you gave it and as it rises through the air—assuming you threw it downward—the bounce is forcing it through literally millions of particles of air that creates friction that slow the ball down until it must, inevitably, stop and then begin the movement back down due the forces of gravity. Another important consideration to how far a ball bounces has to do with the material against which it is bounced. For instance, if I were to bet you that I could make a bowling ball bounce higher than a tennis ball you might well take that bet and secretly call me a sucker.

But what if I were to take you to the site where the bet would take place and show you that I would be bouncing the bowling ball of a trampoline and the tennis ball off a pile of sand? The energy behind the tosses in these cases will be directed quite differently. All that elasticity of the tennis ball be wasted because the forces of energy behind the toss will have to displace the sand. The bowling ball will displace hardly any of its energy.

The type of ball used in any particular sport was designed in part because of the need for bounce. Tennis would be exceedingly boring—even more so—if a tennis ball only bounced as well as a baseball. Likewise, a baseball can’t have too much elasticity or else guys who’d never hit more than thirty or so homers in a season would suddenly be hitting sixty or so. And that would be just plain wrong.