Why do balls bounce?

In honor of a new sports romance written by my friend Jim Cagney, I thought I’d talk some about why things bounce. To understand this, let’s look at one of the first popular bouncy substance, a natural material, rubber.It’s a long chain of molecules, a polymer. There are many natural polymers, collagen for example, is a natural protein found in skin.Polymeric things tend to bounce when dropped because the stringy chains are arranged in a somewhat messy structure that’s actually very efficient. When something is dropped it picks up energy and when it hits a surface it absorbs some of the energy into its structure. This energy can be used to create heat, sound, or to become a counter force. As Isaac Newton said, for every action, there’s an equal and opposite reaction. Rigid structures will more likely absorb the impact into their structures and become hot or even break bonds. Not polymers, they are apt to react with bounce because they are elastic. The energy of the drop will compress the molecules and then, the molecules will regain their shape and bounce back. Think about what happens when a ball is dropped on a surface that isn’t hard. Lots of the energy goes into squishing that surface down and the ball won’t bounce as high because it has given away energy.

Basketballs are made a little differently from a golf ball or a bouncy ball. They are a hollow bladder filled with air with a polymer coating. This polymer is often leather which is collagen, the skin protein. The air molecules inside the ball do a great job of moving in response to hitting a surface. But they don’t like to be near each other–air likes to fill space, much like paper on my desk. The ball will distort when it hits the floor but quickly get a push back from the air and bounce. The more force you put into the dribble, the higher the bounce.

Congratulations to Jim on his book, On the Rebound. Available here.





2 thoughts on “Why do balls bounce?

  1. That’s really very cool, Cathy. I have to admit, I never really thought about why balls bounced. I figured it was all because of air. Thanks for the great science lesson. There are a lot of Myth Busters fans in my house, so we love things like this!


    1. Yes as you have pointed out, for some it is about air (and the right type of “skin”). I should have said that if there is not enough air, the molecules will have less reason to push back and the bounce will be much less. And again, congratulations!

      Liked by 1 person

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