How does rubber bounce?

-Evan H., Cimmaron, Kan.

Whether it comes from trees or is made by scientists in a lab, rubber can really bounce. Well, a rubber band or rubber on your shoes might not be very bouncy. But a super bouncy rubber ball? It can really catch some air.

The bounciness of a rubber ball has to do with very tiny pieces of the material, as well as its shape. My friend Jimmy Zheng, a graduate student engineer at Washington State University, offered to help out with your question.

In the lab, he is working on a gum-like material to help the parts inside a battery stick together better. If you rolled this material into a ball, it would bounce just a little.

He said what rubber and this material have in common is that they are both polymers. You can think of polymers being like a bowl of cooked spaghetti. Each noodle is a chain of different chemical parts.

Sometimes those spaghetti noodles are tangled up and tighter. Other times they are less tangled. The thing about polymers is that they can do different things, depending on their shape and how they are put together.

gray cat named dr. universe standing with hands on hips wearing white lab coat, red shirt, and yellow pantsWhen a rubber ball hits something, it absorbs energy and releases energy really fast, Zheng said. If you throw the ball, the energy might come from your hands. If you drop it, gravity gives the ball energy. And when the ball moving, it carries the energy in the form of kinetic energy. When it hits something, it stops, and the kinetic energy is stored inside the ball as internal energy. Then it can quickly convert the internal energy back into kinetic energy which allows the ball to bounce back.

You can see the ball bounce. But at the same time, there is a shift in the shape of those polymer chains. There is something happening in the parts that make up the rubber material. The polymer chains sort of relax and then take their shape once again.

If polymers are tighter together, the ball bounces higher. If the polymers are looser, there isn’t quite as much bounce. Knowing this, you might be able to predict what the polymers are like in, say, a basketball, a ping pong ball, or a racquetball.

But what about a steel ball? It has different polymers than a rubber ball. Believe it or not, a steel ball actually bounces higher than a rubber ball. When it hits the ground, the steel snaps back faster. The steel is better at storing energy, which helps give it a lot of bounce power.

You can investigate bounciness with an activity at home. Here are some instructions to make your very own bouncy ball. Cats sure can get a lot of entertainment from them—here’s video proof.

The recipe doesn’t call for rubber, but you will get to play with some polymers. How high will your bouncy ball go? What happens if you warm it up? What happens if you cool it down? Tell me about it sometime at Dr.Universe@wsu.edu.

Sincerely,
Dr. Universe