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Friday, June 11, 2010

That's the Way the Ball Bounces

Why do some balls bounce more than others? Why do they bounce at all? The following experiments, sure to please the child and adult sports fans alike, explore the science of bounce.

Experiment 1: What happens when you freeze a rubber ball?

You will need:

solid rubber ball (super balls work well
a freezer
a yardstick  (optional)

Drop the ball from a given height, say, waist level, and note how high it bounces (use a yardstick if you wish, or just make an eyeball measurement). Now, put the ball in the freezer a few hours. When it’s good and cold take it out and drop it again. How high does the ball bounce?

What's Happening?
Rubber is made of tiny strings called elastomers. At room temperature the elastomers are stretchy, like rubber bands. Squeeze a warm rubber ball and the elastomers compress, flattening the ball. Let go, the elastomers relax and the ball returns to its round shape. The same thing happens when a dropped rubber ball hits the floor: the elastomers compress on impact, then snap back to their original shape. In so doing, they launch the ball upwards, giving it bounce. But at cold temperatures the elastomers become rigid and inflexible. The ball can’t compress, has no way to push off the floor and loses its bounce.

Experiment 2: Do All Balls Have the Same Bounce?

You will need:
different types of balls (baseballs, tennis balls, soccer balls, rubber balls, ping pong balls…)
A yardstick
Paper and pencil to record measurements

This  works best with two people: one to drop the ball, and one to measure its bounce. Prop up the yardstick against a wall so that it is flush with the ground, with the 0-inch mark facing down. To do the experiment, the first person should drop one of the balls from one yard up, and the other person should squat in front of the yardstick so she can measure how high the ball rises on the first bounce (measure from the ball’s bottom; it may take 2 or 3 tries to get an accurate measurement). Record the results, and repeat the experiment with other ball types. Do all balls have the same bounce?

What's Happening?

When a ball hits the floor the impact flattens it a bit. It then snaps back to its original shape, which pushes it off the floor and causes it to bounce. How much a ball bounces depends on how much it squishes on impact, and how well it snaps back to its original shape. Bowling balls don’t bounce very well because they aren’t very squishy. Deflated balls are really squishy but they don’t bounce well because they don’t snap back to a round shape. 

More bounce isn’t always better, though. If you try playing tennis with a baseball, or baseball with a tennis ball, you will realize that a certain amount of bounce is just right for a certain kind of ball.

Did you know?
On cold days golfers put their balls in a pocket to keep them warm. Why do you think they do this? 


  1. Andrea,
    This looks like a great blog! Congratulations on the excellent science and activities. You may want to check out Growing with Science and Wild About Nature, too.

  2. Hi, Andrea. I saw your introduction on the Kidlit community. I love your focus on science and arts and crafts. I'm sure my two boys, ages 4 and (almost) 7, will enjoy some of these.

  3. What a great idea, and I'm sure it was popular!