Bouncy Balls

Have you ever bought a bouncy ball from one of those vending machines? Slam that ball into the ground and watch how high it goes! What if that ball was made of a different material? Is rubber really the bounciest material around? It sure seems to be. Let’s find out.

Problem:

Which material bounces the highest?

Materials:

• 1 rubber bouncy ball (about 1” in diameter)
• 1 round wooden ball (about 1” in diameter)
• 1 ping pong ball
• 1 round rock or stone (about 1” in diameter)
• Video camera
• Notebook
• Measuring tape
• Tape
• Pen

Procedure

1. Find an area outside where the ground is flat. This area should have a wall, fence, basketball hoop, or other vertical object that you can use to place your measuring tape perpendicular to the ground. Don't use your school's gymnasium—your P.E. teacher isn't going to like the idea of students smashing hard objects into the gym floor! An ideal spot would be a basketball or tennis court.
2. Hang your measuring tape against your wall, fence or basketball hoop so that it measures out 5' perpendicular to the ground. Some objects will probably bounce higher than this when we throw them, but don't worry! We just want to make sure the measuring tape is visible so that we have a sense of scale when we record our footage with the video camera.
3. Set up your video camera about 30' away from the spot you're filming. Check to make sure that when you bounce your objects, their height will be visible relative to the measuring tape.
4. Which object do you think will bounce the highest? Use this time to write down your guess, also called a hypothesis, in your notebook.
5. Start recording. Stand about five feet in front of the measuring tape and throw each object straight down into the ground as hard as you can.
6. Watch the video footage you took for each object and see how high each object bounced. Using the measuring tape as a visual reference, take a rough measurement of the distance each object travelled vertically into the air. How many lengths of measuring tape does it look like the object traveled? Record your results in your notebook.

Results:

The bouncy ball should bounce the highest. The ping pong ball will bounce the second highest. The wooden ball will bounce the third highest. The rock will bounce the least high.

Why?

The reason a rubber ball bounces so high is because of its high elasticity. Elasticity is what allows an object to both absorb energy when it strikes the ground and use that energy to regain its shape rapidly, which propels the object off of the surface it strikes. Rocks are very dense and cannot change shape easily, lowering their ability to push off from a surface and bounce. Wood can slightly contort its shape, allowing more bounce than a rock. A ping pong ball is very uncompressed, with more space between its material. This gives it higher elasticity and a better bounce than a rock or wooden ball.