Key concepts
Physics
Energy
Collisions

Introduction
How many ball sports can you name? How many of those have several balls at once in the game? Almost none, right? Games that do use several balls at a time most likely use balls of the same mass, volume and material. Would having two balls of different masses make a game very difficult? In this activity you will explore what might happen if you were to add a tennis ball to a basketball game or a tiny ping-pong ball to a tennis game or any other combination. Ready to be surprised? Try it out!

Background
Balls used in sports come in all sizes, masses and materials. A lot of them bounce. Whether it is a basketball bouncing on the ground, a tennis ball bouncing off a racket or pool balls bouncing off one another, the bounce can be seen as a collision. Let's see if physics can explain what happens.

When a bouncing ball falls, it initially gains speed or kinetic energy—the energy of motion. When it reaches Earth, it collides head-on with an incredibly massive object that is, from your perspective, at rest. The ball slows down, deforms temporarily and shoots back up. The air in the ball acts like a spring—it gets compressed and expands again. During the collision, some of the ball's energy is converted into heat. As a consequence, the ball shoots up with less energy than it had when it reached Earth. Our planet, being so massive, does not move as a result of the collision.

It is interesting to compare a heavy and a light ball as they fall from the same height. Both balls will fall at a similar speed, but because kinetic energy is proportional to the mass of the object, the heavy ball reaches Earth with more energy. It will not necessarily rebound higher, as it also needs more kinetic energy to reach a specific height again. What if we could give the kinetic energy of the heavy ball to the lighter ball? When two balls collide, they exchange energy. Can we let one ball fly off with the energy of the other—and if so, how? Do this activity to find out!

Materials

• A basketball or other large bouncing ball
• A tennis ball or other bouncing ball that is smaller and lighter than the bigger ball
• A ping-pong ball, smaller and lighter than both other balls (optional)
• A hard surface to bounce balls, preferably outdoors, surrounded by a large open area where a ball can fly without colliding with people or objects
• A helper (optional)
• Camera that films video (optional)
• Yard stick that measures meters (optional)

Preparation

• Gather your test balls in the open area where you will perform the activity.
• Note: If your ball does not bounce well, it might be deflated or old. You can add air to some balls like basketballs, soccer balls, volleyballs, etcetera. A bicycle pump with a special piece to inflate balls can help you do the job. Other balls, such as tennis balls or baseballs, wear out with use. Why do you think a deflated ball does not bounce well? Why would a well-used tennis ball not bounce as well?

Procedure

• First, hold the larger ball in front of you at shoulder height and let it fall without giving it a push. How high does it bounce back—almost up to your shoulder, your hip, your knee?
• Repeat this a couple more times. Does the ball bounce back to approximately the same height each time?
• Repeat the test with the smaller ball. Do you think it will bounce higher or lower then the larger ball? How high does this ball bounce back?
• Repeat a couple more times. Are your results consistent?
• Do you think the ball can bounce back higher than the height at which you released it?
• Hold the larger ball in front of you at shoulder height like you did initially. Place the smaller ball on top of the larger ball so they just touch, and hold it there. You might need a helper to hold the balls in place. In a moment, you will release the two balls at the same time. What do you think will happen? Will the larger fall faster or will both stay together as they fall? How will they bounce back? Will a ball bounce higher, the same or lower than before?
• Try it out and observe. You might need to try a couple of times to get the timing right. Was your prediction correct? Can you explain what you observe?
• Repeat previous step a couple more times. Are the results consistent?
• Compare the balls’ masses. Was the top ball (the smaller one) heavier or lighter than the bottom ball?
• Hold the smaller ball in front of you at shoulder height like you did initially and place the larger ball on top. What do you think will happen when you release both balls at the same time?
• Perform the test a few times. Was your prediction correct? Can you explain your observations? Note that although before the larger bottom ball was probably heavier than the smaller top one, this time the top ball is heavier.
• Extra: If you have more balls available, try other combinations such as a ping-pong ball on a basketball or a tennis ball. If you can, try stacking three balls, such as a ping-pong ball on top of a tennis ball that is resting on a basketball, and release all at the same time. Be sure you have lots of free space for the balls to fly!
• Extra: If you would like a detailed view of what happens, you can use a camera to film the experiment. Later, you can study the moving balls on a screen in slow motion. Note that you can calculate the speed (in meters per second) at which a ball travels in the video. To do so, multiply the number of frames per second by the distance (in meters) the ball traveled between two frames. Place a meter stick in your frame or use your height as a calibration of distance in your video.

Observations and results
Did you find that a single ball never bounced back to the height at which you released it, regardless of the ball you used? During a collision, some of the ball’s energy is converted into heat. As no energy is added to the ball, the ball bounces back with less kinetic energy and cannot reach quite the same height. Had you given the ball an initial push, you would have added energy, and the ball might have bounced back higher.

A well-inflated ball bounces better because it has more air inside. This allows it to push back faster, reducing the contact time and contact area in a collision and thus reducing the heat produced. Tennis balls also have air inside but they cannot be reinflated.

Did you also see how a lighter ball shoots high into the air when released at the same time on top of a heavier ball? Both balls fall at the same speed but the heavier ball gains more energy during the fall. When the lighter ball bounces on the heavy ball they exchange energy, and the lighter ball flies off with some of the energy of a heavier ball. It reaches way higher than from the height it was released. The heavy ball, on the other hand, is left behind with little energy and does not move much.

More to explore
Surface Science: Where Does a Basketball Bounce Best?, from Scientific American
Snappy Science: Stretched Rubber Bands Are Loaded with Potential Energy!, from Scientific American
Bouncing Balls, by Scientific Explorations with Paul Doherty
Momentum and Collisions, from Physics for Kids

This activity brought to you in partnership with Science Buddies