Energetic Two-Ball Bounces
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 a different mass 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!
This activity is not appropriate for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.
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 each other, 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 the 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. The earth, being so heavy, 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!
Extra: If you have more balls available, try other combinations like a ping pong ball on a basketball, or a ping pong ball on a tennis ball. For the brave people, stack three balls, like a ping pong ball on top of a tennis ball which is resting on a basketball, and release all at the same time. Be sure to have some 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 a single ball never bounce back to the height at which you released it, regardless of the ball you used? This is to be expected. During a collision, some of the ball’s energy is converted into heat. As no energy is added to the ball, the ball shoots off 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 of it. 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 the energy of a heavier ball. It reaches way higher than it was released. The heavy ball, on the other hand, is left behind with little energy and does not move much.
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Sabine De Brabandere, PhD, Science Buddies
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