# Surface Science: Where Does a Basketball Bounce Best?

A physics problem from Science Buddies

Key concepts
Energy
Gravity
Physics
Sports

Introduction
Playing basketball can be hard work. Players not only have to run around the court, but just dribbling the basketball takes some serious effort, too. Have you ever wondered why that is? The challenge has to do with how the basketball bounces. When the ball hits the court, its bounce actually loses momentum by transferring some of its energy—into a different form. This means that to keep the ball bouncing to the same height, players must continually put energy into the ball with each bounce. In this activity you will explore how high a basketball bounces on different surfaces compared with the height from which it was dropped. Which surface lets the basketball bounce the highest? Grab a basketball and try this activity to find out!

Background
Playing a game of basketball can be great exercise, and one part of that workout comes from just dribbling the ball. Why is this? When a basketball hits the ground (and as it flies through the air), it actually transforms some of its energy to another form. If players do not put enough energy back into the ball, they will not be able to dribble it effectively.

When a basketball bounces, it has two different types of energy: kinetic and potential. Kinetic is the energy an object has due to its motion. Potential energy is that which is stored in an object—its potential for motion—such as due to its height above the ground. For example, when you hold a basketball at waist level, it has some potential energy. If you drop the basketball, the force of gravity pulls it down, and as the ball falls its potential energy is converted to kinetic energy. When a basketball hits a court floor, a part of the kinetic energy gets converted into sound or heat, some of it briefly changes the ball’s shape (flattening it slightly) and a portion is absorbed by the floor surface.

Materials

• At least two different surfaces to bounce a basketball on, with at least one hard surface and one soft surface (For example, you could use carpet, concrete, grass, linoleum and a basketball court. The surface needs to be flat and next to a wall or other large perpendicular surface.)
• Tape measure or yardstick
• Painter's tape or masking tape
• A helper
• Video camera and access to a computer or a large screen on which to watch your recorded video (optional)

Preparation
• Prepare the walls or other vertical surfaces next to the floor types you want to test so that you can estimate the height of the basketball's bounce. To do this use a tape measure or yardstick along with the painter's or masking tape to mark every eight inches, starting from where the wall meets the floor and going up to 40 inches high on the wall. You should end up with five tape marks on each wall.
• If you are using a video camera, ask your volunteer to set it up so that all of the marked wall measurements as well as the floor are in view. When you're ready to test the basketball on a surface, ask your volunteer to start the video camera.
• If you are not using a video camera, ask your volunteer to get ready to watch when you drop the basketball to see roughly how high it bounces after it first hits the ground.

Procedure
• Hold the basketball so that the bottom of it is lined up with the top edge of the highest tape mark you made.
• Drop the ball. (Do not push it down.)
• Let the basketball bounce back up and then hit the ground a second time before you catch it in your hands (then stop recording if you were doing so). How high does your volunteer say the basketball bounced after hitting the ground the first time?
• Repeat this, dropping the basketball on the same surface a few more times to give you a good idea of just how high it bounces when being dropped from a certain height.
• Repeat this entire process with the other surface(s) you want to test. How high does the basketball bounce off another surface compared with the first one you tested? Why do you think this is?
• Tip: If you are testing a surface that is at a very different temperature (such as concrete outside on a cold day), you will want to do your testing quickly so that the ball does not change temperature too much. A change in the ball's temperature can also affect how it bounces.
• If you videotaped the basketball bounces, watch your videos to try to more closely estimate the basketball's bounce height on the different surfaces.
• Extra: Try to quantify your results from this activity. To do this, you'll want to videotape your bounce trials and closely watch the videos on a large screen to determine the exact height of the basketball before it was dropped and the highest point of its first bounce. You could even graph your results. Exactly how high does the basketball bounce on each of the different surfaces?
• Extra: A basketball loses kinetic energy by transferring it into other forms when the ball bounces. But just how many bounces can a basketball make before losing all of its kinetic energy and stopping bouncing? And how does this change if you alter some factors, such as the type of surface the basketball bounces on or the drop height? Design an experiment to investigate how many bounces a basketball can make and how various factors affect that number, and then try it out!

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