Under Pressure: Ball Bouncing Dynamics

Objective

In this experiment you will test if air pressure will affect the dynamics of ball bouncing.

Introduction

Bouncing is what makes playing with a ball so much fun. But what makes a ball bounce? If you were to see a ball drop to the ground in slow motion, you would see its shape change dramatically. When it hits the ground, the bottom of the ball becomes flat against the floor. Then when it bounces up, it returns to its original shape. The changes in shape are due to a balance of forces and energy:

"When you drop a ball, gravity pulls it toward the floor. The ball gains energy of motion, known as kinetic energy. When the ball hits the floor and stops, that energy has to go somewhere. The energy goes into deforming the ball-- from its original round shape to a squashed shape. When the ball deforms, its molecules are stretched apart in some places and squeezed together in others. As they are pushed about, the molecules in the ball collide with and rub across each other." (Doherty, 1991)

In this experiment you will use a ball filled with air. When it is dropped, the air molecules will squish together and then spring apart. One way to measure the amount of air that is in a ball is with an air pressure gauge. You will use an air pump, a basketball, and an air pressure gauge to test if changing the air pressure will change the bounciness of the ball.

Terms, Concepts and Questions to Start Background Research

To do this type of experiment you should know what the following terms mean. Have an adult help you search the internet, or take you to your local library to find out more!

air pressure
forces
energy
motion
gravity

Questions

How high will a ball bounce?
Will the air pressure change the bounciness of a ball?
How can you measure the height a ball bounces?

Bibliography

Doherty, P., 1991. "That's The Way The Ball Bounces," The Exploratorium Museum, San Francisco, CA. [accessed August 4, 2006]
http://www.exploratorium.edu/sports/ball_bounces/index.html
Here is another activity that tests temperature and bounce using golf balls and baseballs:
Exploratorium Staff, 1998. "Science Activity: Bouncing Balls," The Exploratorium Museum, San Francisco, CA. [accessed August 4, 2006]
http://www.exploratorium.edu/baseball/bouncing_balls.html
There are many other great ball projects in this fun book:
Goodstein, M., 1999. Sports Science Projects: The Physics of Balls in Motion. Berkeley Heights, NJ: Enslow Publishers.

Materials and Equipment

ball air pump
ball pressure gauge (available at a sporting goods store)
basketball or soccer ball
stepladder
video camera with a tripod
paint
tape measure (preferably metric)
large roll of paper
volunteer

Experimental Procedure

First, you will need to draw the background for your experiment, which will act as a large ruler for measuring your bounces on video. Use the paint and a tape measure to make a series of lines on your roll of paper. It should be tall enough to hang up behind your ladder, about 5 feet tall. Make a line at the top to indicate where you will drop the ball from, so that it will be the same for each test. Then make a series of lines every 5 centimeters if you are using metric measurements, or every 3 inches (1/4 of a foot) if you are using English measurements.
Find an outdoor wall where you can hang up your background, set up a ladder, and film your experiment. It should be on a smooth cement surface for optimal ball bouncing. Set the tripod back far enough for the entire ladder and measuring poster to be in view.
Fully pump up the basketball and check the pressure with the pressure gauge. Write the pressure in the data table:
Air Pressure in PSI (Lbs) Height of Bounce (cm)
Have your volunteer get the video camera ready. Carefully climb the ladder with the ball. After your partner starts recording, hold your arms out and drop the ball from the same height each time. It is important to let the ball fall out of your hands, and not to push it down with your hands, wrists, or arms.
Let the ball bounce until it stops, and then stop recording.
Use the pressure gauge to release some of the air pressure from the ball. Record the new pressure reading in your data table and repeat steps 4 and 5.
Keep releasing pressure and repeating the experiment until the ball no longer bounces.
Now, take your video and watch it to gather your data. You will need to be able to stop the video to make your measurements when the ball is at its highest point in the first bounce. Write each measurement in the data table.
A ball bounces in an arc, so measure the height at the top of the arc of the first bounce. (Image by Harold E. "Doc" Edgerton)
Make a line graph of your data. Set up the graph by putting a scale of the air pressure on the left side of the graph (Y-axis) and the height of the bounce on the bottom of the graph (X-axis). Now use your data table to put a dot where your data intersect. Then draw a line to connect the dots. Is it a straight line? Does it slope up or down? What do you think this means about your results?

Variations

Does height make a difference? Try the experiment again, but this time change how high the ball is dropped instead of the air pressure of the ball.
Do different kinds of balls bounce differently? Try bouncing different kinds of balls to test this question. Some balls to try are: soccer, basketball, volleyball, golf, tennis, or handball.
Check out these Additional Sports Project Ideas that explore the physics of bouncing balls.
Does the weather make a difference? Try your bounce test in different weather conditions. Try comparing a hot day to a cold day. Try comparing two days with different atmospheric pressure. You can check the weather stats at http://www.weatherunderground.com
There are many other great ball projects in this fun book by Madeline P. Goodstein, Sports Science Projects: The Physics of Balls in Motion published by Enslow Publishers. Check it out from your local library!

Credits

Sara Agee, Ph.D., Science Buddies