Bouncing Ball

Q:
How does the surface on which a ball bounces effect its bounce? What forces act on the bouncing ball?
- Samaja (age 16)
Avondale College, New Zealand
A:
Samaja,

That’s a great question for two reasons:
1) It involves interesting physics.
2) You can do experiments by yourself to verify the stuff that I will write below (please try it).

The best way to think about bouncing balls is to first think about something simpler that is also bouncy: A SPRING.

Suppose you have a spring that looks like |/\/\/\/\/\/\/\/\/\/\|. If you grab both ends and push them together, the spring compresses. When you let go the spring will push itself back out to its original length. The physics that goes on is the following: When you compress the spring you are putting energy into it, energy that it stores. When you let go of the spring, it uses that energy to get back to its original length. The energy stored in the spring is called "Potential Energy" since it has the potential to do stuff (like push your hands apart). After you let go of the spring, and it has expanded back to its original length, all of the potential energy you put into it has been released….the spring is exactly the same as it was before you squeezed.

A ball is a lot like a spring. When you throw a tennis-ball against a wall, it gets squished (in the same way that the spring can be compressed). If you had a high-speed camera and took a picture at the instant the ball was at rest, just before starting to bounce back, you would see that it was quite deformed. This deformation stores potential energy in the rubber the ball is made of, which is then released back to the ball causing it to bounce back as it becomes round again, having almost the same speed you threw it with.

The key word is ALMOST. The material the ball is made of (rubber or plastic perhaps) is not an ideal spring like the case we discussed above. When we put energy into a ball by deforming it we don’t get all of it back again when it returns to its original round shape…this is just a property of most materials. (Springs are also not truly ideal, but they are usually much better at releasing their potential energy than balls are).

Usually, the more something deforms as it bounces, the worse it is at giving back its energy…a hard ball generally bounces higher than a soft one…you know this is certainly true for soccer balls you have played with. Super-Balls (the ones that bounce really high) are made of very hard solid rubber.

OK, now suppose you throw a ball against a wall of a floor that is itself squishy rather than hard (like a carpeted floor versus a hardwood floor). Now the floor also acts like a spring, and just like the case of a ball, a squishy floor is probably not so good at giving back its deformation energy. If you bounce a soccer-ball off a hard road it will bounce higher than if you bounce it off a lawn.

By the way, just because the potential energy is not all released into the motion of the bouncing ball (called kinetic energy), it is nevertheless not lost…energy can not be created or destroyed, just changed. The potential energy that is apparently not recovered as motion of the ball becomes heat (the ball warms up a bit as you bounce it) and perhaps also sound (the ball makes a loud noise as you bounce it).

Mats

(published on 10/22/2007)