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Q & A: Height of ball bounce

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Q:
How do you determine the height of a balls bounce if you know the height it was dropped from and the balls radius? What does that have to do with the number of times it hits the floor?
- Leslie
A:
The height to which a ball will bounce depends on the height from which it is dropped, what the ball is made out of (and if it is inflated, what the pressure is), and what the surface it bounces from is made out of. The radius of the ball doesn't really matter, if you are measuring the height of the ball from the bottom of the ball to the ground.

A ball's gravitational potential energy is proportional to its height. At the bottom, just before the bounce, this energy is now all in the form of kinetic energy. After the bounce, the ball and the ground or floor have absorbed some of that energy and have become warmer and have made a noise. This energy lost in the bounce is a more or less constant fraction of the energy of the ball before the bounce. As the ball goes back up, kinetic energy (now a bit less) gets traded back for gravitational potential energy, and it will rise back to a height that is the original height times (1-fraction of energy lost). We'll call this number f. For a superball, f may be around 90% (0.9) or perhaps even bigger. For a steel ball on a thick steel plate, f is >0.95. For a properly inflated basketball, f is about 0.75. For a squash ball, f might be less than 0.5 or 0.25 - squash balls are not very bouncy. The steel ball on an unvarnished pine wood floor may not bounce at all, but rather make a dent, and so what the floor is made out of makes quite a lot of difference.

For multiple bounces, it's just like dropping the ball again from a reduced height. If the first height is h, the second will be f*h, the third f*f*h, the fourth f*f*f*h, and so on. So if f is 0.9, the first bounce will be 0.9 times as high, the second 0.81 times as high, the third 0.729 times as high (as the original height), and so on.

Try it yourself! Does f depend on the height? (it shouldn't much, but it might..) Try it for different balls!

Tom

(republished on 07/11/06)

Follow-Up #1: Bouncing balls

Q:
which type of ball bounces the most ? which kind of ball bounces the most ?
- vanessa and tricia (age 12)
chicago il us
A:
The best I have seen is called a Super Ball.  You can buy one in a novelty shop. Tennis balls bounce about 53% of their dropped height, Super Balls about 85 or 90 % of their dropped height. I've never tried golf balls; why don't you try one and let us know.  The differences in bounce height depend on the style of construction as well as the material the balls are made of.

LeeH

(published on 05/11/08)

Follow-Up #2: Golf ball drop experiment

Q:
I did an expieriment dropping a golf ball on cement and it bounced an average of 5.454545... does that mean that that would be the fraction of energy lost, or does it need to be tested on a designated surface?
- Marcus Johnson (age 12)
A:
Hi Marcus.   You don't give us enough information to tell what is going on.  For example what does the number 5.45... mean?.  Is that in inches, centimeters, per cent...?  How high was the drop?  What kind of surface?
Give us some more details.

LeeH 

(published on 09/16/09)

Follow-up on this answer.