Q & A: Temperature affecting bouncy balls

Q:
I understand the difference between a hot or cold basketball bounce; but I was wondering if I have a bouncy ball that holds no air, what effect would temperature have then, if any?
- Garrett Russell (age 13)
Lee Central School, Lee, Ma
A:
How bouncy a ball is depends on the fraction of the energy which is lost in the collision between the ball and the floor. For very hard balls, this depends at least as much on the floor as it does on the ball.

Balls with more air pressure in them bounce better because air, when compressed, will uncompress (spring back) with little or no energy loss, while the rubber the ball's made out of isn't quite as nice. When the rubber flexes, it heats up and makes a noise, dissipating energy. A ball that has higher air pressure in it will not squish as much during the collision, and so less energy will be lost.

A hot ball will usually have more air pressure in it than the same ball, colder (unless it's leaky), because air likes to expand when hot. That's one reason it's bouncier. A second reason is that the rubber may be less stiff when hotter, and dissipate less energy when it squishes.

If your ball gets very very cold (like a racquetball dipped in liquid nitrogen), it may even shatter into lots of pieces when it hits a hard floor. Not too bouncy. But if you don't throw it so hard at the floor, it may bounce instead.

Now if your ball is made out of something solid, the effects depend on the materials. If the ball is made of rubber, then the stiffness argument above would incline me to predict that the ball will still be less bouncy when cold.

Some balls are not very bouncy to begin with, like squash balls. Some kinds of stiff materials do not dissipate energy very much, like steel. A steel ball on a steel floor is amazingly bouncy. But drop it on an unvarnished wood floor and it will just go thud and make a dent in the floor. You might be able to make a squash ball "bouncier" by cooling it to liquid nitrogen temperatures and dropping it on a hard but springy floor. Don't drop it from so high that it shatters or even cracks, but my guess is that while a squash ball may become less bouncy as it becomes colder, it will then become more bouncy as it freezes and turns into a rigid solid. Then it probably depends on how springy the floor is.

Tom

(published on 10/22/2007)

Follow-Up #1: hot and cold bounces

Q:
I have a science project on "How does temperature affect a bouncy ball bounciness". Can you give me some advice on how to freeze or heat a bouncy ball. For freezing I can place it in my ice box. Where can I buy liquid nitrogen? For hot temperature, i can place the ball in boiling water. Do you have any other suggestions. Thanks
- noah wong (age 8)
san jose, ca United States
A:
Noah- We get many questions on this project, which seems to be a favorite assignment around the country. Although it's hard to imagine why so many teachers would choose such a boring project, your questions at least are about how to do experiments. Your ideas about how to get things hot and cold are very good, except that you should just use warm hot water, not boiling water, to avoid getting hurt. Liquid nitrogen is hard to get outside labs.  You could get a nice range of temperatures using

3. room temperature
4. warm water
5. hot water from the tap.

Mike W.

(published on 05/16/2013)

Follow-Up #2: comment on bouncing balls

Q:
It's not a question, but a comment....teachers do not choose the science project these kids have to turn it, they have to choose and tell the teacher what their project is about. My daughter came up with this why do some balls bounce higher than others and what are they made of...and its seems to be one of the favorites amoungst 13year olds....its rather not boring but rather interesting to watch and learn what she finds out and how they do this research. Just a thought from the comment about teachers picking such a boring topic.....its the exact opposite. from a mom watching her daughter work....;o)
- a mom (age 43)
Fort Lauderdale, FL
A:
Your daughter's project sounds pretty interesting. It's a natural thing to wonder about.

I suspect that the specific question about basketballs and temperature was assigned, because it showed up in the same detailed way from many sources.

Mike W.

(published on 10/24/2009)

Follow-Up #3: not-boring projects

Q:
I would also like to respond to the idea that the bouncy ball project is "boring." Like the other parents said, it is not the teachers that choose the projects, but rather the kids in conjunction with their parents. My 3rd grade son chose this project this year. In any event, it is not at all a boring project for a child interested in mechanics, as my son is. We are learning all sorts of principles related to potential and kinetic energy, compression and rigidity, how to make precise measurements, etc. It is an inexpensive and fairly easy project that offers a lot of learning opportunities. A similar project in that it seems boring but is really very interesting is the question of how salt affects the boiling and freezing points of water, which I did with my other son in the 4th grade. We learned all about vapor pressure of water versus water solutions. I think that really, there has been no project that any of my children has done that is "boring." This is because the world around us is a fascinating place. The most mundane events of a child's (or adult's) life are opportunities for wonder and learning. I've always told my children that boredom only exists in the person experiencing it. There is really nothing boring in the world.
- Hina (age 43)
Austin, TX USA
A:

You make a convincing case. Probably my view on what's interesting or boring is distorted by seeing the same questions come in over and over again from around the world. It sounds like your son and you and his teacher have done a good job of making this bouncy-ball problem interesting by following up on the reasons.

The boiling/freezing point problems sound interesting even to me. We've got a lot of discussion of them on the site.

Mike W.

(published on 01/11/2014)