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Q & A: Baseballs and temperature and humidity

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Most recent answer: 10/22/2007
Q:
’m doing my science fair on "What affects the bounce of a baseball" and I already did all of the experimentations. Now I need to write the paper, and for that I was wondering if you could tell me about what humidity does to the bounce of the ball and what heating and cooling the ball would do to the bounce. If you can help in any way please do. Thanks!
- Brent 17
Dayton Christian High, Dayton, Ohio, USA
A:
Many people have done experiments on what makes a baseball bounce better. Heating them does make the balls bounce higher in general, although the actual behavior of the ball when heated depends on what it is made out of. Tennis balls, basketballs, racquetballs, etc. are all easier to think of than baseballs because they are hollow with air inside. A baseball is made from a small rubber core with many layers of yarn wrapped around it, and has a stitched leather outside. I suspect (I haven't done the experiment) that when the ball is warmer, the components are less stiff. If you have ever tried flexing leather boots on a cold, cold day, you will notice that it isn't as easy as on a hot day. There are different kinds of stiffness in the world -- there are springy things that give you back the energy you put in them (like rubber and springs, and they bounce well if they are stiff or not), and then there are things that just deform and stay that way, like leather and yarn. I suspect that the stiffer leather and yarn just absorb the mechanical energy of the bounce more because it is harder to flex them when cold.

The humidity (of the air) should have almost no measureable effect on the bounce of a baseball (higher humidity may increase air drag for those long fly balls however). It will take a long time for a change in the air to "soak into" the ball because it is so tightly wound and sealed up. If you keep your baseballs in a bucket of water overnight however, my guess is that they will not bounce as well the next day (again, I have not done the experiment, except to have played with some old, torn softballs on a rainy day and being disappointed in their performance). The yarn will soak up lots of water and it will squeeze out of the threads when the ball hits the ground or bat, robbing the ball of some energy to do all this. But for normal humidity conditions I suspect that the mechanical properties of the yarn will be affected only a very tiny amount, and that you will have to wait a long time before the increased water content of the air shows up inside the ball.

Tom

(published on 10/22/2007)

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