|MadSci Network: Physics|
Erin, Your experiment has puzzled several people for some time now. We were intending to try some rubber balls ourselves but our environmental chamber has been in constant use and we're not too sure even how to design a simple experiment to test this. A few observations: There are many different types of rubber each naturally having or designed to have a set of properties. These properties are reasonably well but not rigorously defined; they can vary with time, temperature, history and other conditions in hard to measure ways. You may have found some of these conditions Rubber is a complex material consisting of long molecular chains with freedom of movement between the chains which is restricted by crosslinks between the chains. The chains also have a loose or folded structure which means that they can be lengthened without actually stretching the molecule itself. The freedom of movement between the chains means that when the stretching force is released the rubber can return [eventually] to its original shape. This stretching and other molecular movement takes time to happen. In the confines of your test you are requiring that all the stretching and returning happen while the ball is in contact with the floor and that this contact happens in the same amount of time or that any time differences are somehow compensated. [This is not meant to belittle your experiment because the important thing is to make good accurate observations and think about them which you have done very well. A really good experiment doesn't answer questions! it gives you data that makes you think about new questions that you[and our team of experts] haven't thought of yet.] Time of contact and the amount of movement of the rubber molecules are possible hidden variables. This can be investigated, i think, by varying the height the ball is dropped and measuring the percent of rebound as a function of height[total energy]. You can imagine the difference in hitting a ball with a bat. i\If you hit it just right it goes far if you hit it too easy or too hard, hit the cover off the ball, the results are not as good. A second variable is the surface you are bouncing the ball off. The ability of the floor to rebound also effects the bounce, as you cool the ball the ball gets harder and the floor deforms more in the contact causing different behavior. Rubber as it is cooled becomes harder and its modulus or recovery from deformation increases. it bounces higher, however, the range of deformation in which it bounces higher may change. As it cools more there may be internal crystallization, you do notice that it becomes cloudy, again changing the properties. It will, at some low temperature, pass thru a transition know as the glass transition where it becomes hard and brittle or glasslike. If you reached this temperature the ball should bounce higher but the time constant and the deformation of the bounce surface may also change. This makes a definable measurement more difficult. Another theory that was presented is that the short cool hardens the surface and increases its modulus over the center and the composite structure behaves differently from the original one temperature ball. As you see I have no real answers. What you can do is record your conditions and your measurements accurately. If possible check out the other variables such as the type of ball, the height, and time-temp in the cooler and possibly the bounce surface. In any case please do continue to choose projects and record good observations that raise questions rather than give answers. If you have questions or more data don't hesitate to email me. JimG email@example.com
Try the links in the MadSci Library for more information on Physics.