Re: Why does a rubber ball bounce higher when only partially frozen/very cold?

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 jgriep@msn.com