Re: Rubber band length and elasticity

Dear Jonathan,

You want to measure how the elasticity of a rubber band changes with rubber band length. A good first step would be to define what you mean by "elasticity". We know it has something to do with "stretchiness", so a good definition might be "the change in length of the rubber band due to a given force". So, if you have two rubber bands, and one stretches more than the other when you apply the same force to both, then the one that stretches more is the one that is more elastic. This isn't exactly the textbook definition of "elasticity", but I think it describes what you want to measure.

How would you do an experiment to measure this? Well, you need to apply a force to rubber bands of different lengths, and measure the change in length, that is, how much the bands stretches from its resting position. A good way to apply a force is to use gravity: hang a weight on the end of the rubber band, and then measure the new length of the band. The change in length from the resting position is how much the band has stretched.

You should first try this with the same kind of rubber band, cut to different lengths. Then try it with different kinds (long, skinny ones, short wide ones, etc.) The reason for this is that the stretchiness of the rubber band will probably depend on three things: its length and width, and its composition (what the band is made of). By measuring the elasticity for bands of different width, length, and composition you should be able to see the effect of all three factors.

(If you really want to know, the equation that describes the stretching of a rubber band is (delta L)=(FL)/(EA) where (delta L) = length change, F=force, L=length, E=Young's modulus, A=cross-sectional area. Ask your teacher for help in understanding this.)

For more information, try looking up "elasticity", "Hooke's Law", or even "Young's modulus" in a science book. This is your science project, and one thing that's important to learn about science is where to find information..