Hookes law or simple dynamics?

1. Nov 19, 2005

~Josh [G]

Hi, I just have a problem dealing with the background of a project im working on in physics...
I basically have a rubber band fixed at a point above the ground, from which I have masses suspended from it. So obviously, as I add more mass, the elastic is strained more and more, and stretches to a point where it finally just snaps. Anyhow, in this particular situation, how exactly would I figure out the maximum tension (or ultimate strength) before it fractures? Is it just a matter of T=mg? Or F = k/\L? (Hookes law) if you argue that elastics are considered springs...in which case, how would I determine a constant of proportionality?
thanks

2. Nov 19, 2005

lightgrav

You use F = mg to find out what Force you've applied to the band.
You can "try" to use F = k/\L to determine the k - value for the band.

As you stretch the material, the "spring konstant" will gradually change
(it is getting thinner, so the Pressure = F / cross-section Area changes).
As you get CLOSE to breaking it - the "Ultimate Tensile Strength" -
k will start to change drastically (some molecule bonds ARE breaking).

3. Nov 19, 2005

~Josh [G]

oooh so is it safe to say tension = F applied up until the elastic breaks?

4. Nov 20, 2005

lightgrav

well, if the mass is bouncing, then its acceleration is nonzero.

But if a=0 , then the hanging thing has mg (down) and T (up) only.

The elastic will be more likely to break if the hanging thing bounces,
because its Temperature will cycle rapidly (bad for macromolecules).