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Hookes law or simple dynamics?

  1. Nov 19, 2005 #1
    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?
  2. jcsd
  3. Nov 19, 2005 #2


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    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).
  4. Nov 19, 2005 #3
    oooh so is it safe to say tension = F applied up until the elastic breaks?
  5. Nov 20, 2005 #4


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    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).
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