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Elasticity of metals springs and rubber bands

  1. Apr 3, 2008 #1
    How come both spring and rubber band are elastic whereas spring are contracted and rubber bands are stretched? I understand that elasticity of an object depends on it ability to reform from deform. I also get that spring works b/c of normal force(too much contraction...right?-being the reason for rebound) But what does a rubber band works on that makes it snap back to its original position?(what force propels it?)

    Thanks.
     
  2. jcsd
  3. Apr 3, 2008 #2
    Both the spring and the rubber have the tendency to return to the original shape after being either stretched or compressed. If you didn't realize that fact about compressing rubber then you forgot about bouncing a rubber ball.

    In a solid the bonds between neighboring atoms or molecules are visualized like an array of springs in three dimensions, like in a mattress. The bonds are electrical. When the material is stretched or compressed the network of bonds is distorted into a shape that's unstable. It finds equilibrium as it returns to the original shape.
     
    Last edited: Apr 3, 2008
  4. Apr 3, 2008 #3

    Andy Resnick

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    When an object is strained, it develops internal stress. The stress energy stores the energy of deformation and can be released (elastic deformation), dissipated (plastic deformation), or some combination of the two (viscoelasticity, viscoplasticity, creep, etc. etc.). The relationship between stress and strain is properly a 4-th rank tensor, but that usually gets simplified considerably by taking into account various material invariances- crystal structure, isotropy, etc.
     
  5. May 3, 2008 #4
    Andy please could you provide a comment on the effect of elasticity when a ball is struck with the same force but with objects of different impact surface areas. Does the smaller impact surface area not poke into the ball further than the larger impact surface area. If this is the case would the ball not get greater potential energy from the smaller impact area based on the equation .5 * K * X * X where X=distance of displacement ( ie the distance that the impact area projects into the ball)

    Many Thanks

    Lucio
     
  6. May 5, 2008 #5

    Andy Resnick

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    I don't understand what you are asking. Elasticity is a material property, not a property of an interaction.
     
  7. May 6, 2008 #6
    Clarification of question

    Andy thanks for taking the time to respond.

    My basic question is this. If a spherical inflated ball (with a standard inner rubber ball which holds the air and a outer casing of either rubber/leather/plastic) were projected towards a flat surface and then projected at the same speed towards a ridged surface would the ball rebound with more speed from the ridged surface. My observation is that a ball rebounding from a ridged surface seems to be punched back at a greater pace than the ball rebounding from a flat surface. Not knowing the physics of elasticity to any depth, my thoughts were that the ball on striking the the ridged surface has the ridges poke deeper into the ball than the flat surface and therefore the elastic properties of the ball produce a greater elastic effect.

    Many thanks for any guidance.

    Lucio
     
  8. May 6, 2008 #7

    Andy Resnick

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    That may be true; the ridges act to decrease the contact area between sphere and wall, so the local deformation may be greater. Contact time and dissipation may be higher as well, though, so it's not clear if there is a single result that would occur.
     
  9. May 6, 2008 #8
    Thanks Andy. If contact time and dissipation are higher does that increase the energy of the ball's rebound?

    Also, which physics equations would apply when quatifying this effect?

    Thanks again
     
  10. May 7, 2008 #9

    Andy Resnick

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    If the contact time increases, the dissipation also increases, so the stored elastic deformation energy deceases.

    As for equations... nothing simple enough for me to write here. Landau and Lif****z's "Theory of Elasticity" is ok, but doesn't take the correct viewpoint, IMO. Better are books like Green and Zerna's "Elasticity" or Marsden and Hughes "Mathematical foundations of elasticity" and the like which take a continuum approach.
     
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