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Homework Help: Inelastic Collisions and ratio of their masses

  1. Oct 14, 2011 #1
    1. The problem statement, all variables and given/known data

    Two objects moving in opposite directions with the same speed undergo a totally inelastic collision, and half the initial kinetic energy is lost. Find the ratio of their masses, m1/m2.

    2. Relevant equations

    The objects are moving in opposite directions with the same speed, so v1=-v2=v

    3. The attempt at a solution

    The part that gets me is the part that says half the kinetic energy is lost. Could someone explain how I factor that into the equations? If I just solve for m1/m2, plugging in 1/2v for vf(I don't know if that's right) I get 3, which is not the correct answer.
  2. jcsd
  3. Oct 14, 2011 #2
    If the collision were perfectly elastic, kinetic energy is conserved. But it is totally inelastic and 1/2 of the energy is lost. Therefore you can say:

    .5 * m1 * v1^2 + .5 * m2 * v2^2 = (.5 * (m1 + m2)* vf^2) * .5

    Then start cranking on the algebra to get m1/m2.
  4. Oct 14, 2011 #3
    That makes sense. However, when I solve for m1/m2 I get (.5vf^2 + v^20/(v^2-.5vf^2) when I need an actual ratio.
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