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Homework Help: 2D inelastic collision and K of CM

  1. Feb 20, 2009 #1
    1. The problem statement, all variables and given/known data
    Car 1 (1500kg) is heading north 80km/h, Car 2 (3500kg) is heading east 50km/h. They collide inelastically.
    a.) Speed and direction of the wreck (car 1 + car 2)?
    b.) The kinetic energy lost?
    c.) What is the kinetic energy of the two-particle systems center of mass before and after the collision?
    d.) Where was the lost kinetic energy hidden before the collision?

    2. The attempt at a solution
    a. is just vector sum p1+p2 where I can get the velocity by dividing it with the masses (about 12m/s), I can get the angle too (34,438... deg)
    b. wrecks K minus K of car 1 and car 2 = - 360532,40... J so about 360 kJ lost
    c. But this one I don't understand. I'd say it stays the same, because... well, that's what I calculated (CM from the v-vectors = v of CM), yet I don't really belive it...
    d. ... after all shouldn't the K be lost to deformation of bodies (+heat etc.)? Does the K of CM stay the same anyways?
  2. jcsd
  3. Feb 21, 2009 #2
    Ok, I found something useful on the net:
    http://courses.washington.edu/ph122mo/A08/MT3info.html" [Broken]
    # Inelastic collisions

    * momentum is still conserved
    * kinetic energy of center of mass motion cannot be lost
    * maximum energy loss for items sticking together after collision

    And that is really all I needed to know. :approve:
    Last edited by a moderator: May 4, 2017
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