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Completely Inelastic Collision

  1. Nov 6, 2013 #1
    I'm currently trying to make a proof to convince myself that when two object collide and stick afterwards, there is maximum energy loss. I've been thinking about it and trying to come up with a mathematical proof to solidify the idea in my head.
    Please tell me if there's any errors in my explanation or if there's anything that should be added.

    Case 1: object 1 is moving and object 2 is stationary (with no external forces, a frame of reference can always be used in which the motion is 0 m/s, and I realize this is a proof in itself, but I want to come up with something mathematically instead of intuitively)

    Ki = (1/2)m1vi2 [1]

    Taking the derivative and solving for 0 will give me an extreme value for the kinetic energy.

    Kf = (1/2)(m1+m2)vf2 [2]

    and if vf=0 m/s, then this system will have 0 J (which, using the right frame of reference, is possible in any situation where the velocity of the two "stuck" objects is constant"

    pi = pf since there is no net external force
    vf=m1vi/(m1+m2) [3]

    Plugging [3] into [2] and dividing by [1], to see the ratio between Kf and Ki



    I'm just confused now since this ratio doesn't seem to tell me much about two kinetic energies, does it? What else should I do now?
    Last edited: Nov 6, 2013
  2. jcsd
  3. Nov 6, 2013 #2


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    Staff: Mentor

    Try working the problem in a frame in which the total momentum is zero. Then, before you go through the work of finding the extreme value, look carefully at the pre-collision and post-collision state.
  4. Nov 6, 2013 #3
    Firstly, where K is kinetic energy, m is mass, v is velocity, and p is momentum, it could be technically debated that, if:
    Kf = (1/2)(m1+m2)vf2 [2]
    K'fp = (m1+m2)vf
    What has K been differentiated with respect to?

    Another more easily addressed problem appears to be;
    pi = pf
    This is true if momentum is conserved, as is the case with ideal elastic collisions. In an inelastic collision, KE from equation [1] in the OP is lost to heat and other processes involved in the coalescing of the two objects, regardless of whether the final velocity; has been fixed to be zero, is measured to be zero, or otherwise. So pi ≠ pf , and therefore equation [3] is incorrect.

    Lastly if K is dependent on v, and the final velocity of the coalesced objects is 0m.s-1, it becomes clear that the ratio Kf / Ki provides no useful information.
  5. Nov 6, 2013 #4


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    Staff: Mentor

    Momentum is always conserved, in both elastic and inelastic collisions, whether ideal or not.
  6. Nov 7, 2013 #5
    Use the center of mass reference frame. In this frame the objects are at rest after the collision (by momentum conservation) which means the energy is a minimum (since its zero, and cannot be negative).
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