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Completely inelastic collision

  1. Oct 6, 2007 #1
    1. The problem statement, all variables and given/known data
    Is it true that a completely inelastic collision can be elastic (i.e. KE could be conserved) such as when you fire a gun at a metal ball at rest and the bullet and the metal ball stick togethor? Isn't completely inelastic somewhat of a misnomer since it implies that the collision is not elastic?



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Oct 6, 2007 #2

    Doc Al

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    Staff: Mentor

    No. What makes think KE is conserved in that situation? Do the calculation and find out for yourself.

    Well, it's not elastic--they stick together! KE is not conserved.
     
  4. Oct 6, 2007 #3
    Why does the fact that they stick togethor require that KE is not conserved?
     
  5. Oct 6, 2007 #4

    Doc Al

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    Just do the calculation! Use conservation of momentum:

    [tex]m_1\vec{v}_1 + m_2\vec{v}_2 = (m_1 + m_2)\vec{v}_f[/tex]

    Then compare initial and final KE. Keep it simple if you like, let v_2 = 0 and assume that all motion takes place along a single dimension.
     
  6. Oct 6, 2007 #5
    OK. I did the calculation and I see that KE gets lost. But, in general, in a closed system, energy must be conserved, so that KE gets transformed to the energy in chemical bonds that holds the two units together, correct?
     
  7. Oct 6, 2007 #6

    Doc Al

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    When we say KE is "lost" we are talking about macroscopic translational KE. Of course, that energy isn't really lost--it's transformed mainly into thermal energy.
     
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