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Conservation of energy or conservation of momentum

  1. Feb 16, 2008 #1
    OK this is about a bullet hitting a block on wheels (inelastic collision), no friction or energy losses in the problem.

    I'm trying to find Velocity of the system after collision

    Why is it that if I use conservation of momentum I get a different answer of V than if I used conservation of Kinetic energy.

    Momentum
    m1*v1=(m1+m2)*v2

    K energy
    0.5*m1*v1^2=0.5*(m1+m2)*v2^2

    You can make up your own numbers....I just dont understand the physics!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
     
  2. jcsd
  3. Feb 16, 2008 #2

    Doc Al

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    Staff: Mentor

    It's an inelastic collision--energy is not conserved. Mechanical energy will be lost.
     
  4. Feb 16, 2008 #3

    G01

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    Homework Helper
    Gold Member

    The reason you get different numbers is that mechanical energy is NOT conserved in an inelastic collision, while momentum is.
     
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