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Homework Help: Conservation of momentum and energy problem

  1. Nov 9, 2005 #1
    I read the answer of it, but I don't get why the answer is that.
    And the answer is

    1/2 M gun x (M bullet * V bullet / M gun)^2
    - 1/2 M gun+you x (M bullet * V bullet/ M gun+you) ^2

  2. jcsd
  3. Nov 9, 2005 #2


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    Homework Helper

    What have you tried?

    After the impact, energy is conserved (you can't assume conservation during the impact though).
  4. Nov 9, 2005 #3
    Before I checked the answer, I calculated the recoil speed of the gun. And I set the equation using conservation of momentum to find the velocity of "you+gun".
    i.e (M bullet * V bullet / M gun) * M gun = M you+gun V you+gun

    But I don't know why the final answer is the Kinetic energy from the gun after the bullet is fired minus the Kinetic energy of "you+gun" after the impact.
  5. Nov 9, 2005 #4
    By conservation of momentum, you have calculated the the momentum of the system directly after the after this inelastic impact. By looking at the kinetic energy before & after the collision you will notice that energy is lost somewhere; ask yourself where this energy has gone.

    You also have to assume that no energy is lost during the time it take the rifle to you.

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