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Homework Help: Work energy theorem question

  1. Feb 22, 2012 #1
    problem -
    a bullet of mass .012kg and horizontal speed 70meters per second strikes a block of mass 0.4 kg and instantly comes to rest with respect to the block. the block is suspended from the ceiling by means of thin wires . calculate the height through which it rises.

    attempt -
    total initial energy = 1/2(mv^2)=1/2*.012*70*70
    total final energy = mgh=(0.012+.4)*10*h
    equating the 2 .
    i get 7.5 meteres but the answer given is 21.2 cm .

    pls help .
  2. jcsd
  3. Feb 22, 2012 #2

    Doc Al

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    Staff: Mentor

    Mechanical energy is not conserved during the collision of bullet and block. But what is?
  4. Feb 22, 2012 #3
    Can you please tell me how it is not conserved and how to go about this problem ?
  5. Feb 22, 2012 #4


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    Staff Emeritus
    Science Advisor
    Gold Member

    This is an inelastic collision, which means that kinetic energy is not conserved. Use conservation of momentum to find the final velocity of the bullet+block just after the collision. Use that to find the new kinetic energy after the collision, and then assume all of this is converted to potential energy as the block swings upward.
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