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Homework Help: Law of conservation of energy problem

  1. Apr 6, 2009 #1
    1. The problem statement, all variables and given/known data
    A .5 kg block is dropped from 15m. A 5 gram bullet hits the block at 233 m/s. At what height must the block be hit to completely stop the blocks freefall momentarily.

    2. Relevant equations

    3. The attempt at a solution
    bullets ke
    When the bullets kenetic energy is equal to the blocks potential it will momentarily pause in mid air. Atleast i thought.

    bullet's ke = 135.72 joules= blocks pe = 27.698m
    The block would have to be 27 meters above the ground to have as much potential energy as the bullet described.

    Then i was thinking of solving it using momentum but im not sure how that would work out.
  2. jcsd
  3. Apr 6, 2009 #2


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    Homework Helper

    Equating momentums of the block and the bullet, you can find the velocity of the block.
    Then by using kinematics you can find h.
  4. Apr 6, 2009 #3


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    Hi leapoldstotch! :smile:
    Energy is not conserved in a collision unless the question tells you it is …

    in this case, the bullet will embed in the block, and have the same speed as it, so this is a perfectly inelastic collision and energy is obviously not conserved.

    You can only use conservation of momentum (for the collision itself).
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