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Homework Help: Physics : Work Energy Power Question

  1. Jun 20, 2007 #1
    1. The problem statement, all variables and given/known data

    A bullet of mass 0.0035 kg is shot into a wooden block of mass 0.121 kg.

    They rise to a final height of 0.547 m as shown. What was the initial speed (in m/s) of the bullet before it hit the block?

    http://www.physics247.com/physics-homework-help/conservation_nrg_quiz1.php" [Broken]

    There is a picture there on how the setting looks like.

    2. Relevant equations

    3. The attempt at a solution

    K.E of bullet = P.E of the total mass at its final height
    1/2mv(square) = (m+M)gh
    (1/2)(0.0035)v(square) = (0.0035 + 0.121)(9.81)(0.547)
    v = 19.5m/s

    The actual answer is 116.5m/s.

    Where did I go wrong? :confused:

    Ya. Thanks for any help provided. And since this is my first time posting forgive me if there are any mistakes in the way I posted the question. Thankx again. =)
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Jun 20, 2007 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Here's where you went wrong: Mechanical energy is not conserved during the collision (some of the bullet's kinetic energy becomes thermal and deformation energy). But another physical quantity is conserved during any collision--what is that?

    After the collision, the mechanical energy of "bullet + block" is conserved.
  4. Jun 20, 2007 #3
    Thanks man. =) I got it. So the momemtum of the system is conserved. So by using that I can solve this. Hm..Thank you very much for clearing my misconception. :)
  5. Mar 24, 2011 #4
    please help me answer this question..
    A boy holds a 40-N weight at arm's length for 10 seconds. His arm is 1.5m above the ground.The work done by the force of the boy on the weight while he is holding it is?
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