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Homework Help: Impact of a Bullet into a Block

  1. Jun 1, 2010 #1
    So I checked out the physics class I was taking on ratemyprofessor.com and John Ross was the guy who was teaching it. He had good reviews, so I figured I'll take his class. Well, a TA is teaching it, and I won't name the person, but he isn't really a teacher--even with his PhD in astrophysics. Needless to say, I'll probably be needing quite a bit of help. Thanks in advance for anyone who does help.

    1. The problem statement, all variables and given/known data
    A 12g bullet is fired in a 87.8g block of wood at rest on a horizontal surface and stays inside. After impact, the block slides 9.8m before coming to rest. The acceleration of gravity is 9.8 m/s^2

    If the coefficient of friction between the surface and the block is .5, find the speed of the bullet before impact.

    Answer: 81.5033 m/s

    2. Relevant equations
    Normal Force = mg
    Work = Distance * Force
    Another one with velocity in the variable

    3. The attempt at a solution
    The farthest I got was finding the frictional force that opposes the motion of the block.

    Normal Force = (.012+.0878)(9.8) = .97804 in the upward direction
    Friction Force = (.5)(.97804) = .48902

    I suppose I could turn that into work, which would be:

    Work = (.48902)(9.8) = 4.79240 in the westward direction

    That gives me the force that opposes the motion of the block, but I don't think that gets me anywhere closer to the velocity. This is where I am stuck, and although I know the answer, I will have to know how to do this for the test. Any guidance will be much appreciated.
  2. jcsd
  3. Jun 1, 2010 #2


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    Science Advisor
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    Gold Member

    Yes, good so far. Now find the velocity of the mass-bullet system immediately after impact by using the work-energy theorem . Then it's conservation of momentum.
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