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Homework Help: Momentum + Friction force = me depressed

  1. Oct 20, 2004 #1
    Hey guys, I need help on some physics problems again.

    A 15g bullet strikes and becomes embedded in a 1.10kg block of wood placed on a horizontal surface just in front of the gun. If the coefficient of kinetic friction between the block and the surface is 0.25, and the impact drives the block a distance of 9.5m before it comes to rest, what was the muzzle speed of the bullet.

    A gun is fired vertically into a 1.4kg block of wood at rest directly above it. If bullet has mass of 21g and speed of 310m/s, how high will the block rise into the air before the bullet becomes embedded in it?

    Please give me hints on how to solve these two problems.
  2. jcsd
  3. Oct 20, 2004 #2


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    Homework Helper

    Use conservation of momentum, then work the initial speed for the block.
    Last edited: Oct 20, 2004
  4. Oct 20, 2004 #3
    Sorry, some people (like myself) aren't well equipped with physics skills and know right off the bat what " Use conservation of momentum, then work the final speed for the block." means totally.

    Cyclo you've helped me out many times man, can u explain just a bit more. It is a new topic that was taught just yesterday in class. Thanks.

    I have no idea how the friction force fits in there....
  5. Oct 20, 2004 #4


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    Well you should use your knowledge that change of momentum will be 0, thus it will be conserved.

    [tex] m_{bullet}v_{bullet_{o}} + m_{box}v_{box_{o}}= (m_{bullet} + m_{box})v_{both}[/tex]

    The box was at rest

    [tex] m_{bullet}v_{bullet_{o}} = (m_{bullet} + m_{box})v_{both}[/tex]

    Now you need to know the speed of the both, use the rest of the information, Newton's 2nd law and kinematic.
  6. Oct 20, 2004 #5
    thx alot man :)
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