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Bullet hits a block and it slides.

  1. May 25, 2009 #1
    1. The problem statement, all variables and given/known data
    A 7 g bullet is fired into a 13.43 kg wood block that is at rest on a wood table. The block, with the bullet embedded, slides 5.92 cm across the table. What was the speed of the bullet?


    2. Relevant equations
    Ki=Kf
    ma=N+mg+fs
    Xf=Xi+(Vf^2-Vi^2)/2a


    3. The attempt at a solution
    I am not sure how to solve this because i am not given a coeffieicent of friction of wood on wood. I tried one that i found online (.48) but it didnt work then i tried .25 and that didnt work either. I thought i need this so i can find the acceleration so i can find out delta t so I can find the velocity of the wood with the bullet in it. But i came up with this equation to solve for the velocity of the bullet......

    PHysics10.jpg
     
  2. jcsd
  3. May 25, 2009 #2
    Mb= mass of bullet Mw= mass of wood
    Vb= velocity of bullet Vw=Velocity of wood
     
  4. May 25, 2009 #3
    A 7 g bullet is fired into a 13.43 kg wood block that is at rest on a wood table. The block, with the bullet embedded, slides 5.92 cm across the table. What was the speed of the bullet?

    You know that during a collision, momentum is conserved:

    p = p' (' means final)

    (Mbullet)(Vbullet) = (Mblock+bullet)(Vblock)

    Also, AFTER the collision, conservation of energy takes place, so:

    Et = Et', initially, the block only has kinetic energy, and finally, it comes to rest, so kinetic is converted to thermal:

    1/2(Mblock+bullet)(Vblock)^2 = (mew)(Mblock+bullet)(9.8)(0.0592)

    I don't know why wou werent given the coefficient. But if you find it, you can get the final speed of the block, and sub it back to find initial speed of bullet. I don't know, that is the only thing I can think of...
     
  5. May 25, 2009 #4
    So i do need it, there is no other way to solve this without being given the mu.
     
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