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Homework Help: A bullet and a block (Momentum)

  1. Dec 11, 2009 #1
    A bullet of mass 7.00 g is fired horizontally into a wooden block of mass 1.29 kg resting on a horizontal surface. The coefficient of kinetic friction between block and surface is 0.180. The bullet remains embedded in the block, which is observed to slide a distance 0.230 m along the surface before stopping.

    What was the initial speed of the bullet?



    Okay, first off, I know the block is sliding after the collision so that is internal energy.

    [tex]\Delta[/tex]Uinternal = Friction Force* Distance

    =(12.7106N)(.180)(.230m) = 0.52621884J

    From here, i do not know what to do.

    Here was what I attempted:

    KE=[tex]\Delta[/tex]Uinternal

    so

    .5(1.297kg)v^2 = 0.52621884J

    Velocity of the block was 0.9007m/s

    Then I used Conservation of Momentum

    Pi = Pf

    .007kgV = (1.297kg)(.9007m/s)

    V=166.9m/s


    I don't know, seems slow for a bullet.
     
  2. jcsd
  3. Dec 11, 2009 #2

    AEM

    User Avatar

    Yes, it seems slow for a bullet, but then 23 cm doesn't seem very far for the block to slide. Your approach looks correct to me.
     
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