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Bullet Passes Through Block

  1. Mar 5, 2006 #1
    I have a problem where a 20 gram bullet is fired at 1200m/s throguh a 45 gram block and emerges at 900m/s. I am asked to find the time that the bullet took to pass through the block which leads into a follow-on question about finding total force applied. I do not even know how to approach this problem to find an answer. Please help!

    I believe that I need to use a kinetics equation but the subsequent parts of the question ask for the speed of the block after the collision, using conservation of momentum, which I have already solved. Itried the conservation of momentum delta rho=force x delta time= mass times delta veloity but keep coming up with missing conversion factors.
     
    Last edited: Mar 5, 2006
  2. jcsd
  3. Mar 5, 2006 #2
    First, I am assuming the block to be stationary? If that is so, then momentum is NOT conserved.

    No matter how I approach this I cannot get an answer without more information. Is this the whole problem statement?

    -Dan
     
  4. Mar 5, 2006 #3
    Here is the actual problem statement: In a ballistics test, a 20g bullet traveling horizontally at 1200m/s goes through a 40 cm thick 450kg stationary target and emerges with a speed of 900m/s. The target is free to slide on a smooth horizontal surface.
    a.) how long is the bullet in the target?
    b.) What average force does the bullet exert on the target?
    c.) What is the targets speed right after the bullet emerges?
    I have solved c. I cannot figure out how to approach a. And b obviously depends on my answer to a.
     
  5. Mar 5, 2006 #4
    did you try using this?

    Vf**2 = Vi**2 + 2ad, where Vf is the final velocity of the bullet relative to block (Vf of bullet - Vf of block) and Vi is the initial velocity of the bullet and d is the thickness of the block.

    After you get a:

    a = (Vf - Vi)/t and you get the time

    To get the force:

    F = am or Ft = change in momentum
     
  6. Mar 6, 2006 #5
    That was the ticket. Thank you very much. I knew that there was a kinetics equation that applied, but always forget the one you provided.
     
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