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Homework Help: Find the speed of the bullet as it emerges from the block

  1. Nov 5, 2007 #1
    1. The problem statement, all variables and given/known data
    A 10.0g bullet is shot through a 1.0 kg wood block suspended on a string 2.0m long. The center of mass of the block rises a distance of 1.0cm. find the speed of the bullet as it emerges from the block if its initial speed is 450m/s.

    2. Relevant equations
    I think this has to do with linear momentum, p(i)=p(f)(mv=mv). Either that or conservation of energy: mgh +1/2mv^2=mgy +1/2mv^2

    3. The attempt at a solution
    Im confused as to how to go about solving this problem. for the initial mv, I plugged in the values for the bullets mass and used 450 m/s as the initial velocity. for the final mv, i plugged in the the mass value for the block and bullet, and used that to divide my initial mv, which was good for linear momentum but still not good enough, i dont think its right. so when i use the conservation of energy,i know my h=2.0m and y=.01m

    Am I on the right track. Which equation should i use???? Thanks
    Last edited: Nov 5, 2007
  2. jcsd
  3. Nov 6, 2007 #2


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    Homework Helper

    The idea is to use:

    mbullet*vi = mbullet*vf + mblock*vblock

    where vf is the velocity of the bullet right after it goes through, and vblock is the velocity of the block right after it goes through.

    so you need to find vf. but before you can do that you need vblock... do you see a way you can find the velocity of the block right after the bullet goes through?
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