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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
    1. The problem statement, all variables and given/known data[/b]
    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 by a moderator: May 12, 2015
  2. jcsd
  3. Nov 5, 2007 #2
    Okay think about you started out with this much energy then you had to spend that energy on other things then how much energy do you have left?
  4. Nov 5, 2007 #3
    I think its this equation: mgh +1/2mv^2=mgy +1/2mv^2, using my kinetic energy as my velocity
    should be:(I THINK) v=radical of 2*g*h

    Is this correct?
  5. Nov 5, 2007 #4
    okay so, you start out with kinetic energy of the bullet then you hit the wood of certain mass, that slows down the bullet, but it slows down by raising the wood and bullet up some hight. then after that the bullet comes out of the wood at lower kinetic energy thus comes out slower then ititiallly started. Can you form an equation from that info?
  6. Nov 6, 2007 #5
    Is it mv(initial)=mv(final)?
  7. Nov 6, 2007 #6
    No. Read again, this part. It must have done some work? What did the bullet do work on? What kind of collision is it when this occured?
    Last edited: Nov 6, 2007
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