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Homework Help: Bullet Impulse

  1. Feb 28, 2009 #1
    1. The problem statement, all variables and given/known data
    In a ballistics test, a 25.0 bullet traveling horizontally at 1100 goes through a 20.0 -thick 450 stationary target and emerges with a speed of 800 . 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 target's speed just after the bullet emerges?

    2. Relevant equations

    impulse= F*change in time

    3. The attempt at a solution

    I have no idea how to do part a.
    I used the impulse equation for b but got the wrong answer.
    For c, I think I use the impulse equation and then calculate vf from the kinetics equation?
  2. jcsd
  3. Feb 28, 2009 #2


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    Part (a) is a bit tricky. You probably don't have a formula that is perfect for the job, containing Vi, Vf, d and t. May have to use one equation to find the acceleration, then another one to find the time.

    Alternatively, you could sketch the v vs t graph from 0 to t, showing the initial and final v.
    The distance is the area under this graph - a trapezoid. Using this, you could make your own formula relating Vi, Vf, d and t. Or put the numbers on the graph and just find the time geometrically, using the area formulas.
  4. Mar 1, 2009 #3
    ok so i got a and b

    a. t=.00021
    b. average force = 35714.286

    but I still don't know how to get part c...I thought maybe it would be 300 because that is the speed the bullet lost in the target but that is wrong.

    any help??
  5. Mar 1, 2009 #4


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    Doesn't b) give you the average force on the target?

    You know that action/reaction thing?

    The target's mass is what?
  6. Mar 1, 2009 #5


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    In the future can you please provide the units your problems are in?

    Units matter.
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