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Homework Help: Bullet Enters/Exits Mass, Mass Slides

  1. Mar 24, 2009 #1
    1. The problem statement, all variables and given/known data

    In a ballistics test, a 25 g bullet traveling horizontally at 1200 m/s goes through a 35 cm thick 350 kg stationary target and emerges with a speed of 850 m/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 it exert on the target?

    (c) What is the target's speed just after the bullet emerges?

    2. Relevant equations

    It would be nice to have some more equations, my professor gave the class four pages of equations scribbled in his horrible hand-writing, to help us... I guess. Instead of giving us the basic equations that we can use to plug-in to create our own equations he already solved them all out into the forms he understands, which means we have to un-solve them to try to make sense of them, so really I have no idea what other equations to put here since he has no clear division between which equations go with which section of the book.

    momentum of system before = momentum of system after ||| Pi=Pf

    3. The attempt at a solution

    I already solved part C using the equation provided.

    Part A I attempted something that I knew wouldn't work but gave it a try anyway and I know I need part A to solve B so I haven't gotten around to trying to solve B yet.

    I seem not to be able to resist applying simple mechanics to complex problems just to see if I can knock it out of the way. Anyway I did this to solve for B, it was obviously going to be wrong but I submitted it anyway.

    B: I tried to say that the


    Vavg/DT=??? Then that gave me seconds so I took s-1. Yea, it obviously was just bs, but that is how lost I am.
  2. jcsd
  3. Mar 24, 2009 #2


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  4. Mar 25, 2009 #3
    Thanks, getting to work on things now. Hopefully I can make some progress without having to ask too much here.
  5. Apr 1, 2009 #4
    My problem is finding the time, the Δp (momentum) is a joke to find.

    0.001seconds doesn't seem like a bad impulse time, considering the average of the problems I've been working with is 0.005 seconds, but it won't accept it, I'm not sure what else to do even since I've posted this because no one mentioned time.

    I'm given it decelerates from 1200m/s to 850m/s in the distance of 0.35m.

    Finding the time should be cut and dry like I found it, but its not, so I'm confused.
  6. Apr 1, 2009 #5

    a) Use the equation Δx=1/2(Vo+V)t, so t=(2Δx)/(Vo+V)


    http://qaboard.cramster.com/physics-topic-5-218055-cpi0.aspx [Broken]

    Got 0.0003seconds, didn't accept it.

    Also read all of


    He seems to think the part I think is hard is elementary =(

    So I learned nothing from that as well.

    I recalculated the time he declared to be right using the formula I tried using on mine(the same one I tried right above this in this very post), it gave his same answer. Meaning I am calculating time right, assuming he was, and he did get the right answer. Anyway, I am getting really annoyed with my "online" system when I am almost positive my answer is correct.. am I wrong?
    Last edited by a moderator: May 4, 2017
  7. Apr 1, 2009 #6


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    Start with determining the deceleration through the material.

    That you can get from Vf2 = Vi2 - 2*a*x

    In your case that would be 12002 - 8502 = 2*.35*a

    Now armed with a you can determine t, from x = 1/2*a*t2

    That arrives at a number a bit larger than .0003 sec.
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