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HELP w/ momentum problem

  1. Feb 9, 2008 #1
    I am having problems with part B of the following question.

    A human cannonball with a mass of 70 kg experiences an impulse of 4000 N/s for 0.35 s
    a) Calculate the force acting on the human cannonball.

    This is my sol'n:

    J = 4000 N/s
    (delta)t = 0.35 s

    J = F(delta)t
    4000 N/s = F(0.35 s)
    F = (4000 N/s) / (0.35 s)
    F = 11429 N

    b) How long was the barrel of the cannon? (Assume the force is applied only for a period of time that the cannonball is in the cannon.)

    For part B I think I just need clarification on what the question is saying. Is it saying to change the time or the force ????
  2. jcsd
  3. Feb 9, 2008 #2
    From my understanding, you have to find the distance the object travelled while it was in the cannon.
  4. Feb 10, 2008 #3
    the time given was 0.35 s. Do i just pick a anumber like 0.1 for the time it was in the cannon ??
  5. Feb 10, 2008 #4


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    Ummm...no. I think you'd stick with 0.35 s, seeing how that's what's given to you.

    btw, I think the units of impulse should be N*s, not N/s
  6. Feb 10, 2008 #5
    I think you need to use kinematics + Force. d = v_1*t +.5at^2. You can find out the acceleration from Fnet = ma and the initial velocity would be 0 since cannon balls usually start from rest before shot out of a cannon.
  7. Feb 10, 2008 #6


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    I don't think kinematics are necessary. From the impulse, you know the change in momentum. That gives you the final velocity. As cse63146 said, the initial velocity is zero. From that, you know the change in kinetic energy, which gives you the work done. If you know the work done, and you have calculated the magnitude of the force that did it, then you know over what distance that force must have acted.
    Last edited: Feb 10, 2008
  8. Feb 10, 2008 #7
    Yeah, that sounds more reasonable.
  9. Feb 12, 2008 #8
    Kinematics does work because you can find the accelatration due to newtons law and plug that into a kinematics equation
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