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Homework Help: Momentum/Impulse force vectors

  1. Jan 16, 2009 #1
    Hi friends,
    I am preparing for my exam and i am stuck with this problem, please help me out.
    It wont take too long, I hope.

    1. The problem statement, all variables and given/known data
    Two bodies A and B have masses mA and mB respectively and collide completely inelastic.
    Given that the mass of body A is 1200 kg and body B is 800 kg determine the avergage force vectors acting on each body during the collision. The collision itself lasts for 0.2 s.


    2. Relevant equations
    Velocity of body A: (5i + 3j) m/s
    Velocity of body B: (-i + 4j) m/s

    Common velocity after collision I found to be: (2.6i + 3.4j) m/s (this eq was derived in a previous question with mA = 3/2 mB)

    3. The attempt at a solution
    Well, I have tried using the momentum impulse theorem only to find out that the total momentum was conserved, which is good indeed. However I dont know how to find the answer I have kind of like used up my knowledge on impulse.
    The answer is (-14400i + 2400j) N.
    I have no clue how this answer was derived.
    Please help.
     
  2. jcsd
  3. Jan 16, 2009 #2

    Doc Al

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    Staff: Mentor

    To find the force on a given body, use the impulse-momentum theorem:

    [tex]\vec{F}\Delta t = \Delta (m\vec{v})[/tex]

    What's the change in momentum of body A?
     
  4. Jan 17, 2009 #3
    I have tried this equation but I just seem to get -4000i+16000j. But this seems to be wrong.
    Any further advice? I mean I substituted the final momentum from the initial momentum of A and divided by time, which however is initial momentum of B.
    Doesnt make sense. Please help
     
  5. Jan 17, 2009 #4

    Doc Al

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    Staff: Mentor

    Do it step by step:
    (1) Find the change in velocity of A. (Final velocity minus initial.)
    (2) Then find the change in momentum.
    (3) Then the force.
     
  6. Jan 19, 2009 #5
    Hi Doc Al,
    I am really sorry for the late response but I was so busy studying that i havent been able to check the post.
    THank you very much for your help, I finally got it (actually right now). I followed your steps and I am really thankful for your help.
     
  7. Jan 19, 2009 #6

    Doc Al

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    Staff: Mentor

    Excellent. (And you are very welcome.)
     
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