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Resultant Impulse

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  1. Feb 7, 2016 #1
    1. The problem statement, all variables and given/known data
    IMG_0051.jpg

    2. Relevant equations
    ΣFΔt = impulse
    mΔv = momentum
    ΣFΔt = mΔv

    3. The attempt at a solution
    IMG_0052.jpg

    I got the first part of the question (I hope I did it correctly), I just don't understand what my professor means by resultant impulse... Do I use the same equation? Is it going to be the same as a) since the boulders aren't moving?
     
  2. jcsd
  3. Feb 7, 2016 #2
    impulse is the change in momentum(FΔt=ΔP-Newtons 2nd law)
    but here ΔP=0=impulse
     
  4. Feb 7, 2016 #3

    haruspex

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    To clarify, that is for the net (i.e. resultant) impulse.

    @x2017 , in the first part, you are correct that they exert the same impulse, but your final sentence is wrong in referring to the weights of the boulders. Why did the boulders not move?
     
  5. Feb 7, 2016 #4
    Hmmmm... because the force applied by each girl isn't great enough to overcome the force of friction acting on each boulder? (I expect this to be incorrect since friction isn't mentioned in he question at all, it's just the first thing that come to mind!)
     
  6. Feb 7, 2016 #5

    haruspex

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    We are not told which way they are pushing, but suppose it is horizontal. Neither are we told whether the boulders are simply lying on flat ground, but suppose they are. If there is no opposing horizontal force then the boulders must move. What could the opposing force be?
    But you don't need to care what the opposing force is, you just need to deduce there is one. Weight acts vertically.
     
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