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Homework Help: Intro momentum problem

  1. Jan 1, 2008 #1
    1. The problem statement, all variables and given/known data
    A 2kg block, initially moving due east at 6m/s, is acted upon by an impulse having magnitude 4Ns and direction due west. What is the final velocity of the block?
    a) 8m/s west
    b) 4m/s west
    c) 4m/s east
    d) 8m/s east

    2. Relevant equations
    change in momentum = F*(time interval) = mass*Velocity(final) - mass*velocity(initial)

    3. The attempt at a solution
    assuming west is the (-) direction
    4Ns = (2kg)(-Vf) - (2kg)(6m/s)
    2Ns = (-Vf -6m/s)
    -8m/s = Vf
    so 8m/s west
  2. jcsd
  3. Jan 1, 2008 #2


    User Avatar
    Homework Helper
    Gold Member

    You have the right idea, but I think you are making a sign error.

    Your relevant equation is correct, so start with that.

    Now, the impulse is directed west, so it should be negative based on your sign convention. Also, I don't know why you multiplied v_f by a negative sign in your first line of work. That is changing your answer. What do you get now that you take those sign errors into account?
    Last edited: Jan 1, 2008
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