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Homework Help: Simple Momentum Question

  1. Nov 7, 2006 #1
    Brett takes a slap shot, exerting an average force of 16.8 newtons on the puck for 0.116 seconds. The puck has a mass of 0.110 kilograms and was initially moving directly away from the goal at 3.4 meters per second. How fast does it move toward the goal?
    21.127 or -21.127 is not the correct answer. I am unsure which direction would be considered negative momentum. Here is my work.
    Impulse= 1.95
    1.95=change in momentum
    1.95= mass X change in velocity
    1.95= .110kg X (vf-3.4)
    I divided 1.95 by .110kg and then added 3.4 to the other side. According to my online homework checker this is wrong. Could someone please help me solve this overthought problem? Perhaps something isn't negative when it should be.
  2. jcsd
  3. Nov 7, 2006 #2

    Doc Al

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    Staff: Mentor

    To keep track of signs, do this: Pick one direction (towards the goal, say) and call that positive. Given that, what's the sign of the impulse? the initial velocity?
  4. Nov 7, 2006 #3
    That's great I think I got it. Here is one problem that makes no sense to me even after using the given hint.
    2. A car moving at 13.7 meters per second crashes into a barrier and stops in 0.277 meters.
    a. How long does it take to stop the car? Hint: This is a constant acceleration problem.

    So I used the const acc eq X=.5(vf+vi)t and got

    This can't be right. Where did I go wrong? I tired just multiplying the two givens together and that didn't work either. Was it wrong to assume the final velocity was 0? Thanks for your help.
  5. Nov 8, 2006 #4
    this is correct.
    why dont you like your answer?
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