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Homework Help: Will the impulse be negative?

  1. Dec 16, 2011 #1
    1. The problem statement, all variables and given/known data
    A car with a mass of 1.0 X 103kilograms is moving with a speed of 1.4 X10 2m/s. The impulse required to bring the car to rest is?

    2. Relevant equations

    impulse = change in momentum...

    3. The attempt at a solution
    the answere is 1.4 X 105 N X S or is it the negative....
  2. jcsd
  3. Dec 16, 2011 #2


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    Homework Helper

    Technically the impulse must be negative to stop the positive velocity.
    But it is so obvious that the minus sign is often omitted in reporting the answer. Like getting a bill for car repairs of $250. Your payment should really be -$250 to cancel it out, but we never bother saying that.

    Velocity is usually defined as being a vector, so you are aware of its direction. But speed is not a vector, only the magnitude of velocity so when asked for speed no sign or direction is required. Impulse, acceleration and momentum don't have separate words for the magnitude so it often isn't clear whether the question requires a sign indicating direction. Both kinds of answer should be accepted in the absence of clarity.
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