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Homework Help: Dynamics 2nd law of newton

  1. Aug 23, 2016 #1
    1. The problem statement, all variables and given/known data
    A skier glides down a slope, and has at the bottom of the slope, at the beginning of a horizontal piece, a velocity of 30 m / s. He begins to brake and after 4 seconds later has a speed of 10 m / s. The friction between the skis and the snow is 400 N during braking.
    a. What is the acceleration of the skier during braking?
    b) What is the mass of the skier??

    2. Relevant equations
    F= m x a
    vf=af x t + v(i)

    3. The attempt at a solution
    10m/s= af x 4s + 30m/s
    af = -5m/s^2
    Thats what i thought answer would be but answer should be +5m/s^2
    Why is that ???
    And for mass F= m x a
    m = F/a
    m = 400N / (-5)
    m=-80 kg
    Maybe I have to exchange value for v (initial ) and v (final )??
  2. jcsd
  3. Aug 23, 2016 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Your answer is perfectly fine. The minus sign means that the skier is slowing down. The magnitude of the acceleration is just 5 m/s^2; that's probably what they wanted for the answer.

    Realize that F and a point in the same direction and thus must have the same sign.

    If you use a = -5 m/s^2, then you must use F = -400 N. (But you can just make them both positive, since they are in the same direction.)

    The main thing is to be consistent with signs.
  4. Aug 23, 2016 #3
    Now i understand . Thank you very much
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