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Homework Help: A sled slides down a hill

  1. Oct 31, 2014 #1
    1. The problem statement, all variables and given/known data
    The length of a slope is 500 meters and it's height is 10 meters. 60 kg sled went down the slope. Find the average friction force if it's speed in the end = 8 m/s (the starting speed = 0)

    2. Relevant equations
    Ft=mv-mv0
    t=S/v

    3. The attempt at a solution
    Ft=mv-mv0=60*8=480
    Average speed = (8+0)/2=4
    t=500/4=125
    F=480/125=3.84 N
    Weight=mg=600
    600-friction=3.84
    friction=596.16 N

    Is it correct?
     
  2. jcsd
  3. Oct 31, 2014 #2

    haruspex

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    You have calculated a propulsive force (F), right?
    What direction does that act in?
    What direction does mg act in?
    (You might have noticed that you have not used the 10m height.)
     
  4. Oct 31, 2014 #3
    1.I think so, it's the force that accelerates the sled
    2.Downwards
    3. I do not really see use for the height of the slope

    It looks like it makes sense, since I know the mass and acceleration I can calculate the overall force F, and friction works against mg, so the equation is correct, right?
     
  5. Oct 31, 2014 #4

    haruspex

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    As in vertically down? You obtained it by dividing momentum by time. What direction was the momentum in?
     
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