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Inclined plane problem strange result

  1. Oct 8, 2016 #1
    1. The problem statement, all variables and given/known data
    7. A boy drags his 60.0-N sled at constant speed up a 15.0° hill. He does so by pulling with a 25.0-N force on a rope attached to the sled. If the rope is inclined at 35.0° to the horizontal,
    (a) What is the coefficient of kinetic friction between sled and snow?
    (b) At the top of the hill, he jumps on the sled and slides down the hill. What is the magnitude of his acceleration down the slope?
    2. Relevant equations
    See Below
    3. The attempt at a solution
    For question b, I ended up getting a result that should mean the sled is accelerating up the hill at ##2.29m/s^2##. Can someone look over my work? I was not sure what "If the rope is inclined at 35.0° to the horizontal" meant. I assumed it meant that the angle was relative to tilted axes.
  2. jcsd
  3. Oct 8, 2016 #2


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    The tilted axis is not horizontal so that's not right.

    The hill is 15.0° relative to the horizontal.
    The rope is 35.0° relative to the horizontal.
    So the rope must be at what angle relative to the hill?
  4. Oct 8, 2016 #3
    Okay, that makes more sense.
  5. Oct 9, 2016 #4


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    That's right.
    Ignoring that your ##\mu## is wrong your working looks right for b) and your solution is valid.
    Remember than generally:
    ##f_{kinetic} < f_{static}##
    ##f_{static} \leq \mu N ##

    In words, the sled wouldn't accelerate up the hill, it'd just sit there without moving. The boy would have to find a steeper hill or fix his sled.
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