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Homework Help: Free-body Diagram Problem

  1. Jun 19, 2009 #1
    1. The problem statement, all variables and given/known data
    A dog pulls a sled with unspecified force at 25° above horizontal. Mass of sled-passenger-rope partile is 80 kg and there is negligible friction. Find the maximum magnitude of force F that can be applied to the rope without lifting the sled off the surface.

    2. Relevant equations
    sum of forces = ma
    normal force is Fn
    mass of particle is m

    3. The attempt at a solution

    Fxnet = Fcos(25) = max
    Fynet = Fsin(25) - 9.8*m + Fn

    In any case where the sled is not lifted, Fynet = 0, since acceleration is in direction of x only (for this problem).

    However, I am confused about how to find maximum force before sled is lifted.
  2. jcsd
  3. Jun 19, 2009 #2
    When you start to lift the sled you have: Fsin(25)= 9.8*m. In that moment, you have no Fn because the sled is not touching the ground.
  4. Jun 19, 2009 #3
    Don't think of being lifted off the ground in terms of the net force, but in terms of the normal. When the normal is 0 or negative, then you have lift-off.

    Though the two approaches are completely equivalent, I find that, personally, the latter is simpler to follow.
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