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Homework Help: Sled on a hill with friction

  1. Sep 26, 2010 #1
    1. The problem statement, all variables and given/known data
    A loaded penguin sled weighing 66 N rests on a plane inclined at angle θ = 18° to the horizontal (Fig. 6-23). Between the sled and the plane, the coefficient of static friction is 0.27, and the coefficient of kinetic friction is 0.18. (a) What is the minimum magnitude of the force , parallel to the plane, that will prevent the sled from slipping down the plane? (b) What is the minimum magnitude F that will start the sled moving up the plane? (c) What value of F is required to move the sled up the plane at constant velocity?


    2. Relevant equations
    Fk=uFn
    Fsmax=uFn

    3. The attempt at a solution
    a) I know the acceleration of the sled would be negative but if I set it up like
    Fa-Fs=-ma... it can't be solved

    b) Fsmax=uFn
    =.27(6.73)(9.8)
    =17.81

    c)Fa-Fk=ma
    Fa-Fk=0
    Fa=Fk
    Fa=uFn
    =11.87N
     
  2. jcsd
  3. Sep 26, 2010 #2

    Doc Al

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    Staff: Mentor

    What forces act on the sled parallel to the plane? There are three, one of them being the applied force that you need to solve for. What are the other two? What must they add to for the sled to not move?
     
  4. Sep 26, 2010 #3
    I don't see the three forces that act on it....the applied force, the static friction force, and?
     
  5. Sep 26, 2010 #4

    Doc Al

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    Staff: Mentor

    What force is always there, acting on any mass? (What force acts to make it slide down the hill?)
     
  6. Sep 26, 2010 #5
    okay so ...
    Fa-Fs-Fgcostheta=-ma

    I don't see how this can be solved though
     
  7. Sep 26, 2010 #6
    you need to sum the forces in the Y as well, so you can find the normal force
     
  8. Sep 26, 2010 #7

    Doc Al

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    Staff: Mentor

    What direction does friction act? What does the friction force equal? What's the 'cos theta' business? What's the acceleration? (Remember--you are preventing it from sliding.)
     
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