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Homework Help: Finding Minimum force

  1. Dec 4, 2008 #1
    1. The problem statement, all variables and given/known data

    A Small boy is sledding on a snowy slope and looks very weary as he drags his sled up again after each run. Each time he pulls the sled up he makes an angle of 30 degrees with respect to the ground. The mu of kinetic friction is 0.10. At what angle should the boy be pulling to exert minimum force?

    2. Relevant equations

    3. The attempt at a solution

    By way of plugging and chugging various numbers, I believe that the angle should be 0. But to get a difinitive answer,our teacher hinted that I would need to use calculus, but I'm not readily famailar with exactly how to apply a sum of all forces equation in that manner.
  2. jcsd
  3. Dec 5, 2008 #2


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    Homework Helper

    Welcome to PF.

    I would suggest that you first draw a diagram of the sled/boy/slope.

    Consider that the slope is at angle φ and let the boy be pulling (relative to the slope) at angle θ, such that the normal force on the sled would be given by m*g*cosφ (less the θ component of lift from the boy) and the downward force of gravity would be given by m*g*sinφ less the cosine force of the pull from the boy. .

    From that you should be able to create an equation that identifies the Work done to carry himself up the hill, and the work needed to pull the sled up the hill against friction. (Don't forget he is adding to his weight and hence his work in getting up the hill as he pulls at angle θ.)

    Ultimately you will want to identify, for any particular φ the minimum of the function of θ in the expression of work.
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