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Homework Help: Two friction system

  1. Sep 25, 2010 #1
    1. The problem statement, all variables and given/known data
    A man of 72.0kg is pushing a heavy box along a flat floor. The coefficient of sliding friction between the floor and the box is 0.20, and the coefficient of static friction between the man's shoes and the floor is 0.65N.

    a) If the man pushes downward on the box at an angle of 30 degrees, what is the maximum mass of the box be can move?

    b) If the man pushes upward on the box at an angle of 30 degrees, what is the maximum mass of the box he can move?


    2. Relevant equations
    fk=ukN


    3. The attempt at a solution
    Ok, so I've set up the free body diagrams for the man and box.

    For the man:
    Fy= N - mg + Psin(30)
    Fx= Pcos(50) - ukN, subbing in N= mg-Psin(50)
    Fx= Pcos(50) -ukmg + ukPsin(50)=ma

    The equations for the box are all similar. Lacking for what to do next!
     
  2. jcsd
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