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Stuck on ladder problem, know the solution but need an explanation

  1. Dec 7, 2008 #1
    1. The problem statement, all variables and given/known data
    An 85 kg person stands on a uniform ladder that is 4.3 m long and weighs 65.0 N. The floor is rough; hence it exerts both a normal force, f1, and a frictional force, f2, on the ladder. The wall, on the other hand, is frictionless; it exerts only a normal force, f3. Use the dimensions in the figure to find the following. (a = 3.8 m.)

    2. Relevant equations

    I know the solution but I don't understand why f3(3.8)-mg(2.15)=0. How is this a correct expression of the torque?

    3. The attempt at a solution
  2. jcsd
  3. Dec 7, 2008 #2


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    It is not. The correct expression for torque about the base of the ladder must include f3, the man's weight, and the ladder weight, using the appropriate perpendicular diatances from those forces to the ladder base to calculate the respective torques and signs of the torques. You are missing a few dimensions.
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