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Really need help with this Free Response question for AP Physics!

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  1. Jan 4, 2014 #1
    1. The problem statement, all variables and given/known data

    1. An 80-kg man is one fourth of the way up a 10-m ladder that is resting against a smooth, frictionless wall. If the ladder has a mass of 20 kg and it makes an angle of 60° with the ground, find the force of friction of the ground on the ladder.



    2. Relevant equations

    [itex]T[/itex] = Torque

    ∑[itex]T[/itex] = [itex]T[/itex][itex]_{man}[/itex] + [itex]T[/itex][itex]_{ladder}[/itex] + [itex]T[/itex][itex]_{wall}[/itex]

    3. The attempt at a solution

    Well I tried to find the Force of Friction F[itex]_{f}[/itex] = μn; F[itex]_{f}[/itex] = μmgcosθ:

    mg = μmgcosθ
    (20 kg)(-10 m/s^2) = μ(20 kg)(-10 m/s^2)cosθ
    μ = 1.05

    and

    [itex]T[/itex] = F*r*sinθ
    [itex]T[/itex] = (800 N)(10 m)(sin 60°)
    [itex]T[/itex] = -2438.5 N


    I have no idea which of that is right. I only got 2 out of like 10 points for this Free Response question and I need to fix it, but I'm horrible at Physics and really need some help figuring it out. Thank you!
     
  2. jcsd
  3. Jan 4, 2014 #2

    Doc Al

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    Staff: Mentor

    (1) Start by identifying all the forces acting and drawing yourself a free body diagram.
    (2) Careful! μN gives the maximum possible friction between surfaces, which is not what you want; You want the actual friction, which will be lower. Just label it Ff; you'll solve for it.
    (3) Apply the conditions for equilibrium for both translation and rotation.
    (4) Pick your 'axis' for calculating net torque wisely.
     
  4. Jan 4, 2014 #3
    I'm sorry but that didn't help at all. I don't even know how to draw a free body diagram. I have absolutely no idea what to even do.
     
  5. Jan 4, 2014 #4

    Doc Al

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    Staff: Mentor

    Do you have a textbook?

    Draw a diagram of the ladder showing all the forces acting on it.
     
  6. Jan 4, 2014 #5
    Yes and I've tried to use it but just makes no sense to me.

    What are the forces? My guess would be normal force and the force of weight. What about tension? I never know whether to include that or not.
     
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