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Difficult Ladder Against Wall Torque Problem

  1. Sep 26, 2011 #1
    Difficult Ladder Against Wall Torque Problem!!

    1. The problem statement, all variables and given/known data
    An 85 kg person stands on a uniform 6.6 kg ladder that is 3.9 m long, as shown. 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. Using the dimensions in the figure, find the magnitudes of f1, f2, and f3.

    2. Relevant equations
    All forces and torque formulas.

    3. The attempt at a solution
    I found f1 - 898.6 N. I can't find f2 and f3, which I think are supposed to be equal. No matter what I try, I can't seem to get an answer. Here's my work:

    16hok91.jpg

    Help please? Thanks in advance.
     
  2. jcsd
  3. Sep 26, 2011 #2
    Re: Difficult Ladder Against Wall Torque Problem!!

    Have you tried applying f = μN, where f is the frictional force and N is the normal force? Where you given the coefficient of friction, μ?
     
  4. Sep 26, 2011 #3
    Re: Difficult Ladder Against Wall Torque Problem!!

    No; I wasn't given the coefficient of friction. My instructor, for all of our problems relating to torque, has made it clear that we do not need to apply f = μN.
     
  5. Sep 26, 2011 #4
    Re: Difficult Ladder Against Wall Torque Problem!!

    Alright. At the end of your work it looks like you wrote "f3 = 698.1N". Is this not what you were looking for?
     
  6. Sep 26, 2011 #5
    Re: Difficult Ladder Against Wall Torque Problem!!

    That is what I was looking for, but that's an incorrect answer.
     
  7. Sep 26, 2011 #6
    Re: Difficult Ladder Against Wall Torque Problem!!

    Got it. Just one last question; how far up (or down) the ladder does the problem say the man is standing?
     
  8. Sep 26, 2011 #7
    Re: Difficult Ladder Against Wall Torque Problem!!

    That information is not given.

    Here is the diagram that goes along with the problem:

    11-3ae.gif
     
  9. Sep 28, 2011 #8
    Re: Difficult Ladder Against Wall Torque Problem!!

    Great. It is hard to see your work in the picture, but try solving for the torque around the top of the ladder. Show me the equation you set up. I got to a different answer.
     
  10. Sep 28, 2011 #9

    SammyS

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    Re: Difficult Ladder Against Wall Torque Problem!!

    Find the torque (moment) about the contact point at the bottom of the ladder. It doesn't involve f1 or f2 and it doesn't depend
    upon the person's distance up the ladder.
     
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