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Homework Help: Finding minimum angle of friction.

  1. Oct 4, 2007 #1
    1. The problem statement, all variables and given/known data
    A uniform ladder of mass m and length L leans at an angle of theta against a frictionless wall. If the coefficient of static friction between the ladder and the ground is mu, determine a formula for the minimum angle at which the ladder will not slip.

    I have added my own additional information in red.

    2. Relevant equations
    various equations perhaps.

    mu= Ffr/FN
    Ffr is friction force.
    FW is force by the wall

    3. The attempt at a solution
    http://img171.imageshack.us/img171/1830/s8sv4.jpg [Broken]
    this is what I have done. I took my ref point at the top of the ladder.

    (FN)(Lcostheta) + (Ffr)(Lsintheta) = (mass*g)(L/2 costheta)
    I don't know if this approach is correct. I doubt it.

    Can someone give me head start here?...I have been stuck on this problem for a while now. How do I set this up?

    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Oct 5, 2007 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Nothing wrong with this approach. Except for a sign error: the second term is a counter-clockwise torque.

    Fix that error and keep going. Combine this with the friction equation. And with the fact that vertical and horizontal forces must add to zero.
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