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Friction and Normal Force

  1. Nov 24, 2013 #1
    1. The problem statement, all variables and given/known data


    Three points, ABC and a downward force exerted at O. C is pinned to a vertical wall. C to B extends horizontally for 15m. At B, a bend occurs downwards to point A on the ground. The downward force is applied between points B and A at point O. The horizontal distance from B to O is 5m and from O to A is 2m.

    The force applied at O is 40N downwards towards the ground.

    i) Determine the friction and normal force acting at A
    ii) Determine the minimum uk required for the bar not to move.

    2. Relevant equations

    Sum of all Fx and Fy must equal 0. Friction=(uk)Fn, Moment=F*d?

    3. The attempt at a solution

    I am just learning this topic and have spent a great deal of time looking at friction on inclined planes. When I came to this question, I simply am confused where to begin. Calculating the moment calculates a force representing what when it comes to friction? Looking for a solution to guide me though how to begin analyzing these types of problems. Thank you.
  2. jcsd
  3. Nov 24, 2013 #2


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    Are we to assume (or were you told) that there is a frictionless hinge at B?
    Otherwise friction at A doesn't seem to come into the problem.
  4. Nov 25, 2013 #3
    Unfortunately, it does not say. So yes, I suppose we assume there is friction at B. But we do not know the coefficient? Or does it matter?
  5. Nov 25, 2013 #4


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    The question 'works' if it's a rigid bar, not hinged at C, and assuming same coefficient of friction at both contacts.
    It also works if hinged at B and C, but then you'd also need to know the height.
  6. Nov 25, 2013 #5
    So it's not hinged at c, but where does one begin the analysis?
  7. Nov 25, 2013 #6


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    Assign (unique) names to the unknown normal forces. If it slips, it must slip at both contact points, so in the limiting case you can assume each is at the limit of static friction. Now you can write down the three usual statics equations - horizontal, vertical and torque. But the torque equation will involve the height, another unknown, so that won't be useful here.
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