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Homework Help: Hanging sign, torque and pivot points

  1. Nov 14, 2007 #1
    1. The problem statement, all variables and given/known data

    There is a sign of mass M that is attached to a rigid bar perpendicular to the ground. There is also a rope attached to the sign at the same point, then pulled out at an angle 45 degrees below the horizontal up to the ceiling.

    a) Consider the rigid bar to be at equilibrium while the sign is hanging. What are the net force (in Newtons) and the net torque (in N m) acting on the bar?

    b) Identify all forces that act on the bar. Write out an equation for the net force on the bar.

    c) Choose the point where the bar meets the wall as a pivot point. Write out an equation for the net torque on the bar about that point.

    d) Determine the magnitude of the tension in the rope. Take the mass of the sign to be 25kg, the mass of the bar is 5kg, and the length of the bar is 2m.

    e) We could choose any point as the pivot and still get the same net torque. Why was the choice in part c a good choice for the pivot?

    2. Relevant equations

    T_net = Ia
    when in equilibrium,
    -- F_net = 0
    -- T_net = 0

    3. The attempt at a solution

    I got that, for (a), these are both equal to zero, as the bar is in equilibrium...don't know where to go from here.
  2. jcsd
  3. Nov 15, 2007 #2

    Shooting Star

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    Homework Helper

    If you give a rough diagram, I'm sure you'll get some responses. I am unable to visualize the whole picture.

    Anyway, in all these kinds of statics problems, the way to proceed is to equate the sum of the horizontal and vertical forces individually to zero; and to equate the moment of all the forces about a conveniently chosen point to zero. Then solve the resulting equations.
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