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Force and Torque

  1. Nov 9, 2008 #1
    1. The problem statement, all variables and given/known data
    A 10 kg plank leans against a frictionless wall and floor at an angle of 60 degrees with the horizontal. A horizontal rope connects the center of mass of this uniform plank with the wall.
    a- Use dynamics to find the normal force between the floor and the plank.
    b- Find the normal force between the wall and the plank.
    c- Determine the tension in the rope.
    d- Show the net torque is zero when the pivot is located where the plank meets the wall.

    2. Relevant equations

    torque = Fsin(phi) radius

    3. The attempt at a solution
    a- normal floor = weight = 10kg (9.8) = 98 N
    b- Using torque, I end up with -56.6.

    -torque of normal floor + torque of normal wall = 0
    98 N (sin 210) length/2 = normal wall (sin 120) length /2

    normal wall = -56.6
    What am I doing wrong?
    Last edited: Nov 9, 2008
  2. jcsd
  3. Nov 9, 2008 #2
    Using the point where the plank rests on the ground as the point of rotation, a torque due to tension and one due to gravity act in the negative direction, both of which are negated by a normal force imposed upon the plank by the wall in the clockwise direction. Does this help?
  4. Nov 10, 2008 #3
    If I use that as the rotation spot, I don't think I will have enough info to solve. I don't know the tension force or the length of the plank. I was using the center as the pivot to avoid lookig at tension and weight.
  5. Nov 10, 2008 #4
    I tried using that spot and this is what i came up with:

    torque tension + torque weight - torque normal floor = 0
    56.6 N * sin 240 * length/2 + 98 N * sin 150 * length/2 - 98 N * sin 210 * length = 0

    But this equation leaves me with 49N * length, not 0.
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