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Homework Help: Torque and static equilibrium beam with weights

  1. Oct 31, 2014 #1
    1. The problem statement, all variables and given/known data
    The answer is to be given in terms of L and w. The picture shows a beam of weight w and various blocks on it also each of weight w. It also shows the gravitational force acting downward on the center of gravity of the beam. The beam rests on a triangular pivot point.

    2 blocks of weight w are stacked and sit 2L to the left of the pivot. 1 block sits 1L to the right of the pivot. The center of gravity force is also shown at 1L to the right of the pivot. Another block sits at 3 to the right of the pivot. I am to state why the beam is or is not in equilibrium.

    2. Relevant equations

    3. The attempt at a solution
    The answer key shows that it is in equilibrium. I don’t understand why it is. To me it seems that the spot that has the center of gravity will actually have a force of 2w at the center of gravity-one for the block and one for the beam. This gives -3wL + -2wL +4wL or a net torque of 1wL in the clockwise direction. Where am I thinking incorrectly about this? (I have only a small hope that I am right and the answer key is wrong).

    Thank you!
  2. jcsd
  3. Nov 1, 2014 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Why haven't you written the equations of static equilibrium to determine whether the beam is in equilibrium or not?
  4. Nov 1, 2014 #3
    I thought that is what this is: -3wL + -2wL +4wL=-1wL and since it isn't zero, it isn't in equilibrium.
  5. Nov 1, 2014 #4
    My EE degree son is home for the weekend and he thinks that I am right and the key is wrong. If you disagree please post.
  6. Nov 1, 2014 #5

    Doc Al

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    Staff: Mentor

    Assuming I understand your description of the problem, you are correct and the answer key is wrong.
  7. Nov 1, 2014 #6
    Thank you!
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