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Homework Help: Forces and Torque on a block

  1. Apr 6, 2009 #1
    1. The problem statement, all variables and given/known data
    The drawing shows a rectangular piece of wood. The forces applied to corners B and D have the same magnitude and are directed parallel to the long and short sides of the rectangle. An axis of rotation is shown perpendicular to the plane of the rectangle at its center.


    The magnitudes of the forces at corners B and D are each 17 N. The long side of the rectangle is 2.5 times as long as the short side. What are the magnitude and direction of the force FA applied to corner A? Assume that the direction of FA is the same as in part (b).

    2. Relevant equations

    Torque = Force * length

    3. The attempt at a solution

    So originally, I thought that to solve this problem, the torque at A would be equal in magnitude to the net torque from B and D. So I did the following:

    let x = the length of the shorter side; therefore, 2.5x is the length of the longer side
    let n = the force applied at A

    n*x = 17x + 2.5*17*x
    solving this, we can cancel out x, and get that n = 59.5; this is incorrect. I tried a few other things, including n*x + 17x = 2.5*17*x to get n=25.5, but that is also incorrect.
    So I'm confused here, am I misreading the question? I know you guys can't solve the question for me, but maybe you can point me in the right direction...I can only submit the answer to this question one more time before webassign locks it out.

    Thanks :)
  2. jcsd
  3. Apr 6, 2009 #2
    You did not say so, but I presume that this piece of wood is in equilibrium.

    What is part (b)?

    There is no force shown in the figure at A; are to understand that there is one at an angle?

    Is there also a force applied at C?
  4. Apr 6, 2009 #3
    I am also assuming that the wood is in equilibrium, I posted all the information I was given that was pertinent to the question.

    part b was just a multiple choice question, it asked what direction the force should be applied on A. The answer was in the direction to B.

    There is no force on C, it never stated that there was and the diagram doesn't show it.
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