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Static, Moment

  1. Sep 8, 2008 #1
    1. The problem statement, all variables and given/known data
    The wall crane in Fig. 3 supports a load of 800 N. Determine the horizontal and vertical
    components of reaction at the pins A and D. Also, what is the force in the cable at the winch
    [URL=http://imageshack.us][PLAIN]http://img182.imageshack.us/img182/992/tutorialse0.jpg[/URL] http://g.imageshack.us/g.php?h=182&i=tutorialse0.jpg [Broken][/PLAIN]

    2. Relevant equations
    Newton's Laws of Motion, Moment, Equilibrium of Forces

    3. The attempt at a solution
    First I draw a free body diagram at B. The vertical forces involved are the y component of the tension in rod BD and the y component of the tension of cable WB.

    Since the tension of a massless cable is the same throughout the string, the tension of the cable is 400N. Substituting the value of TWB to the above equation, I get TBD=490N.

    However, a problem occurs when I attempt to find TBD by taking the moment at A. The solution for TBD turns out to be 2752N.

    What cause the difference in the answer?
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Sep 8, 2008 #2


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    Homework Helper

    I think it will be easier for us to see where you went wrong if you would show some work with regard to the step on substituting the value of Twb.

    My suggestion to get Tbd, now that you have the tension in the rope right is to take the moments at A. The sum of moments must be zero. You have a 800N weight acting at 2 meters CW, and Tbw acting at 1 meter at a 60 degree angle CW. The counter moment CCW then must be Dx.
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