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Statics - Moment using scalar analysis

  1. Oct 21, 2009 #1
    1. The problem statement, all variables and given/known data

    Statics4.png

    2. Relevant equations

    I think that I need to find the moment of the force F about the line BC first.

    Then, using that moment, find the "projection" of [itex]M_{BC}[/itex] onto the X axis to find the answer. [itex]M_{BC}\sin \left(45^{o}\right)[/itex]

    However, I am not getting anything near the solution value.

    3. The attempt at a solution

    My only attempt at solving this that came close to correct was to use the distance from point B to F. That was obviously wrong (but yielded 20 lb - which was extremely close to the 20.2 given by the author.)

    (I know that is wrong...) As soon as I sat the problem down to go to the next one, I immediately realized my method was incorrect, but I have tried and tried to get a correct answer and nothing has yielded in 1.5 hrs, so I digress.

    I have a test this afternoon, so I cannot waste a lot of time on one that is holding me up. Two more chapters to review...

    Any hints will be great!

    TIA
     
  2. jcsd
  3. Oct 25, 2009 #2

    PhanthomJay

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    The moment of a force about the x axis is [tex]F_zy + F_yz[/tex]. Since only F_z is acting, there is no F_y term, so you just have to compute the y distance (the perpendicular distance from the x axis to the line of action of the force). You need to watch your geometry and trig to get the corect y value which yields the correct answer of F = 20.2 N.m.
     
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