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Statics Question / Understanding Direction

  1. Sep 30, 2012 #1
    1. The problem statement, all variables and given/known data

    I'm trying to understand how to determine the sign (+/-) of a given couple. In the sample problem below it shows the 30 lb couple (on the zy-plane) having a negative value for it's force. Is this found by using the RHR? And if so, in which direction do I orient my hand? In the positive direction of the axis it's parallel to?

    C3kmH.jpg

    2. Relevant equations

    [itex]M = Fd[/itex]

    Right Hand Rule

    3. The attempt at a solution

    I tried to depict the two orientations inside of the picture above.

    Thank you for any help / feedback is appreciated.
     
  2. jcsd
  3. Oct 1, 2012 #2
    I think you meant to say that it has a negative value for its moment? If you look at the couple you'll see that it causes a clockwise moment in the z-y plane which means it causes a negative moment in the z-y plane. This means that your thumb should be perpendicular to the z-y plane in order for the RHR to be obeyed and keep the same sign of the moment created.
     
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