Statics Question / Understanding Direction

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Homework Statement



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


Homework Equations



[itex]M = Fd[/itex]

Right Hand Rule

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.
 

Answers and Replies

  • #2
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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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