# Statics Question / Understanding Direction

1. Sep 30, 2012

### Screen

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?

2. Relevant equations

$M = Fd$

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. Oct 1, 2012

### cmmcnamara

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.