Understanding Moment Equations in Rigid Body Equilibrium

[cursive]

I have uploaded the question with solution at this address: http://i41.tinypic.com/o5xm5j.jpg

All I need to know is why in the moment equation "NA*a - F*b = 0" the forces Nb and Nc aren't included. Is it because the moment is being calculated about the dot in the middle, and the other two forces are pointing at it?
If the Nc force wasn't pointing directly at that center point (and was pointing slightly above), would I have to calculate the net moment including the horizontal and vertical components of Nc from point C?
When do I know not to include a force in the moment calculation?

Thanks!

 

[Thaakisfox]

If the of the force passes through the axis then it doesn't exert any torque on the axis. Think of it as how would the torque rotate about the given axis ;)

 

[srvs]

Your assumption is right, you do not include it because the line of action of that force goes through the point around which you calculate the moment. If it goes through it then the perpendicular distance from the line of action to the point, moment arm, would be zero. Intuitively you can also see that it just wouldn't cause any rotation.

 

[cursive]

I guess I got so caught up in formula's 'n such that I forgot what was really going on.

Thank you both!