Understanding Statics Diagram Problems: Moment Calculations Explained

[Jason03]

I have a diagram below for the problem...what I am trying to understand is when the moments are found about point A as they are in the solution...the 800N force downward is considered positive and the Cy reaction is considered negative...and the Cx is considered positive...

What confuses me is why the 800N is positive and the Cy negative...they are in opposite directions but they would cause the same rotation direction in my eyes...I must be looking at it wrong...

 

[Doc Al]

What confuses me is why the 800N is positive and the Cy negative...they are in opposite directions but they would cause the same rotation direction in my eyes...I must be looking at it wrong...

You must be. If you imagine a pivot at end A, which direction would the 800 N force cause the rod to rotate? Since Cy is on the same side of the pivot but points in the opposite direction, it must cause it to rotate in the opposite sense.

 

[Jason03]

No i see now...For some reason I wasnt seeing A as the pivot point...when I thought of the 800N downwards I was seeing it as going counterclockwise but that would not be true because point A is a pivot holding it in place causeing it to rotate clockwise...