Understanding Statics Diagram Problems: Moment Calculations Explained

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SUMMARY

This discussion focuses on understanding moment calculations in statics diagrams, specifically regarding the forces acting on a pivot point. The 800N downward force is classified as positive, while the Cy reaction force is negative due to their opposing directions relative to the pivot at point A. The confusion arises from misinterpreting the rotation direction caused by these forces; the 800N force induces clockwise rotation, while the Cy force, acting in the opposite direction, induces counterclockwise rotation. Clarifying the pivot point is crucial for accurate moment analysis.

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Students and professionals in engineering, particularly those studying mechanics or civil engineering, will benefit from this discussion. It is also valuable for anyone involved in analyzing forces and moments in static systems.

Jason03
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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...
 

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Jason03 said:
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.
 
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...
 

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