Understanding Statics Diagram Problems: Moment Calculations Explained

In summary, the conversation is about understanding the moments found about point A in a problem and how the 800N force downward, Cy reaction, and Cx are considered positive or negative. The confusion arises from the fact that the 800N force and Cy are in opposite directions, but would cause the same rotation direction. However, this misunderstanding is cleared up by realizing that point A is the pivot and the direction of rotation would be opposite for the 800N force and Cy.
  • #1
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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  • #2
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.
 
  • #3
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...
 

1. What is a moment in statics?

A moment in statics is a measure of a force's tendency to cause rotation around a specific point or axis. It is calculated by multiplying the force by the distance from the point or axis of rotation.

2. How do I calculate the moment of a force?

To calculate the moment of a force, you need to first determine the magnitude and direction of the force. Then, you need to choose a point or axis of rotation and measure the perpendicular distance from that point to the line of action of the force. Finally, multiply the force by the distance to calculate the moment.

3. What is the difference between a positive and negative moment?

A positive moment indicates a clockwise rotation, while a negative moment indicates a counterclockwise rotation. This is based on the right-hand rule, where the direction of rotation is determined by the direction of the force and the direction of the moment vector.

4. How do I solve moment equilibrium problems?

To solve moment equilibrium problems, you need to set up an equation where the sum of the moments of all forces acting on the system is equal to zero. This equation can then be solved for an unknown force or distance.

5. What are common mistakes to avoid when solving statics moment problems?

Some common mistakes to avoid when solving statics moment problems include not properly identifying the point or axis of rotation, not considering the direction of the moment, and not using the correct units for force and distance. It is also important to check your calculations and make sure they are accurate before proceeding with the problem.

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