Understanding Net Force and Net Moment in Statics

AI Thread Summary
The discussion focuses on understanding the differences in net force and net moment between two free body diagrams (FBDs). One participant calculates a moment of 120N-mm from a 10N force but struggles to see why the net moments differ. It is clarified that the net moments can be the same if the same axis is chosen for calculation, but they may vary with different axes. The conversation emphasizes that both net force and net moment can differ between the two FBDs. Overall, the key takeaway is that the choice of axis significantly impacts the calculation of moments in statics.
WesTech
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Homework Statement

statiics.PNG

Homework Equations


I can see how the net force is not the same but I'm not getting the net moment.

The Attempt at a Solution


12mm x 10N = 120N-mm. How do these two FBDs not have the same net moments.

Thanks for any help!
 
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WesTech said:

Homework Statement

View attachment 110384

Homework Equations


I can see how the net force is not the same but I'm not getting the net moment.

The Attempt at a Solution


12mm x 10N = 120N-mm. How do these two FBDs not have the same net moments.

Thanks for any help!
Welcome to the PF. :smile:

What is the actual problem statement? Something about showing the two situations do not have the same net forces and net moments?
 
WesTech said:
How do these two FBDs not have the same net moments.

If I'm reading this right, it looks like the FBD on the right is just the FBD on the left, except it's showing you the moment caused by the 10N force. Or in other words, the 10N force is still there they're just not showing it and the net moment is the same on both. Of course I'm not sure what the actual problem statement is, so...
 
WesTech said:
How do these two FBDs not have the same net moments.
In general, moment depends on the chosen axis. If you choose the left hand end of the bar as the axis then the moments will be the same, but maybe not for other choices.
 
I think haruspex may be correct. They are two separate FBD and the answer to the question is that neither their net force or moment are the same. I may need to over this again.

If no new comments are posted, thanks for your time!
 

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