Understanding Net Force and Net Moment in Statics

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Homework Help Overview

The discussion revolves around understanding the concepts of net force and net moment in the context of statics, specifically through the analysis of free body diagrams (FBDs). Participants are exploring why two FBDs do not yield the same net moments despite having differing net forces.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants are attempting to understand the relationship between net forces and net moments in the context of two FBDs. Questions are raised about how the moments can differ based on the chosen axis and the implications of the forces represented in the diagrams.

Discussion Status

The discussion is ongoing, with participants providing insights and questioning the assumptions made about the FBDs. Some guidance is offered regarding the dependence of moments on the chosen axis, but no consensus has been reached regarding the specific problem statement or the differences in net moments.

Contextual Notes

There is a lack of clarity regarding the actual problem statement, which may be contributing to the confusion about the net forces and moments. Participants are also reflecting on their understanding and may need to revisit the concepts discussed.

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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