Understanding Net Force and Net Moment in Statics

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SUMMARY

The discussion centers on the differences in net force and net moment between two Free Body Diagrams (FBDs) in statics. A participant calculates a moment of 120N-mm using a 10N force applied at a distance of 12mm. It is concluded that the net moments can be the same if the axis of rotation is chosen appropriately, but the net forces will differ between the two FBDs. The importance of the chosen axis in determining moments is emphasized.

PREREQUISITES
  • Understanding of Free Body Diagrams (FBDs)
  • Knowledge of net force and net moment concepts
  • Familiarity with moment calculations in statics
  • Basic principles of equilibrium in mechanics
NEXT STEPS
  • Study the principles of moment calculation in statics
  • Learn about the significance of the axis of rotation in determining net moments
  • Explore examples of Free Body Diagrams and their applications
  • Investigate the relationship between net force and net moment in static systems
USEFUL FOR

Students and professionals in engineering, particularly those studying mechanics, statics, or structural analysis, will benefit from this discussion.

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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