Moment of a Force About a Point

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SUMMARY

The discussion clarifies the conditions under which a force creates a moment about a point, specifically addressing the force F in relation to point A. It establishes that force F does not produce torque at point A when the line of action passes through A, despite the presence of an offset pivot point O. The opposing force at O is crucial in determining the torque around point A, as it influences the direction and magnitude of the moment. The discussion emphasizes the importance of understanding the geometry of forces and pivot points in three-dimensional scenarios.

PREREQUISITES
  • Understanding of basic mechanics principles, including force and torque
  • Familiarity with the concept of moments in physics
  • Knowledge of pivot points and their role in rotational motion
  • Ability to interpret 3D diagrams and forces in a spatial context
NEXT STEPS
  • Study the principles of torque and its calculation in various scenarios
  • Learn about the role of pivot points in rotational dynamics
  • Explore the concept of line of action and its impact on force application
  • Review examples of 3D force diagrams to enhance spatial reasoning skills
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Students studying physics, particularly those focusing on mechanics, engineers dealing with rotational systems, and anyone interested in understanding the dynamics of forces and moments in three-dimensional space.

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Homework Statement



I'm a bit confused on when a force creates a moment about a point, and when it does not. In particular, in the attached diagram, would F be able to produce a moment about point A? I initially thought that wouldn't be possible as A lies on the same line as F, but since F does not seem to pass directly through A, I'm not so sure.
 

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The force, F, alone does not produce torque at A. The fact that there is an offset stationary pivot point, O, means that there is a force at O opposing F which produces a torque around A. (You can see that if the pivot point, O was moved to the other side of the force at A, then the torque would switch to the opposite sign. So it is the opposing force at O that is controlling the torque.)
 
Note that the line of action of F does pass through A. A is the point or hole where the rod meets the bar. 3D drawings can get very confusing.
 

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