Moment reaction with rotation and offset CG

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Discussion Overview

The discussion revolves around the calculation of moment reactions in a mechanical system where the center of gravity (CG) is offset. Participants explore the implications of this offset on reaction forces and moment calculations, focusing on the accuracy of hand calculations compared to program outputs. The context includes theoretical and practical aspects of mechanics, particularly in relation to rotational dynamics.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested, Conceptual clarification

Main Points Raised

  • The original model with a uniform CG produced accurate moment reactions, verified through hand calculations and cross products of reaction forces.
  • With the CG offset, the participant observed discrepancies between hand calculations and program predictions, noting a significant error margin of 55% in the moment calculation.
  • There is uncertainty about whether additional reaction cross products are needed due to the new configuration of forces with the offset CG.
  • The participant expresses confusion regarding how to account for the added rotation of the box resulting from the offset CG.
  • One participant inquires about the presence of an upward reaction force, suggesting a need for clarification on the system's constraints.
  • The participant confirms the presence of a normal force reaction and mentions that the box is sliding on a frictionless surface.
  • Another participant questions whether the thread is posted in the appropriate forum, indicating a concern about the framing of the problem.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the correct approach to calculating moments with an offset CG, and multiple viewpoints regarding the implications of the offset and the necessary calculations remain. The discussion is unresolved, with ongoing inquiries and clarifications sought.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about the system's constraints, such as the frictionless nature of the surface and the implications of the offset CG on rotational dynamics. The exact nature of the forces acting on the body and their contributions to the moment calculation are not fully detailed.

abrooski
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Hey folks,

I created and verified a model with hand calculations, and decided to re-use my same logic.. Which now is giving me false numbers. Was wondering if you guys could help me out.

Setup/Verification (See attached Reaction_Forces picture):
  • Body in motion being held down by straps.
  • Uniform Center of Gravity (CG) in model.
  • Reaction forces (in red) are found in top 4 corners, x,y,z for each.
  • Moment Force is verified by summing cross product of each force reaction (x,y,z) and its relative locations from body CG. Resulting moment is 0.5% off my program approximation. Great!
    • [reaction 1 cross product] + [R2 crossP] + [R3 crossP] + [R4 crossP] = One axis moment reaction. Perfect.
Problem (See attached NEW_CG):
  • Same body in motion. CG is now offset from exact middle of body. New CG.
  • Reaction forces adjust accordingly as expected. Nice.
  • Moment is predicted by program with a sizeable addition in one of the 3 axis.. Roughly equivalent total though.
  • Hand calculated Moment, using same method as above with cross product of reaction forces and relative locations from NEW CG, is completely wrong. 55% wrong.
    • Are there more than 4 reaction cross products now?
Is there something I'm not taking into account? The body is now rotating slightly, I understand there will be 2 axis forces for moment now.. But my hand calculations are so far off I feel like I'm missing something.

Any help would be appreciated!
 

Attachments

  • Reaction_Forces.jpg
    Reaction_Forces.jpg
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  • New_CG.jpg
    New_CG.jpg
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I presume the box is standing on something? I cannot see an upward reaction force.
 
I apologize, yes, there is a normal force reaction and the box is sliding across a frictionless floor.
 
Did I post this in the correct forum? I haven't been able to figure this out yet.. Approaching the problem correctly is my greatest concern. I don't know how to account for the added rotation of the box do to the offset center of gravity. Anybody have any advice for me?
 

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