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Moment reaction with rotation and offset CG

  1. Mar 11, 2015 #1
    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!

    Attached Files:

  2. jcsd
  3. Mar 11, 2015 #2


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    Gold Member

    I presume the box is standing on something? I cannot see an upward reaction force.
  4. Mar 11, 2015 #3
    I apologize, yes, there is a normal force reaction and the box is sliding across a frictionless floor.
  5. Mar 16, 2015 #4
    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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