Calculate Resultant Couple from 3 Couples in 3D

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Discussion Overview

The discussion revolves around the calculation of a resultant couple from three given couples in a three-dimensional space. Participants explore how to specify the magnitude and direction of the resultant couple's axis using angles relative to the positive x, y, and z axes. The context appears to be homework-related, focusing on the application of vector mathematics in mechanics.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant expresses uncertainty about how to resolve moments into force components without specific measurements, noting the importance of setting a position vector 'r'.
  • Another participant clarifies that the moment of a couple is independent of 'r' and can be represented by a vector perpendicular to the plane of the couple, following the right-hand rule for direction.
  • A participant provides calculated components for the couples and derives the resultant couple's components, magnitude, and angles with respect to the axes.
  • Concerns are raised about the sign conventions used in the calculations, particularly regarding the direction of the y component.
  • A later reply acknowledges the sign error and appreciates the correction made by another participant.

Areas of Agreement / Disagreement

Participants generally agree on the methodology for calculating the resultant couple, but there is a disagreement regarding the sign conventions for the components, particularly the y component. The discussion remains unresolved regarding the implications of these sign conventions.

Contextual Notes

There are limitations related to the assumptions made about the direction of the axes and the interpretation of positive and negative components, which have not been fully resolved in the discussion.

wannawin
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Homework Statement


Replace the three couples with a single resultant couple. Specify its magnitude and direction of its axis using angles to the positive x, y, and z axes.

yphi0.jpg



Homework Equations



M = r x F (r cross F)

The Attempt at a Solution



I'm pretty sure I want to resolve the moments into their force components, but to do that I would need to set an r. But since I have no measurements other than the angle of inclines I don't see how I can do that.
 
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wannawin said:

Homework Statement


Replace the three couples with a single resultant couple. Specify its magnitude and direction of its axis using angles to the positive x, y, and z axes.

yphi0.jpg



Homework Equations



M = r x F (r cross F)

The Attempt at a Solution



I'm pretty sure I want to resolve the moments into their force components, but to do that I would need to set an r. But since I have no measurements other than the angle of inclines I don't see how I can do that.
The nice thing about couples is that they are independent of 'r'...the moment of a couple about any point is the couple itself. The couple can be represented by a vector pointing perpendicular to the plane of the couple, following the right hand rule method for its direction (+ or - ) perpendicular to the plane. Then solve the resultant couple as a vector located anywhere on the plane, but with a certain magnitude and direction.
 
Then I guess this question isn't that complicated at all. If you number the couples 1-3 going from left to right:
M1x=0
M1y=1.5cos20
M1z=1.5sin20

M2x=0
M2y=1.5
M2z=0

M3x=0
M3y=1.75cos25
M3y=-1.75sin25

Therefore MRx=0
MRy=4.496
MRz=-0.2265

MR has a magnitude of 4.5012

And the angles are 0 with the +x axis, 2.91 with the +y axis, and 87.1 with the -z axis (using simple trig)

Barring any issues with significant digits, I think that makes some sense since the resultant is almost vertical.
 
You may have your plus/- signs mixed...the y component points down, are you calling that the positive y axis? Otherwise, your work is very good.
 
PhanthomJay said:
You may have your plus/- signs mixed...the y component points down, are you calling that the positive y axis? Otherwise, your work is very good.

I did. Good catch and thanks a lot
 

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