Finding moment, relative to a different axis

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SUMMARY

The discussion focuses on calculating moments in a three-dimensional system involving two cords attached to point C, with tensions |TCB|=380N and |TCA|=450N. The user successfully derived the moments MTCA and MTCB using the formula M=MTCA+MTCB, where MTCA=A×TCA and MTCB=B×TCB. The challenge lies in determining the moment relative to the axis OB, which requires establishing a new axis system where OB is the y-axis. The user proposes using angles and distances to calculate the moments around OB, specifically questioning the validity of their approach regarding the contributions of the x-axis.

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given the following system:
2 cords are attached to point C,
http://picasaweb.google.com/devanlevin/DropBox?authkey=Gv1sRgCL_4l4PpvP_YsQE&pli=1&gsessionid=oBksCECRwcHSRoX-oRPaRA#5329455433140212770

and knowing that
|TCB|=380N
|TCA|=450N

find:
1) the moment relative to x,y,z
2) the moment relative to the axis 'OB'

for the 1st part, what i did was
M=MTCA+MTCB
and i found that
MTCA=A×TCA
MTCB=B×TCB


now comes the problem, how do i find the momentum relative to OB, i know that i need to make a new axis system, where OB is my "y" axis, and an axis 90degrees to it(63.43° from z axis) will be my new "z" axis, while x will stay the same,
i found the angle of my new axis relative to the original system

α=angle between 'y' and OB
α=26.565°

to find the moment, i need

1) force Tx (which is the same force Tx i used before)
2) the shortest distance dz between Tx and my new z axis
3) force Tz
4) the shortest distance dx between Tz and my new x axis

i am not at all sure that this is correct, if you can see a better way let me know.
 
Last edited by a moderator:
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am i right in saying:

since the axis OB passes through the origin O, the moment relative to axis OB will be the same as the moment relative to O ?
BUT THAT DOESNT LOOK RIGHT TO ME,
i don't think that Mob should have any magnitude on the X axis since the line OB is on the y-z plane:

i thought that maybe i could do this:

since i can find the angle of OB, (comes to 26.565 degrees) i can say that since OB is on the YZ plane, the moment can only be My and Mz, so the moment around OB is:

(My/cos(26.565))j + (Mz/sin(26.565))k

is this correct??
 
Last edited:

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