# Moment relative to a different axis

1. Apr 28, 2009

### Dell

if i have a case of a number of forces all working in 3d space, for example the following diagram of 2 tension forces

and i am asked to find the total moment relative to an axis different to (x, y, z) axis, in this case, i am asked to find the moment relative to the 'OB' axis.

can i say that since the line OB passes through the origin O, the moment relative to OB will be the same as the total moment relative to my original axis'?

what i did was say that the moment is the cross product of ==> R x F
R= distance from the axis OB to the line of the force
F= Force

so i could take any point on the axis and any point on the line of the force, for example, take point O from axis OB, and point C on the line of the force and i get R=(OC) now that is the exact cross product i used to find the moment around O,
in the end i get x, y, z, display for my moment,

M=1619i - 2699j + 2258k

is that the correct way to display my answer or is there some way i can get it to one number which is the total moment only on the OB axis

BUT THAT DOESNT LOOK RIGHT TO ME,
i dont 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 by a moderator: May 4, 2017