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Homework Help: Moment relative to a different axis

  1. Apr 28, 2009 #1
    if i have a case of a number of forces all working in 3d space, for example the following diagram of 2 tension forces

    http://picasaweb.google.com/devanlevin/DropBox?authkey=Gv1sRgCL_4l4PpvP_YsQE&pli=1&gsessionid=oBksCECRwcHSRoX-oRPaRA#5329455433140212770 [Broken]

    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

    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
  2. jcsd
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