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The direction of moments (no specific problem, just a confusion)

  1. Feb 19, 2014 #1
    Hi.

    For whatever reason I'm really struggling with figuring out the direction of a moment in two dimensional statics problems. How can I tell if it's CW or CCW? The right hand rule just confuses me even more and I cannot find any explanation that breaks it down simply enough for me to understand.

    I thought I understood it in that I could just base the direction of the moment on the quadrant it was located in, but that doesn't seem to be consistent. I feel like I'm just missing something very fundamental about the way moments work in relation to their corresponding point and it's throwing me off entirely. Please help! Explain like I'm five!
     
  2. jcsd
  3. Feb 19, 2014 #2

    rude man

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    The moment or torque on a particle a distance r from the origin of your coordinate system is given by

    τ = r x F

    where τ is the torque vector,
    r is the position vector from the coordinate system origin to the particle,
    and F is the force vector applied to the particle.

    The right-hand rule says, "hold your right hand so the 4 fingers all point in the direction of r; hold the thumb up so it forms a right angle to the fingers; now curve the fingers around towards the direction of F, and the thumb will point in the direction of τ."

    If this confuses you, you can always revert to using a cartesian coordinate system:

    r = rx i + ry j
    F = Fx i + Fy j

    then set up the determinant with
    i j k in 1st row,
    rx ry rz in second row
    Fx Fy Fz in third row.
     
  4. Feb 19, 2014 #3

    collinsmark

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    In addition to rude man's advice,

    If you are working with 2-dimensional statics problems, such that all beams/girders/trusses/etc. lie on the x-y-pane of the paper, the direction of the moment vector will always be either into the paper or out of the paper. You can use rude man's explanation of the right hand rule to determine of the direction of a given moment is into or out of the paper.
     
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