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Homework Help: Moment diagram by parts

  1. Apr 23, 2012 #1
    Urgent Help Please!

    Just wanna ask some rules/ tips in drawing MOMENT DIAGRAM BY PARTS and COMPUTING FOR THE MOMENT OF THE AREA OF THE MOMENT about a particular axis.

    Here's the step I typically do:

    1. Get the reactions.
    2. Plot the moment diagram of EVERY force. (that's why it's called by parts)
    3. Take the moment of THE AREA OF THE MOMENT from (2) to either the LEFT or RIGHT end of the beam.

    Here's what confuses me. In (2) , can I take the moment of the forces at ANY PART OF THE BEAM? Does the result will be the same if I take moment on OTHER PART of the beam?

    Then, what is the general rule and would you give me some tips in doing this stuff. Thanks a lot! :D
  2. jcsd
  3. Apr 23, 2012 #2
    One never takes moments at a point. What one does is to take moments about an axis. In two-dimensional work the relevant axis for bending moments is perpendicular to the plane of the paper. The bending moment on the left of a given section should be exactly the same as the bending moment to the right of that section. This fact follows from the definition of bending moment: "the algebraic sum of the moments on one side (or the other) of a section." Given a bending moment diagram, and the need to take moments of the area about an axis for the purpose of obtaining a deflection, the axis of reference is still perpendicular to the plane in which the problem is posed, but many people find it easier to take moments about an axis which lies in the plane of the paper. The question you pose is "which axis is the correct one to take?" I would urge you to return to the theorem on which this is based and answer back in this forum where you find the difficulty in interpretation. Once we know you have understood the theorem and can manage it in a cumbersome way, then that is the point where tips are appropriate.
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