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Moments question

  1. Jun 24, 2016 #1
    1. The problem statement, all variables and given/known data
    VEB6QiA.png
    o93AW8C.png

    2. Relevant equations
    tau = r x F

    varignons theorem

    3. The attempt at a solution
    I drew the diagram and the moment arm method and the component of the force perpendicular to vector OA seemed hard to find, so I used varignons theorem.

    tau = -abs(5000*5)+abs(6000*-4) = -16000 (which seems to be correct)

    I don't understand the second part of the question however.
    Is it asking me to find new points for the axis of rotation (one on the y axis and one on the x axis) such that the moment about each point is zero? The answers suggest that I'm just finding the x and y intercepts.


    e.g. if I find some point on the y axis, won't the x component of the force always cause a clockwise rotation, and the y component won't cause any rotation because it is parallel; so how can the moment be zero?
     
  2. jcsd
  3. Jun 24, 2016 #2

    haruspex

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    There will be a point on the y axis such that F has no moment about it. If you do it by breaking F into x and y components, each component will have a moment about the point, and the two will cancel.
     
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