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Can you do sum of all moments on a point that's not a rotational axis?

  1. Jun 23, 2011 #1

    Femme_physics

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    Can you do "sum of all moments" on a point that's not a rotational axis?

    For instance, in this structure, with the two wires holding the beam

    http://img228.imageshack.us/img228/3687/twowiresj.jpg [Broken]

    Can I do sum of all moments on B? Or is it not allowed because it's not a pivot point?
     
    Last edited by a moderator: May 5, 2017
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  3. Jun 23, 2011 #2
    Re: Can you do "sum of all moments" on a point that's not a rotational axis?

    You can take moments about any point in a plane or any line in 3D.

    Some are more useful than others. We usually try to eliminate the effects of one or more external load by taking moments about a point or line that the line of external load passes through.
     
  4. Jun 23, 2011 #3

    Femme_physics

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    Re: Can you do "sum of all moments" on a point that's not a rotational axis?

    Can I even take a sum of all moments on a wire?
     
  5. Jun 23, 2011 #4

    SteamKing

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    Re: Can you do "sum of all moments" on a point that's not a rotational axis?

    It depends on what you are looking for. Wires are good at carrying tension loads, but they tend to kink when subjected to moments or torques.

    Although you have not stated a full problem, it appears that you have been asked to do a static analysis of this structure, to determine the tension in the wires given the weight of the rod and the magnitude of the applied load F. In addition to the tension, there will be a reaction at A, the nature of which will depend on whether A is fixed or pinned in some manner.

    Hint: When writing your equations of static equilibrium, if you use point A as the reference point for moment calculations, then the reaction force at point A will not contribute to the sum of the moments, because the moment arm from A to A equals 0. You will, however, obtain a moment expression involving T, W, and F, with W and F being known. This makes it easier to substitute for T in the other equation of static equilibrium involving just the summation of forces.
     
  6. Jun 23, 2011 #5

    I like Serena

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    Re: Can you do "sum of all moments" on a point that's not a rotational axis?

    As Studiot already said: "You can take moments about any point in a plane or any line in 3D."

    So yes, you can take the sum of moments on a wire.
    You can even take the sum of moments somewhere in thin air, although that will usually not be very useful. :wink:

    You may also note his careful distinction of a plane and of 3D.
    In 3D you're actually always taking the sum of moments with respect to a line.
     
  7. Jun 23, 2011 #6
    Re: Can you do "sum of all moments" on a point that's not a rotational axis?

    When you are finding the centroid of an odd shaped object such as an L shape or the letter O you will find that the centroid actually is in thin air.
     
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