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Definition of a moment in statics? Please help

  1. Feb 7, 2009 #1
    1. The problem statement, all variables and given/known data

    How do i know how to calculate it properly ? Like i Know M=Fd but, how can i tell if its positve counterclockwise or negative clockwise. Expecially when you have components.

    hibbeler_chapter4.jpg

    Something Like this thats broken into compoents how can i calculate the moment ? Im totally confused on how it goes
     
  2. jcsd
  3. Feb 7, 2009 #2

    LowlyPion

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    Homework Helper

    Let's assume you are talking about Torque, which is force taken through a moment arm.

    Take 1 direction to be positive. I use the right hand rule generally - which means counter-clockwise with the Torque arrow pointing up.

    Resolve the force arrows with respect to x,y so that you have a representation that looks like

    F = a i + b j

    Then the point at which a line along and through the force intersects the x or y axis, then the product of the distance along the axis times the component perpendicular to the axis is your Torque.

    For instance in your example at B the Torque will be + FBy*B along the x axis.

    Alternatively, you can project the line of the Force vector to a line that is perpendicular with the pivot and then just take the product.

    A quick search yielded this link that more or less says the same thing.
    http://www.physics.uoguelph.ca/tutorials/torque/Q.torque.intro108.html
     
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