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Couple forces

  1. Feb 17, 2009 #1
    Hey, i'm having some difficulty calculating couple moment. i understand that [tex]\sum[/tex]M = [tex]\sum[/tex]Fd. here's an example:
    question.jpg
    it asks to resolve the couple forces.

    the answer is:
    (4/5)(60lb) = 48lb
    and
    (40lb)(cos30)

    [tex]\sum[/tex]M = -(48lb)(4ft) + (40lb)(cos30)(8ft)

    Now, i don't understand why "-(48lb)(4ft)" is negative. I'm not sure when to use a negative value. Can someone explain this to me?

    Cheers
     
  2. jcsd
  3. Feb 17, 2009 #2

    stewartcs

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    It's a matter of convention. Typically, the clockwise direction is negative and counter-clockwise is positive. So if the applied force causes rotation in the clockwise direction, the moment is negative and vice-versa.

    CS
     
  4. Feb 17, 2009 #3

    Andy Resnick

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    2016 Award

    I don't see where '8 ft' comes in. Shouldn't that be 'd ft'?
     
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