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Homework Help: Assignment On Couples and moments

  1. Sep 20, 2007 #1
    [Urgent]Assignment On Couples and moments

    1. The problem statement, all variables and given/known data
    Two coplanar couples equal and opposite moments balance each other
    Case1: lines of action constituting the couple intersect

    Case2: lines of action constituting the couple do not intersect

    Above is what the lecturer gave us. I have really no clue what to do. I have to submit this assignment on monday :S

    2. Relevant equations
    Equations on moments and couples i guess

    3. The attempt at a solution
    Have tried to draw vector diagrams but didnt go too far :(
  2. jcsd
  3. Sep 20, 2007 #2


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    What is the question?
  4. Sep 20, 2007 #3
    Good question!
    lol kidding
    In fact im not sure, the lecturer only gave what ive written above
    so i guess i have to show two cases where

    "Two coplanar couples equal and opposite moments balance each other"

    Thats my problem its supposed to be kinda basic but am not sure on how to show it :S
  5. Sep 20, 2007 #4


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    When a force couple has the line of action of the two forces intersect... then the moment is 0. The forces act along the same line... eg: two people pulling at a rope.

    first case: So he's asking for a case with two force couples where the two couples each have equal and opposite forces acting along the same line? How about two pairs of people... each pair pulling on a rope in opposite directions? seems a strange case... I'm wondering if I'm understanding the question.

    The second case is more significant. when the lines of force constituting a couple aren't intersecting, that means the moment is nonzero. Can you think of a real life case with two opposite couples balancing each other?
    Last edited: Sep 20, 2007
  6. Sep 20, 2007 #5
    Im same here. I got this question since monday and ive tried to think about it but if ever i get an answer im not even sure that its good. Or else just as u said before, the case appears to be strange...

    Im going to ask the lecturer for more details about the question and im gonna post whatever she told me this afternoon.
  7. Sep 21, 2007 #6


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    What I've investigated so far is to see what happens to the resultant moment on a system in the first case.

    If you draw a diagram with two sets of parallel lines (each set separated by a different distance, say [tex]a[/tex] and [tex]b[/tex]) intersecting each other at some arbitrary angle, say [tex]\theta[/tex], and the sets of forces for the couples , [tex]F_a[/tex] and [tex]F_b[/tex], you can get an equation for the resultant if you slide the force pairs along the lines of action till they meet at the intersection points. The resultants, [tex]R_a[/tex] and [tex]R_b[/tex] (same magnitude), form another couple. I have not solved for the distance between the resultants in terms of [tex]a[/tex] and [tex]b[/tex] yet though.
    Last edited: Sep 21, 2007
  8. Sep 21, 2007 #7
    thx a lot :D
    But concerning couples, isnt it that the moment=Fd (where d is the distance between the two forces) whatever the distance is and whatever the angle is?

    Im asking that coz i must start the assignment by defining what moment is and then what a couple is...
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