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Homework Help: [Guidance needed]-Compute the Magnitude and Direction of the Resultant Couple

  1. Nov 9, 2009 #1
    Hello again :). here is a very interesting exercise, I attached a picture of the exercise, its a Circle with Forces.

    1. The problem statement, all variables and given/known data

    Compute the Magnitude and Direction of the Resultant Couple acting on the body Shown:

    2. Relevant equations

    M=F x D

    3. The attempt at a solution

    Can you provide some guidance, so far I believe this is 1 part of the process but the rest I am not sure about.

    M= 50(3)+50(3)+30(3)+30(3)

    M= 50(6)+ 30(6)= 480 ft*lb

    Attached Files:

    Last edited: Nov 9, 2009
  2. jcsd
  3. Nov 9, 2009 #2


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    M= F x perpendicular distance from line of action of force to the point about which you are summing moments.
    That is the process and the resultant couple, however, your units are wrong and you must specify a direction.
  4. Nov 9, 2009 #3
    Ah ok so I was on the right path there, it should be lb right?...

    M= 50(3)+50(3)+30(3)+30(3)

    M= 50(6)+ 30(6)= 480 lb

    How can I specify the direction?, can someone help me with that please.
    Last edited: Nov 9, 2009
  5. Nov 9, 2009 #4


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    if M = F x D, and F is in pounds and D is in feet, then the unit for the moment is _______???
    One way is to specify clockwise or counterclockwise? Or, since moment is a vector, its resultant is either into or out of the page (the 3rd dimension or Z axis), perpendicular to the plane that the couple is in, representing the axis about which the moment is applied,according to the Right Hand Rule.
  6. Nov 9, 2009 #5
    Then it should be ft-lb???????????

    Ok in terms of specifying the direction you're saying find the resultant?, then find theta which that would be direction right?, how is that even possible with this circle?.

    Do need to take some one of these vector and try to find its resultant or something?, clear up and keep in mind I am on a basic level here.
    Last edited: Nov 9, 2009
  7. Nov 9, 2009 #6
    Seems like Jay went offline.......

    Can another helper chime in please, specially about the whole direction part of the exercise.

    He said I got this part of the process good

    M= 50(3)+50(3)+30(3)+30(3)

    M= 50(6)+ 30(6)= 480 ft-lb

    The next step is about direction and he said something there I did not understand.
    Last edited: Nov 9, 2009
  8. Nov 9, 2009 #7
    About direction imagine how it will move. Will it go up down right or will it execute a circular motion?
    IT's your job to define the possitive and the negative direction. That means you can say whether clockwise is possitive or negative.
  9. Nov 9, 2009 #8
    It will do a circular motion TO THE LEFT, the force of 30 lb will push to the left and then the bigger force of 50lb will make the whole rotation happens. Is that a good way of seeing it???, I would need to show that in a calculated manner thought?, since it does say "compute the magnitude(done) and direction)"

    Oh btw what about that whole units situation, did I get it right?, its ft-lb correct?...
    Last edited: Nov 9, 2009
  10. Nov 9, 2009 #9


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    Suppose the circle in the problem is the top of a tap, and you are opening it with some force. In which direction the top of the tap moves?
    That is the direction of the resultant moments.
  11. Nov 9, 2009 #10
    You open everything by turning to the left, so it moves in the left direction.

    Do I just state it like that with words or would I need to do something like this???

    [tex]\Sigma[/tex]F= F1 + F2+ F3 + F4

    30lb + 50 + 30+ 50=

    Or Is that^ just a wrong turn?....

    What's confusing me is that it states "calculate the magnitude & direction", its making me
    think I need to use the forces, ratio and that 60 degree angle lol, damn I need some serious guidance here.
    Last edited: Nov 9, 2009
  12. Nov 9, 2009 #11


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    In your problem, the circle is being turned to the left, so how does the top of the tap move? Up or down? Whatever the answer is, you can just say "out of the page" or "into the page" unless the teacher is obsessed with fancy notation.

    We're talking about direction here, so you don't need to do that.

    Every vector has a magnitude and direction, so the fact that the question asks for magnitude and direction is being F cross d is a vector. (By the way, ft*lb is the right unit.)
  13. Nov 9, 2009 #12
    Oh so its just about stating the direction, I guess I can talk to him about it but I would put
    "out of page" in this case right?, that's what I vision it doing.

    I was just throwing that out there in the myst of confusion, sorry....

    Oh ok, thanks for clearing that up :)

  14. Nov 9, 2009 #13


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    Yeah, it's out of the page.
  15. Nov 9, 2009 #14
    Cool, thanks for the help, I appreciate it.
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