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Solving 2D moments about point O

  1. Jun 2, 2012 #1
    (a) The moment the force, F, makes about O is 15 N-m clockwise, and θ = 30˚, find the magnitude of F.
    (b) If the force F = 100 N, determine the angle θ so the force develops a clockwise moment about point O
    of 20 N-m. Limit your answers to a range of 0º ≤ θ ≤ 90º.

    I'm a little lost can someone please help me with how to set up equations so i can figure out the answers. Thanx
     

    Attached Files:

  2. jcsd
  3. Jun 3, 2012 #2

    tiny-tim

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    welcome to pf!

    hi majunee! welcome to pf! :smile:

    start by completing the diagram

    continue the line of force backwards, and find its perpendicular distance from O :wink:
     
  4. Jun 3, 2012 #3
    thanx so is this correct so far
    -15 KN-m = F[-(sin30°)-(50mm)-(300sin60°)]
     
  5. Jun 3, 2012 #4

    tiny-tim

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    hmm … you're going to need another hint:

    call that corner "C"

    draw the line G through C parallel to F

    draw the two perpendicular lines from O to G and from C to F …

    how long are they? :wink:
     
  6. Jun 3, 2012 #5
    Sorry Im really having trouble understanding the concept...
    wouldnt 0-G be 300sin60 and c-f be 50 + xsin30
     
  7. Jun 3, 2012 #6

    tiny-tim

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    ahhh, you seem to be measuring vertical distances

    no, to find the moment of a force, you need to measure distance(s) perpendicular to the line of the force :wink:

    start again :smile:
     
  8. Jun 3, 2012 #7
    how? if force has continuing line of action shouldnt we measure along the yaxis which would be like i did?
     
  9. Jun 3, 2012 #8

    tiny-tim

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    why the y axis? :confused:

    (are we using the same diagram? … the force is at 30°)
     
  10. Jun 3, 2012 #9
    yes we are
    so would it be -300sin60 + 300 cos 60 - 50 + xsin30 - x cos 30
     
  11. Jun 3, 2012 #10

    tiny-tim

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    why the y axis? :confused:

    and what is the "x" in your equation?
     
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