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Moment of force about a point

  1. Oct 17, 2012 #1
    1. The problem statement, all variables and given/known data

    The moment of force Fa about point O

    2. Relevant equations



    3. The attempt at a solution

    I know how to setup with breaking the force into x,y component but how to tell if its rotating cw or ccw about the point. I think y is ccw but i cant tell on x. I think i would use right hand rule but need to know how to do it correctly.
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     

    Attached Files:

  2. jcsd
  3. Oct 17, 2012 #2
    Right hand rule would work here. How did you come to conclusion that Fy
    "rotates" ccw? If You know the distance and direction of the force, it's easy to determine the direction of moment.

    The distance has to be perpendicular to the force, and your fingers have to be directed in the direction of the force, i.e. the direction of rotation. Your palm shows moment direction.
     
  4. Oct 17, 2012 #3
    Well to setup the prob it would be like -200cos(60)(9in)+200sin(60)(30in), but why is the first term negative if x pulls straight out? My prof. didnt pull this from the book. Maybe I'm not understanding the principle behind moments etc.
     
  5. Oct 17, 2012 #4
    Please check the photo that i attached. If you try to rotate Fx around point P, your thumb would be faced down which is the negative z direction, and therefore that moment is also negative.
     

    Attached Files:

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