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Homework Help: Force vectors

  1. Sep 13, 2010 #1
    The three concurrent forces acting on the post produce a resultant force of 0. If F2 = 0.5F1, and F1 is to be 90 degrees from F2 as shown, determine the required magnitude F3 expressed in terms of F1 and the angle theta.


    I am having no troubles finding the magnitude of F3, however, I am having trouble understanding how to get theta. Apparently the angle formed by F1 and F2, plus ninety degress, gives you the answer. However, I'm not seeing the relationship. Help?
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  2. jcsd
  3. Sep 13, 2010 #2

    Doc Al

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    Staff: Mentor

    Use theta to find the angle that F2 (and F1) makes with the horizontal.
     
  4. Sep 13, 2010 #3
    Wouldnt you want to find the angle that F2 (and F1) makes with the vertical, then add 90 degrees to find theta?
     
  5. Sep 13, 2010 #4

    Doc Al

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    Staff: Mentor

    That also would work. Give it a try.

    Express the angle that F1 and F2 make with the vertical or the horizontal in terms of theta. Then you can solve for theta.
     
  6. Sep 13, 2010 #5
    For the vectors to cancel out, the combination of F1 and F2 must result in a vector having the same magnitude as F3 but having opposite direction.

    So, to start, find the x-components of F1 and F2. Add them together to get the magnitude of F3. Then substitute what you know about the angles.
     
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