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Homework Help: Force Resolution

  1. May 27, 2009 #1
    1. The problem statement, all variables and given/known data

    Force Resolution Question-

    3 forces act on something,
    -a 600N force which is 50 degrees from the +x axis
    -a 300N force which is 85 degrees from the +x axis
    -a 700N force which is 50 degrees from the -x axis

    Resultant Force and angle

    3. The attempt at a solution

    Yes, very basic question, not hard to get, but i had a question about the angle im getting.

    This is the set up to a much larger more complicated problem, but i figured id post it here because this part of the question is basic.

    I resolved the forces and got Fx=-38.13 and Fy=1294.7, now my question comes in here. For angle i did theta= arctan(1294.7/-38.13) which is equal to -88 degrees... That doesnt seem right to me and if i go with the wrong angle for this, ill bomb the whole question.

    If i plot Fx and Fy on an xy plane its clear that it is slightly past 90 degrees.An angle of -88 degrees would hint that the resultant is in Quadrant 4, when it is in fact in quadrant 2. I noticed if i add 180 to it, i get roughly 91 degrees, which sounds like it would be along the same line of action as the actual resultant. Can someone tell me where im going wrong here? Im either missing something very obvious or what, im not sure
    Last edited: May 27, 2009
  2. jcsd
  3. May 27, 2009 #2


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    It is always good to draw a sketch of the problem; otherwise, the plus and minus signs will drive you crazy. The resultant is in Quad 2, 88 degrees clockwise from the negative x axis, or 92 degrees counterclockwise from the positive x axis.
  4. May 27, 2009 #3
    Thanks jay,

    Thats what i figured, I had the picture all drawn out and thats what I was getting. It was just odd to me that I wasnt getting theta= 92 degrees. I guess it just gave me the reaction force for some reason. My teacher is very nit picky so he would probably screw me for the whole problem if i just left the angle as -88 without explaining why.
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