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Finding direction after adding/subtracting vectors?

  1. Sep 28, 2011 #1
    1. The problem statement, all variables and given/known data

    For the vectors given in Fig. 3-32 (|A| = 67.0 and θ = 55.5°), determine the following.

    Image for the problem (http://www.webassign.net/giancoli5/3_35alt.gif)

    Need to find:

    C-A-B

    Direction (counterclockwise from the +x axis is positive)



    2. Relevant equations




    3. The attempt at a solution


    I understand how to find magnitude when adding and subtracting vectors. And I understand that the angle is found by tan^-1 (ry/rx). I got 117 for the magnitude. My rx is -36.5 and my ry is -111.22. So I took the inverse tangent and got 71.8, which is incorrect. I know that sometimes you have to subtract that angle from 180 or something, but I honestly don't know why or when you do that.

    If anyone can explain this to me, I would greatly appreciate it!
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Sep 28, 2011 #2

    SammyS

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    Welcome to PH Forums.

    Make a rough sketch of C + (-A) + (-B) to give you an idea of whether you need to add or subtract 180° .
     
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