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Homework Help: Subtracting this vector - as opposed to adding it

  1. Jan 22, 2012 #1
    1. The problem statement, all variables and given/known data
    Sketch the vector difference D = A - B

    I have found F = A + B.

    2. Relevant equations

    A(x) = AcosTHETA
    A(y) = AsinTHETA
    B(x) = BcosTHETA
    A(y) = BsinTHETA

    R = sqrt(rx^2 + ry^2)

    3. The attempt at a solution

    As opposed to the F = A + B I have tried changing the signs of B or multiplying B(x) = -BcosTHETA etcetera and I can't come with the right solution. Do I need to put the angle 37 degrees in the second quadrant? I am confused as how to find -B as A is going to be the same as the first equation (F = A + B).
     

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  2. jcsd
  3. Jan 22, 2012 #2

    SammyS

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    attachment.php?attachmentid=43021&d=1327282200.jpg
    Hello rpgnick85. Welcome to PF !

    You would have
    -Bx = -Bcosθ

    -By = -Bsinθ​
    That puts -B in the third Quadrant, doesn't it?
     
  4. Jan 22, 2012 #3
    -Bx = -Bcosθ

    -By = -Bsinθ

    Does that mean that I would do -18cos(217) for my first value?
     
  5. Jan 22, 2012 #4

    SammyS

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    No.

    The x component of -B is either 18cos(217°) or -18cos(37°) .
     
  6. Jan 23, 2012 #5
    alright so when i use -18cos37 and -18sin37 for my Bx and By values respectively and 12cos180 and 12sin180 for my Ax an Ay values I am coming up with R= 28.5126 and θ=22.33° which is incorrect
     
  7. Jan 23, 2012 #6
    found the answer- was in the wrong quadrant
     
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