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Vector Problem - Angle between two vectors

  1. Jan 22, 2009 #1
    1. The problem statement, all variables and given/known data

    Vectors A-> and B-> have equal magnitudes of 55.0. If the sum of A-> and B-> is the vector 17.7 j, determine the angle between A-> and B->

    2. Relevant equations

    cos(theta) = {[A]^2 + ^2 - [A-B]^2}/2[A]

    (sorry, not sure how to type out that formula!)

    tan(theta) = (Ay + By)/(Ax + Bx)

    R =[(Ax + Bx)^2 + (Ay + By)^2]^1/2

    3. The attempt at a solution

    This one I am not sure about how to start, at all. I plugged what I could into those formulas, but any answers I got were incorrect. I don't understand how I can find out any more information with what is given, namely the other components of the vector (the i for each, as well as discerning which values for j each vector has).
  2. jcsd
  3. Jan 22, 2009 #2


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    Homework Helper

    Since resultant is 17.7j, it is along y-axis. So its magnitude is 17.7. Magnitude of A ans B 55.0 each.
    Use the formula R = [ A^2 + B^2 + 2ABcos(theta)]^1/2 and find theta.
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