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Homework Help: Direction Angle of Vector

  1. May 31, 2009 #1
    1. The problem statement, all variables and given/known data

    Find the direction angle of vector <-2,-5>

    2. Relevant equations

    The components of a vector is v=<magnitude*cos ø, magnitude*sin ø>
    Magnitude of a vector: √(a^2+b^2)

    3. The attempt at a solution

    I found the magnitude of the vector, which is √29. Then I set -2= √29 cos ø and solved for ø. It was 111.891 degrees but the answer to the problem was 360 MINUS 111.891 degrees. I thought it would be 180 PLUS 111.891.
  2. jcsd
  3. May 31, 2009 #2
    Ok you could draw the point in the plane. Make a right triangle with legs 2,5. You could then arcTan of that. That angle is the angle it makes with negative x-axis and negative y-axis. Since you are measuring from the positive x you add 180 to that. That is the angle.
  4. May 31, 2009 #3


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    360-111.891 is logical because of where on the coordinate plane you measure from.

    By the way, welcome to the forums!
  5. May 31, 2009 #4


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