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The angle that a vector makes when it is parallel to x-axis.

  1. Jun 2, 2012 #1
    If there is a (displacement) vector parallel to the x-axis, and this vector is below the x-axis, is the angle (in radians) made by this vector ∏ or -∏? Why?

    If the vector is above the x-axis, what is the angle? Why?
  2. jcsd
  3. Jun 2, 2012 #2
    The angle does not depend on which side of the x-axis it lies on. It depends on what the direction of the vector is. Say if it were pointing towards positive x axis, then the angle would be 0. If pointing towards negative x axis then angle would be ∏.

    PS : ∏ and -∏ are the same thing :wink:
  4. Jun 2, 2012 #3


    Staff: Mentor

    The reference points on the unit circle for the two angles are the same, but the angles are different. An angle of ##\pi## implies counterclockwise rotation; an angle of -##\pi## implies clockwise rotation.
  5. Jun 3, 2012 #4
    Ah, I missed that accidentally. Thanks for the responses.
  6. Jun 3, 2012 #5
    Oh, yes. I was only speaking of them in the sense that they would put you in the same direction after rotation. :smile:
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