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Homework Help: Angle of vevtor in component form

  1. Apr 14, 2012 #1
    Angle of vector in component form

    Find the vector's magnitude and direction. Use right as the +x direction and up as the +y direction.

    [itex]\vec{A} = 3.50 \hat{i} - 7.70 \hat{j}[/itex]

    Find [itex]\theta_A[/itex] degrees below the positive x-axis.

    [itex]\theta_A = arctan\left ( \frac{7.70}{3.50} \right ) [/itex]


    [itex]360^\circ - 65.55604522^\circ = 294.4439548^\circ[/itex]

    But this is wrong. I also tried: -65[itex]^\circ[/itex] and 336[itex]^\circ[/itex]
    Last edited: Apr 14, 2012
  2. jcsd
  3. Apr 14, 2012 #2


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    Homework Helper

    Usually the angle is measured anticlockwise from the x axis, in which case your answer of 294.44 degrees would be correct. When you found the angle below the positive axis, that was an intermediate step? Or is that what the question asked you for? If the question asked for that, then the answer would be 65.56 degrees.
  4. Apr 15, 2012 #3
    No my 294[itex]^\circ[/itex] was the final answer and what was being asked.
    Yes, 65[itex]^\circ[/itex] turned out to be correct but I don't understand why.
    Since the question asked: Find [itex]\theta_A[/itex] degrees below the positive x-axis. I thought it would be -65[itex]^\circ[/itex] or 294[itex]^\circ[/itex]
    By the way are links to a non-copyrighted PDF allowed in the forums ?
  5. Apr 16, 2012 #4


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    I'm not sure about the PDF's... I would suppose so. About the problem, they ask for degrees below the positive x-axis, so this means the angle as measure clockwise from the x-axis, which is why the answer is +65 degrees. In the usual convention of measuring anti-clockwise, the answer would have been 294 degrees, but they specify to not use the convention.
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