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Understanding converting a vector field to cartesian coords

  1. Oct 29, 2012 #1
    1. The problem statement, all variables and given/known data
    Here is the problem and solution but I am confused as to part B
    http://gyazo.com/e77d05fc67cb6ac266ff021ef88052dc


    3. The attempt at a solution
    I understand the first part, but I am totally lost on how they reached their cartesian answer for part B. Firstly why did they do what they did, and secondly where did 36 degrees come from?

    I feel like their is some sort of equation or something I am not understanding.
     
  2. jcsd
  3. Oct 29, 2012 #2

    Dick

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    36 degrees is 0.2π. It's the angle the gave you for phi. Does that help? Other than that they are just using that if u is a unit vector then the component of D along u is (D.u)u.
     
  4. Oct 30, 2012 #3
    Hmm ok but why does he have .5 twice as in .5(ap.ax)ax +.5(ap.ax)ay?
     
  5. Oct 30, 2012 #4

    Dick

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    One term finds the ax component of 0.5ap and the other finds the ay. Are you sure you understood the first part?
     
  6. Oct 30, 2012 #5
    hmm okay thanks I think I figured it out. All I did was use the equations x = p*cosphi, y=p*sinphi and plugged those into the original equation which was already in cartesian for me.
     
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