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Homework Help: Convert this please

  1. Feb 2, 2009 #1
    1. The problem statement, all variables and given/known data

    I had an exam conversion problem:

    A position vector (between the origin and point (5,2,3) is expressed as r = 5x+2y+3z. Express this vector in cylindrical coordinates.

    row = SQRT(5^2 + 2^2) = 5.385

    Fie = tan-1(2/5) = 21.8 degrees

    Z = Z.

    This was a multiple choice test and the answers were:

    A) r=2.232r+0.134(theta)+3(fie)
    B) r=0.5(row)-0.866(fie)+3z
    C) r=5.385(row)+3z
    D) None of the above

    I chose D, because the correct value of fie is not there and A is sypherical.. But, I got it wrong and the correct answer is said to be C.

    I wanted to check with you guys first before going to the professor but is there something I missed??
  2. jcsd
  3. Feb 3, 2009 #2


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    Science Advisor
    Homework Helper

    You could argue that when setting up the coordinates you have the freedom to choose [itex]\phi = 0[/itex] at that point, although I agree that it is rather unusual... normally one takes [itex]\phi = 0[/itex] for points on the x-axis. If you are not sure however, I suggest you go see your professor anyway; the least he can do is explain why the answer should be C and why he thinks you are wrong.

    By the way, [itex]\rho[/itex] is called "rho" and [itex]\phi[/itex] is "phi", just like [itex]\pi[/itex] is spelled "pi" instead of "pie" :wink:
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