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Homework Help: Cylindrical coordinates

  1. Apr 25, 2007 #1
    1. The problem statement, all variables and given/known data

    Say I have line, y = - 5 in cartesian coordinates. How do I express this in cylindrical coordinates?

    Also, if I have a point (0,-5) in cartesian coordinates, how to I express this position vector in cylindrical coordinates?

    2. Relevant equations

    y = r sin (phi)

    j = r(hat)cos (phi) - phi(hat) sin (phi)


    3. The attempt at a solution

    THe relations are right there but I've forgotten howto use them.
     
  2. jcsd
  3. Apr 25, 2007 #2

    mjsd

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    Homework Helper

    do you remember how to turn say [tex](x,y)[/tex] into [tex](r,\theta)[/tex] in 2D? the idea there is similar...
     
  4. Apr 26, 2007 #3

    HallsofIvy

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    Science Advisor

    Cylindrical coordinates? That's in 3d but you only mention 2 coordinates? Perhaps you mean polar coordinates- cylindrical coordinates but ignore the z component.

    You mention [itex]y= r sin(\phi)[/itex] (I would have used [itex]\theta[/itex]). Do you also know that [itex]x= r cos(\phi)[/itex]? Solve those two equations for r and [itex]\phi[/itex] in terms of x and y. The first is easy! You can eliminate [itex]\phi[/itex] by squaring both equations and adding. The second is about as easy: eliminate r by dividing one equation by the other.
     
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