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Homework Help: Writing equations in cylindrical coordinates (need work checked again please)

  1. Sep 9, 2008 #1
    Could someone tell me what I'm doing wrong? thanks!!

    1. The problem statement, all variables and given/known data

    Write the equation is cylindrical coordinates

    7x2 + 7y2 = 2y

    r = ? (has to be in the r = ? format)

    2. Relevant equations

    r2 = x2 +y2
    x = rcos(θ)
    y = rsin(θ)

    3. The attempt at a solution

    7x2 + 7y2 = 2y

    7(x2 + y2) = 2y

    7(r2 = 2rsin(θ)

    r2 = (2rsin(θ))/7

    r = sqrt((2rsin(θ))/7)
  2. jcsd
  3. Sep 9, 2008 #2


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    Science Advisor

    I think it would be better to leave it as [itex]r^2= (2/7) sin(\theta)[/itex] rather than taking the square root, but, yes, that is correct.
  4. Sep 9, 2008 #3
    its an online submission that has r = "enter here" , but i keep getting that its a wrong answer, is there any other way that this could be written?
  5. Sep 9, 2008 #4


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    Homework Helper

    Note that you forgot to cancel out r on both sides of the equation. Don't express r in terms of r in the final answer.
  6. Sep 10, 2008 #5
    Anybody have any idea about tramsforming the momentum equation into 2-D cylindrical co-ordinates...i've already derived the momentum equation from first principle but have difficulty with the transformation to cylindrical co-ordinates
  7. Sep 11, 2008 #6


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    Homework Helper

    Hi, if you have a separate question you should post it in a new thread so that others can aid you. Sometimes people don't bother reading through a thread which already has several replies.
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