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Simple relationship equation major headache

  1. Feb 4, 2013 #1
    1. The problem statement, all variables and given/known data
    The quantity p varies directly with the cube of q and inversely
    with the square of r. If p = 3/2, q = 1/2, and r = 1/3, which of the
    following equations represents the relationship between p, q,
    and r?


    2. Relevant equations
    Here's what I did....
    (3/2)p = (1/2)q^3 / (1/3)r^2


    3. The attempt at a solution
    Could not get the answer, which is: p = (4q^3) / (3r^2)

    Thanks for your help.
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Feb 4, 2013 #2

    haruspex

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    You are quite right to write down an equation of the form p = Aq3/r2, but you have not used the other information correctly. You need to find the value of A such that the given values of p, q and r are a solution to the equation.
     
  4. Feb 4, 2013 #3

    Dick

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    You've got the basic relation right. All that's missing is an unknown constant k. So p=kq^3/(r^2). Just put the given values in for p, q and r and solve for k.
     
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