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Math Help - [SOLVED] Differentiation question

  1. #1
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    [SOLVED] Differentiation question

    All done
    Attached Thumbnails Attached Thumbnails [SOLVED] Differentiation question-problem.jpg  
    Last edited by dumdidum; March 15th 2009 at 01:43 PM.
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by dumdidum View Post


    Thanks for your help

    ...I can tell from the looks of it that it's a fairly simple question, but haven't done high school maths in...well, 5 years

    Do I simply plug p=A/r back into dP/dr....it seems a little too simple

    This was my try:

    ρ = A/r
    dρ/dr = –A/r^2

    dP/dr = – GMρ / r^2
    = – (G x M x –A/r^2) / r^2
    = (G x M x A/r^2) / r^2
    = GMA

    Ps = 0
    (Ps – Pc)(rs – rc) = (0 – Pc) / r
    = –Pc / r
    –Pc / r = GMA
    Pc / r = – GMA
    Pc = – GMAr
    You start with:

    \frac{dP}{dr}=-\frac{GM \rho}{r^2}

    Then as \rho=\frac{A}{r} you have:

    \frac{dP}{dr}=-\frac{GMA}{r^3}

    Now just rework the original argument with the new right hand side.

    CB
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  3. #3
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    Thanks CB. Guess it really was that simple
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