Differential equation- where is my mistake?

Discussion in 'Physics & Math' started by Fudge Muffin, May 24, 2013.

  1. Fudge Muffin Fudge Muffin Registered Senior Member

    Messages:
    148
    "Water flows out of a tank such that the depth \(h\) metres of water in the tank falls at a rate which is proportional to the square root of \( h \) "

    Show that the general solution of the differential equation may be written as \( h = (c-kt)^2 \)

    However, I'm getting something different. I have a 2 in there somewhere. Bear with me as i figure out how to show you what I did with tex, but please show me what I'm doing wrong! Thanks in advance, Fudge.
     
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  3. Fudge Muffin Fudge Muffin Registered Senior Member

    Messages:
    148
    so, \( \frac{dh}{dt} = -k\sqrt h\)

    and \( \frac{1}{\sqrt h} dh = -k dt \)

    intergrating both sides, i get \( 2 \sqrt h = -kt + c \)

    if only this 2 wasn't there, it would be perfect. where did i go wrong?
     
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  5. James R Just this guy, you know? Staff Member

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    39,421
    k and c are just constants, so you're free to replace them with k'=k/2 and c'=c/2.
     
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  7. Fudge Muffin Fudge Muffin Registered Senior Member

    Messages:
    148
    aaah, i had that idea... i thought there was something i was missing but alright, thank you!
     

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