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Homework Help: Setting up a DE

  1. Sep 24, 2013 #1
    1. The problem statement, all variables and given/known data

    Suppose water is leaking from a tank through a circular hole of area Ah at its bottom. When water leaks through a hole, friction and contraction of the stream near the hole reduce the volume of water leaving the tank per second to


    , where c (0 < c < 1) is a empirical constant. Determine a differential equation for the height h of water at time t for the cubical tank shown. The radius of the hole is 2 in., and g = 32ft/s2

    2. Relevant equations

    3. The attempt at a solution

    I am always confused in setting up a DE. Over here, the answer says that V= Aw(area of water). h . The next step written was that dh/dt = 1/Aw * dV/dt ... How does this make sense.. I am confused here. I understand that the "c.Ah.(2gh)^.5" given back in the question is dv/dt . But when you multiply it with 1/Aw, how do you get dh/dt ??? Thanks for any help!
  2. jcsd
  3. Sep 24, 2013 #2

    Ray Vickson

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    The area of the hole is Ah? How can the hole's area depend on the height of water in the tank?
  4. Sep 24, 2013 #3


    Staff: Mentor

    I think the OP means this as Ah, not A * h. Also, I think Aw means Aw.

    You can make what you write clearer by using the features available on this site. For example, to write exponents and subscripts, click the Go Advanced button below the input area, which causes the advanced menu to open across the top. One button is X2, which you can use to write exponents. Another button is X2, which you can use to write subscripts.
  5. Sep 24, 2013 #4


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    If you take the equation V = Aw * h and differentiate with respect to 't', what do you get?
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