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Homework Help: Solve the DE

  1. Oct 14, 2009 #1
    1. The problem statement, all variables and given/known data

    A tank in the form of a right-circular cylinder standing on end is leaking water through a circular hole in its bottom. When friction and contraction of water at the hole are ignored, the height h of the water in the tank is described by:

    [tex]\frac{dh}{dt} = -\frac{A_{h}}{A_{w}}\sqrt{2gh}[/tex]

    where Aw and Ah are the cross-sectional areas of the water and the hole, respectively.

    a) Solve the DE if the inital height of the water is H. By hand, sketch the graph of h(t) and give its interval I of definition in terms of the symbols Aw. Ah, and H. Use g = 32 ft/s^2.

    b) Suppose the tank is 10 feet high and has radius 2 feet and circular hole has radius 1/2 inch. If the tank is initially full, how long will it take to empty?

    2. Relevant equations

    [tex]\frac{dh}{dt} = -\frac{A_{h}}{A_{w}}\sqrt{2gh}[/tex]

    3. The attempt at a solution

    I want to make sure I did this part right before attempting b) as it needs the answer for a).


    [tex]\frac{dh}{dt} = -\frac{A_{h}}{A_{w}}\sqrt{2gh}[/tex]

    [tex]\frac{dh}{dt} = -\frac{A_{h}}{A_{w}}\sqrt{64H}[/tex]

    [tex]\frac{dh}{\sqrt{64H}} = -\frac{A_{h}}{A_{w}}dt[/tex]

    [tex]\int\frac{dh}{\sqrt{64H}} = -\int\frac{A_{h}}{A_{w}}dt[/tex]

    [tex]\frac{h}{\sqrt{64H}} = -\frac{A_{h}}{A_{w}}t[/tex]
  2. jcsd
  3. Oct 14, 2009 #2


    Staff: Mentor

    In the next line, don't put H in yet; leave it as h, the variable.
    I have changed what you had to the following. Can you carry it through?

    [tex]\frac{dh}{8\sqrt{h}} = -\frac{A_{h}}{A_{w}}dt[/tex]

    [tex](1/8) \int h^{-1/2}dh = -\int\frac{A_{h}}{A_{w}}dt[/tex]
  4. Oct 14, 2009 #3
    how is this:

    [tex](1/8) \int h^{-1/2}dh = -\int\frac{A_{h}}{A_{w}}dt[/tex]

    [tex](1/8) 2h^{1/2} = -\frac{A_{h}}{A_{w}}t[/tex]

    [tex]\frac{\sqrt{h}}{4} = -\frac{A_{h}}{A_{w}}t[/tex]
  5. Oct 14, 2009 #4


    Staff: Mentor

    Don't forget the constant of integration in the next and following lines.
    This is fine, except for the missing constant, but why don't you go ahead and solve for h, instead of its square root? I.e., h(t) = ...
  6. Oct 14, 2009 #5
    [tex]\frac{\sqrt{h}}{4} = -\frac{A_{h}}{A_{w}}t + c[/tex]

    [tex]\sqrt{h} = -4\frac{A_{h}}{A_{w}}t + c[/tex]

    [tex]h = (-4t\frac{A_{h}}{A_{w}})^{2} + c[/tex]

    [tex]h - (-4t\frac{A_{h}}{A_{w}})^{2} = c[/tex]

    [tex]h = (-4t\frac{A_{h}}{A_{w}})^{2} + h - (-4t\frac{A_{h}}{A_{w}})^{2}[/tex]
  7. Oct 14, 2009 #6


    Staff: Mentor

    Technically, the c in the next line is a different constant from the one in the previous line, but it's still a constant.
    You can't do what you've done in the next line. You squared the left side to get h; you have to square the entire right side as well.
  8. Oct 14, 2009 #7
    [tex]\frac{\sqrt{h}}{4} = -\frac{A_{h}}{A_{w}}t + c[/tex]

    [tex]\sqrt{h} = -4\frac{A_{h}}{A_{w}}t + 4c[/tex]

    [tex]h = (-4t\frac{A_{h}}{A_{w}} + 4c)^{2}[/tex]

    [tex]c = \sqrt{h} + t\frac{A_{h}}{A_{w}}[/tex]

    [tex]h = (-4t\frac{A_{h}}{A_{w}})^{2} + \sqrt{h} + t\frac{A_{h}}{A_{w}}[/tex]
  9. Oct 14, 2009 #8


    Staff: Mentor

    Or just call it C instead of 4c.

    Now, use your initial condition, that h(0) = H.
    So, when t = 0, you have
    [tex]h(0) = H = C^{2}[/tex]
    From this, you can easily get the constant.
    This last step didn't do you much good, since you haven't really solved for h; it appears on both sides of your equation.
  10. Oct 14, 2009 #9
    [tex]h(0) = H = C^{2}[/tex]

    [tex]c = \sqrt{H}[/tex]

    [tex]h = (-4t\frac{A_{h}}{A_{w}} + \sqrt{H})^{2}[/tex]
  11. Oct 14, 2009 #10


    Staff: Mentor

    OK, you have done part of the a part of this problem. For the b part, replace the symbols Ah, Aw, and H with the given information in this part, and use your functions to find when the tank is empty; i.e., at the time t for which h(t) = 0.
  12. Oct 14, 2009 #11

    [tex]h = (-4t\frac{A_{h}}{A_{w}} + \sqrt{H})^{2}[/tex]

    [tex]h = (-4t\frac{\pi (0.5/12)^{2}}{\pi 2^{2}} + \sqrt{10})^{2}[/tex]

    [tex]t = 1821.47 s[/tex]
  13. Oct 14, 2009 #12


    Staff: Mentor

    I haven't checked your answer. You do a great job at formatting your work in LaTeX, but you could explain what you're doing a little better.

    You're dealing with a function here, so it would be better to indicate that.
    [tex]h(t) = (-4t\frac{\pi (0.5/12)^{2}}{\pi 2^{2}} + \sqrt{10})^{2}[/tex]
    This gives the height of water in the specified tank at time t.

    To find when the tank is empty, you're solving h(t) = 0 for t.
    [tex]h(t) = 0 \Rightarrow (-4t\frac{\pi (0.5/12)^{2}}{\pi 2^{2}} + \sqrt{10})^{2} = 0[/tex]
    from which you get t = whatever.
  14. Oct 14, 2009 #13
    I think this should do it:

    [tex]h(t) = 0 = (-4t\frac{\pi (0.5/12)^{2}}{\pi 2^{2}} + \sqrt{10})^{2}[/tex]

    [tex]4t\frac{\pi (0.5/12)^{2}}{\pi 2^{2}} = \sqrt{10}[/tex]

    [tex]t(0.001736111) = \sqrt{10}[/tex]

    [tex]t = \sqrt{10}/(0.001736111)[/tex]

    [tex]t = 1821.47 s[/tex]

    thanks for the help.
  15. Oct 14, 2009 #14


    Staff: Mentor

    Sure, you're very welcome.
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