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mbigras
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This is a multipart question, I'm working on part (b) but the full question can be found on page 85 at http://www.scribd.com/doc/50357722/Vibrations
(b) Assuming that the water flow is predominantly horizontal, its speed [itex]v[/itex] must vary with [itex]x[/itex], being greatest at [itex]x = 0[/itex] and zero at [itex]x = \pm L/2[/itex]. Because water is incompressible (more or less) we can relate the difference of flow velocitites at [itex]x[/itex] and [itex]x + dx[/itex] to the rate of change [itex]dy/dt[/itex] of the height of the water surface at [itex]x[/itex]. This is a continuity condition. Water flows in at [itex]x[/itex] at the rate [itex]vhb[/itex] ([itex]b[/itex] is the width of the tub) and flows out at [itex]x+dx[/itex] at the rate [itex](v+dv)hb[/itex]. (We are assuming [itex]y_{0} << h[/itex].) The difference must be equal to [itex](b dx)(dy/dt)[/itex], which represents the rate of increase of the volume of water contained between [itex]x[/itex] and [itex]x+dx[/itex]. Using this condition, show that
[tex]v(x) = v(0) - \frac{1}{hL}x^{2}\frac{dy_{0}}{dt}[/tex]
where
[tex]v(0) = \frac{L}{4h}\frac{dy_{0}}{dt}[/tex]
The question gives an explanation about what the continuity continuity condition is. Conceptually I think I understand what going on: Water is flowing into the imaginarily thin container at some rate and is flowing out of it and some slower rate so the water has to go somewhere and ends up flowing up at some related rate. I feel confused about how to actually use the continuity condition to get the equation for the velocity as a function of x. I'm also not sure if all my integration and wiggling around of the continuity condition is allowed.
[tex]
vbh - (v+ dv)hb = b dx \left(\frac{dy}{dt}\right) \\
-dv h = dx \left(\frac{dy}{dt}\right) \\
dv = - \frac{1}{h} \left(\frac{dy}{dt}\right) dx \\
\int dv = \int _{0} ^{\frac{L}{2}} - \frac{1}{h} \left(\frac{dy}{dt}\right) dx \\
v = - \frac{1}{h} \left(\frac{dy}{dt}\right) \frac{L}{2}
[/tex]
Homework Statement
(b) Assuming that the water flow is predominantly horizontal, its speed [itex]v[/itex] must vary with [itex]x[/itex], being greatest at [itex]x = 0[/itex] and zero at [itex]x = \pm L/2[/itex]. Because water is incompressible (more or less) we can relate the difference of flow velocitites at [itex]x[/itex] and [itex]x + dx[/itex] to the rate of change [itex]dy/dt[/itex] of the height of the water surface at [itex]x[/itex]. This is a continuity condition. Water flows in at [itex]x[/itex] at the rate [itex]vhb[/itex] ([itex]b[/itex] is the width of the tub) and flows out at [itex]x+dx[/itex] at the rate [itex](v+dv)hb[/itex]. (We are assuming [itex]y_{0} << h[/itex].) The difference must be equal to [itex](b dx)(dy/dt)[/itex], which represents the rate of increase of the volume of water contained between [itex]x[/itex] and [itex]x+dx[/itex]. Using this condition, show that
[tex]v(x) = v(0) - \frac{1}{hL}x^{2}\frac{dy_{0}}{dt}[/tex]
where
[tex]v(0) = \frac{L}{4h}\frac{dy_{0}}{dt}[/tex]
The Attempt at a Solution
The question gives an explanation about what the continuity continuity condition is. Conceptually I think I understand what going on: Water is flowing into the imaginarily thin container at some rate and is flowing out of it and some slower rate so the water has to go somewhere and ends up flowing up at some related rate. I feel confused about how to actually use the continuity condition to get the equation for the velocity as a function of x. I'm also not sure if all my integration and wiggling around of the continuity condition is allowed.
[tex]
vbh - (v+ dv)hb = b dx \left(\frac{dy}{dt}\right) \\
-dv h = dx \left(\frac{dy}{dt}\right) \\
dv = - \frac{1}{h} \left(\frac{dy}{dt}\right) dx \\
\int dv = \int _{0} ^{\frac{L}{2}} - \frac{1}{h} \left(\frac{dy}{dt}\right) dx \\
v = - \frac{1}{h} \left(\frac{dy}{dt}\right) \frac{L}{2}
[/tex]
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