You are 10 miles upstream from a dam. A 50 sq ft gate 30 feet above the lower water level is opened and water starts to flow from the dam. How long is it until your boat starts moving towards the dam? The lake is 300 feet across, 30 feet deep. You will be pulled towards the dam. Thinking I will need the volume of water to the boat, the cross section area of the lake, area of the gate, height of the gate above the lower level, etc....I can calculate velocity out of the gate, v=sqrt (2gh) and using A1v1=A2v2 find v2 and setting t=10/v2 solve for t. But I don't think it works this way. Perhaps an integral calculus problem.
This is what happens when fish are not biting!!


closed as too broad by John Rennie, rob, Brandon Enright, ja72, Danu Jun 9 '14 at 17:09

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    $\begingroup$ If you want to know when you'll feel the movement of the water, you'll have to quantify your perceptual threshold, too. Is it when sound of the gate opening reaches a hydrophone under the boat? when the water level change is comparable to the surface waves? when a surface current develops above some threshold? when you wake up and you're out of beer and on a mud flat? $\endgroup$ – rob Jun 9 '14 at 14:45

Ok, flow of water is not similar to the motion of a ball from X distance from you that you can say i will feel it at time X/v or so on,, here there is bulk motion,,, as soon as the dam is opened,, the gushing water pushes the neighbouring layer of water and so on, a wave passes through the water to you and you will feel it before the particular gush of water that rushed at the dam actually reaches you,, if the molecules were perfectly rigid,, it would be instantenous ( meaning lesser than the speed of light obviously).. but there is a seperation between the molecules of water so i think it is like the flow of electrons through a wire...colliding and going.. i will try to derive it mathematically and report back


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