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One of my homework tasks is, embarrasing, too hard for me to tackle without some help. So I help somebody can help me with the following problem:

TASK:

"You have a filled glass of water. A straw is stuck partly down into the glass (a depthdfrom the surface), the other end of the straw is on the outside (a distancehbelow the surface). The surface of the water is at a heighty1. The glass' cross section area isA1, and the straw's cross section area isA2. We assume thatA1is much larger thanA2, and that there goes a "flow line" from the water surface until the end (the one with the distancehfrom the surface) of the straw." Illustration:

http://www.student.dtu.dk/~s052861/fysikprob.jpg [Broken]

a)

Decide the velocity of the water, that flows out ouf the straw, when the glass is filled (the surface of the water is aty1.

b)

Show, that the heightyof the water, can be expressed by this equation:dy/dt = - A2/A1 * sqrt(2 * g * (y - y2)).

c)

How long will it go until the water stops flowing out of the straw?

So, my problem is that I really don't now where to start. I've tried to think out a way to apply Bernoullis equation, or to calculate the pressure at the entrance of the straw and using that in some way... But I really need a hint about what I should focus on!

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# Homework Help: Fluid mechanics

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