(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

I've attached an image of a dam. The problem reads as follows: The fresh water behind a reservoir dam has depth D = 15 m. A horizontal pipe 4 cm in diameter passes through the dam at depth d = 6 m. A plug secures the pipe opening. Find the magnitude of the frictional force between the plug and the pipe wall.

2. Relevant equations

gauge pressure = ρgd

3. The attempt at a solution

The answer to this problem seems to entail calculating the gauge pressure and then multiplying it by the area of the pipe to get the force of the water on the plug. I have three questions:

1) Why do we not consider the air pressure above the water here as opposed to only the gauge pressure (ρgd)? Is it because the water in the pipe is not below the air, but rather below rock? I'm not really clear on whether air pressure can only act downward or whether it can "sneak" into crevices like those created by the pipe.

2) If this problem instead involved a giant basin with a plug in the bottom, would we then have to consider the air pressure?

3) I initially tried to approach this problem using Bernoulli's equation, with the velocities equal to 0 on both side. I then realized that Bernoulli's equation reduces to the standard water pressure equation when the fluids aren't moving. Is it fair to say this is true?

Thanks a million!

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

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# Homework Help: Calculating water pressure, gauge pressure

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