# Static fluid mechanics

1. May 22, 2012

### Baxidur

Hi

First off i'm not a student, this is purely a curiosity question

In pascals law classic drawing if you added a piston inside one of the cylinders what would happen?

I've added a drawing to clarify

I'm totally split on what the answer is, Pascals law seems to state the pressure on both sides of the piston will be equal, but real life seems to say the weight of the water in B will push the piston down

Thanks in advance for any help

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2. May 23, 2012

### tiny-tim

Hi Baxidur!
is the weight of the water in B pushing anything else down?

3. May 23, 2012

### Baxidur

Not that I can see

4. May 23, 2012

### tiny-tim

What about the bottom of the tank (on either side of the downpipe)?

5. May 23, 2012

### Staff: Mentor

Neither of your hypotheses is correct. The piston will not move downward, and the piston will not stay stationary. There is a third choice, and this is the correct answer.

The upward pressure on the bottom of the piston is initially higher than the downward pressure on the top of the piston. Since the piston has no weight (and there is no friction), there will be a net force on it. The liquid in column A will move downward (and the piston will move upward) until the pressures on the top and bottom of the piston are equilibrated.

Chet