Fluid, pressure and density

[silverice]

Pressure=rho*g*h=Force/Area
Please see the the piston shown below. If density of the fluid increases, will pressure increase? TBR saids no because neither force or area has changed. But I'm confused because according to the definition of rho g h, pressure should increase.

View attachment pascal.gif

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[milski]

If they were talking about hydrostatic pressure (pressure created from the weight of the fluid), you would be correct. Since they are talking about a piston, I assume that they mean only the pressure created by the piston applying force on the fluid. That pressure stays the same, regardless of the density of the fluid.

If that's what really what's being asked, it's a fairy poorly diagramed question - the piston is missing and having the fluid oriented that way will certainly make me think about including the pressure from gravity.

 

[silverice]

If they were talking about hydrostatic pressure (pressure created from the weight of the fluid), you would be correct. Since they are talking about a piston, I assume that they mean only the pressure created by the piston applying force on the fluid. That pressure stays the same, regardless of the density of the fluid.

If that's what really what's being asked, it's a fairy poorly diagramed question - the piston is missing and having the fluid oriented that way will certainly make me think about including the pressure from gravity.

The original question was form a book and the original picture had piston on it but I was too lazy to take picture to upload and all that. Therefore I stole a picture online.

It's funny you point out the picture is inadequately describing the question. The reason I chose this bad picture simply because I wasn't actively aware of the difference between plain hydrostatic pressure and the pressure in a piston system.

I say, I gotta love TBR!

you always give the best reply, thx!

 

[SaintJude]

I still don't understand it all. If hydrostatic pressure is related to weight, how does density not make a difference?

 

[milski]

I still don't understand it all. If hydrostatic pressure is related to weight, how does density not make a difference?

It does. But the original question is not about hydrostatic pressure. Picture a piston in a horizontal pipe instead of what's posted here.

 

[SaintJude]

Oh, they're just basically saying what goes on in a hydraulic lift.