# Understanding pressure in fluids

1. Feb 6, 2012

### aaaa202

Okay so I already asked a bit about this, but I think I'm gonna need to post several questions on this to understand it, so here goes:

I understand the definition of pressure, just not how it is applied to fluids. How is pressure in a fluid to be interpreted. Is it the force that the particles that make up the fluid exert on an area in the fluid? I think that is correctly understand, but then I just don't understand why the pressure gets bigger the farther you are down a fluid. Why would the total force that the nearby particles exert on your area (we could imagine a thin disk in the fluid) depend on how many particles that are above you?
And further I don't even get how you can say things like this (which my book often does): Every little cube inside the fluid is at rest etc etc. Indeed the fluid is never at rest since, the particles that make it up are whirling around! What am I getting wrong?

2. Feb 7, 2012

### tiny-tim

hi aaaa202!

no, in a stationary liquid, the particles ain't goin' nowhere

they bump up against other particles, and transfer energy to them, but that's it …

the cube stays as it is

(in a flowing liquid, the cube moves and changes shape slightly, but the particles that make it up stay inside it)

(in a gas, i'm not so sure )
yes
it only gets bigger if there's gravity …

look at a a thick disc …

it's in equilibrium, so the force upward (from below) must equal the force downward (from above) plus the weight of the disc! …

ie pressure below = pressure above + ρgh