Questian about static pressure. 1

[mechengfan]

They say that the static pressure at a point is equal from all directions. How can that be possible that the pressure above the point (p) is equal to the weight of water column (h1) which is directed downwards, and the pressure below that point is equal to the weight of water column (h2) which is directed upwards (refer to attached fig.)?

Thank you

 

[zdas04]

A "point" does not have dimensions. It is infintessimally small so there is no appreciable difference between the pressure at the top and the pressure at the bottom. If you considered a "point" to be 3 m across, then your question would be very valid--pressure at the top of the point within a liquid column is measurably less than pressure at the bottom. But a "point" is smaller than the smallest unit of measurement that we can name so hydrostatic variation is irrelevant.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[rb1957]

i think "the pressure above the point (p) is equal to the weight of water column (h1)" is ok, but "the pressure below that point is equal to the weight of water column (h2)" isn't.

like the above post, the pressure at p is rho*g*h1, acting in all directions. thought exercise ... what direction does the pressure inside a ballooon act ?

 

[IRstuff]

Alternately, consider if the pressure was NOT the same in all directions, what would that mean?

A pressure differential in a fluid MUST result in the flow in the direction of least pressure, which is Fick's First Law of diffusion. If there is no flow and there is nothing stopping a flow, then there must not be a pressure difference.

TTFN
faq731-376

 

[Twoballcane]

I like the definition that static pressure is the force excreted against the wall of the container. And remember that total pressure is the summation of velocity pressure and static pressure. Now in the case of incompressible fluids (e.g.. water in a column), using Bernoulli’s equation (with velocity equal to zero and having a delta Z) the total pressure at the top will be smaller than the total pressure at the bottom and since there is no flow, velocity pressure is zero, thus total pressure will equal static pressure thus a delta of static pressure from top to bottom. Now it’s tricky for compressible fluids (e.g. air in a column). The density is so slight that the delta from the top of the column to the bottom is very small and may seem equal, but it is not. It is the same for a balloon. Let’s think in a more grand scale. The air pressure at sea level is 14.7 psi, but air pressure at the top of Mount Everest is 4.4 psi. If we had a balloon as high as Mount Everest and pressurized it, the static pressure at the top of the balloon will be smaller than the static pressure at the bottom. Now with that said, this is an impractical idea to use. So at sea level (or at any level), especially compressible flow, such as AC duct systems or electronic cooling in a chassis or rack, we take static pressure to be equally distributed along the wall.

Tobalcane
"If you avoid failure, you also avoid success."
“Luck is where preparation meets opportunity”

 

[TheTick]

Sum of forces equals ZERO! = not accelerating

 

[Twoballcane]

Ummm...what's not accelerating?

Tobalcane
"If you avoid failure, you also avoid success."
“Luck is where preparation meets opportunity”

 

[mechengfan]

He means that when the sum of forces upon a point=0 the point will not move.

 

[zdas04]

TwoBallCane,
Let's not suck Mr. Bernoulli into a statics problem. If velocity is equal to zero then the assumptions behind Bernoulli's Law are violated and any number that results is meaningless.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[TheTick]

Thinking in terms of neutral buoyancy.

 

[Twoballcane]

OK, I see what you’re saying. My bad. Bernoulli is my go to equation for anything fluid. I deal more with dynamic than static. My little memory trick to get from dynamic to static is to have velocity go to zero and then delta P = rho x g x delta Z which is Pascal’s hydrostatic pressure. But I beg to differ on acceleration. Gravity (which is acceleration) is included in the physics as discussed in my previous post. Also, doesn’t bouncy force have acceleration g in it’s equation?

Tobalcane
"If you avoid failure, you also avoid success."
“Luck is where preparation meets opportunity”

 

[zdas04]

There is a difference between a fluid being subject to a body force and the fluid mass accelerating.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.