What is the direction of pressure force and why is it confusing?

[pyroknife]

I am confused about the concept of pressure force.

Let's say we have some pressure acting on a flat plate. Would the force associated with this pressure be oriented into or out of the plate? Intuition obviously tells me that the pressure force is acting into the plate, but the math appear to be saying it is acting out of the plate.

 

[Svein]

I am confused about the concept of pressure force.

Let's say we have some pressure acting on a flat plate. Would the force associated with this pressure be oriented into or out of the plate? Intuition obviously tells me that the pressure force is acting into the plate, but the math appear to be saying it is acting out of the plate.

As usual, that depends on the situation. If the plate is at rest, the pressure forces on the plate are matched by equal and opposite forces from the plate (I say forces, since I can easily visualize situations where there are different pressures on each side of the plate).

 

[boneh3ad]

Forces associated with fluid pressure act toward the surface in contact with the fluid.

 

[russ_watters]

...the math appear to be saying it is acting out of the plate.

What math? Show us.

 

[A.T.]

If the plate is at rest, the pressure forces on the plate are matched by equal and opposite forces from the plate

It doesn't matter if the plate is at rest, accelerating or whatever. There is always an equal but opposite force by the wall on the fluid per Newtons 3rd Law.

 

[A.T.]

Let's say we have some pressure acting on a flat plate. Would the force associated with this pressure be oriented into or out of the plate?

For positive absolute pressure the force by the fluid on the plate is into the plate.

 

[boneh3ad]

For positive absolute pressure the force by the fluid on the plate is into the plate.

Absolute pressure can only be positive.

 

[A.T.]

Absolute pressure can only be positive.

From: http://www.nature.com/nature/journal/v455/n7210/full/nature07226.html

Strikingly, the absolute pressure of the water within the xylem is often negative

 

[boneh3ad]

From: http://www.nature.com/nature/journal/v455/n7210/full/nature07226.html

Without reading the full article (I won't have access until I get to work) I can't say for certain how these authors define absolute pressure. I suspect it is likely different than the traditional context in fluid mechanics or kinetic theory, as a negative absolute pressure would imply a negative number of molecules of something in a given space.

 

[A.T.]

Without reading the full article (I won't have access until I get to work) I can't say for certain how these authors define absolute pressure.

Does the full sentence help?

Strikingly, the absolute pressure of the water within the xylem is often negative, such that the liquid is under tension and is thermodynamically metastable with respect to the vapour phase

Also:
https://en.wikipedia.org/wiki/Pressure#Negative_pressures

When attractive intermolecular forces (e.g., van der Waals forces or hydrogen bonds) between the particles of a fluid exceed repulsive forces due to thermal motion.

I suspect it is likely different than the traditional context in fluid mechanics or kinetic theory, as a negative absolute pressure would imply a negative number of molecules of something in a given space.

For this discussion it's only relevant that a liquid can be under tension, so the force by the liquid on the walls is into the liquid (it pulls at the walls).

 

[boneh3ad]

Does the full sentence help?

Also:
https://en.wikipedia.org/wiki/Pressure#Negative_pressures

For this discussion it's only relevant that a liquid can be under tension, so the force by the liquid on the walls is into the liquid (it pulls at the walls).

Okay but in the case where you have such a negative pressure, if it is only a result of intermolecular attraction in the fluid, then I would posit that the fluid is not tugging on the wall.

 

[Chestermiller]

Does the full sentence help?
Also:
https://en.wikipedia.org/wiki/Pressure#Negative_pressures
For this discussion it's only relevant that a liquid can be under tension, so the force by the liquid on the walls is into the liquid (it pulls at the walls).

Do you really feel it is worthwhile introducing this into the discussion, considering that the confusion of the OP, who is trying to understand how pressure acts in the other 99.999999999% of practical situations (i.e., not involving xylem) so that he can do his homework?

 

[A.T.]

Do you really feel it is worthwhile introducing this into the discussion, considering that the confusion of the OP, who is trying to understand how pressure acts in the other 99.999999999% of practical situations so that he can do his homework?

My answer to the OP was :

For positive absolute pressure the force by the fluid on the plate is into the plate.

I would have not mentioned negative pressure, if boneh3ad hadn't explicitly denied it's existence.

 

[boneh3ad]

Okay but in the case where you have such a negative pressure, if it is only a result of intermolecular attraction in the fluid, then I would posit that the fluid is not tugging on the wall.

This doesn't make sense either now that I think about it. Essentially, surface tension is the exception to the no negative pressure rule if I am reading correctly. So except in the case of drops and bubbles, for example, negative absolute pressure is essentially not going to occur. I'm not sure it is therefore germane to the discussion, been such a relatively anomalous occurrence.

 

[A.T.]

such a relatively anomalous occurrence.

Trees are all around us.

 

[boneh3ad]

Trees are all around us.

Oh come on, I've already admitted defeat and that you've enlightened me and now you want to go this route? Yes, it is still a relatively anomalous occurrence. Out of all of the fluid dynamics occurring in the universe (or even just on this planet), it is still a very small percentage of these phenomena in which the concept of negative absolute pressure is relevant. The volume of water in trees on Earth is, after all, pretty inconsequential compared to the volume of the water and all of the gases in the universe where this concept is not observed.

So yes, I already conceded that negative absolute pressure seems to be a thing and you've sent me off on a tangent trying to learn more about it, but please don't act like negative absolute pressures are relevant to the vast majority of fluid flows. They aren't.