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

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Discussion Overview

The discussion revolves around the concept of pressure force acting on a flat plate, specifically whether this force is directed into or out of the plate. Participants explore the implications of positive and negative absolute pressure and its relevance in various contexts, including practical applications and theoretical considerations.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants express confusion about whether the pressure force acts into or out of the plate, with intuition suggesting it acts into the plate while mathematical interpretations suggest otherwise.
  • It is noted that if the plate is at rest, the pressure forces are balanced by equal and opposite forces from the plate, although some argue this balance holds regardless of the plate's state of motion.
  • For positive absolute pressure, some participants assert that the force exerted by the fluid on the plate is directed into the plate.
  • There is a discussion about the nature of absolute pressure, with some asserting it can only be positive, while others reference instances of negative absolute pressure in specific contexts, such as within xylem.
  • Concerns are raised about the relevance of negative pressure to the original question, with some participants arguing that it complicates the understanding of pressure in most practical situations.
  • Some participants challenge the notion of negative pressure, suggesting it may not apply in typical fluid dynamics scenarios and is more of an anomaly.

Areas of Agreement / Disagreement

Participants do not reach consensus on the direction of pressure force or the relevance of negative absolute pressure. Multiple competing views are presented, particularly regarding the implications of negative pressure in practical applications versus theoretical discussions.

Contextual Notes

Participants acknowledge that the discussion involves various assumptions about pressure definitions and the conditions under which pressure forces act. The relevance of negative absolute pressure is debated, with some suggesting it is an exception rather than a rule in fluid dynamics.

pyroknife
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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.
 
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pyroknife said:
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).
 
Forces associated with fluid pressure act toward the surface in contact with the fluid.
 
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pyroknife said:
...the math appear to be saying it is acting out of the plate.
What math? Show us.
 
Svein said:
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.
 
pyroknife said:
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.
 
A.T. said:
For positive absolute pressure the force by the fluid on the plate is into the plate.

Absolute pressure can only be positive.
 
A.T. said:
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.
 
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  • #10
boneh3ad said:
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.
boneh3ad said:
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).
 
  • #11
A.T. said:
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.
 
  • #12
A.T. said:
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?
 
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  • #13
Chestermiller said:
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.
 
  • #14
boneh3ad said:
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.
 
  • #15
boneh3ad said:
such a relatively anomalous occurrence.
Trees are all around us.
 
  • #16
A.T. said:
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.
 

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