Direction of pressure force

1. Dec 2, 2015

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.

2. Dec 2, 2015

Svein

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).

3. Dec 3, 2015

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

4. Dec 3, 2015

Staff: Mentor

What math? Show us.

5. Dec 3, 2015

A.T.

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.

6. Dec 3, 2015

A.T.

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

7. Dec 3, 2015

Absolute pressure can only be positive.

8. Dec 3, 2015

A.T.

Last edited by a moderator: May 7, 2017
9. Dec 3, 2015

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.

Last edited by a moderator: May 7, 2017
10. Dec 3, 2015

A.T.

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).

11. Dec 3, 2015

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. Dec 3, 2015

Staff: Mentor

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?

13. Dec 3, 2015

A.T.

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. Dec 3, 2015

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. Dec 3, 2015

A.T.

Trees are all around us.

16. Dec 3, 2015