# Pressure on a curved surface

1. Nov 3, 2013

### Gh778

For compute force from pressure on a surface, do I need take in account the red force ? When an atom press a surface it press atoms at right and at left, this increase the pressure on the curved surface if the volume is closed ?

http://imageshack.us/a/img823/3852/66o4.jpg [Broken]

Or temperature cancel red forces ?

Last edited by a moderator: May 6, 2017
2. Nov 3, 2013

### mikeph

I do not understand the question. What is the red force?

3. Nov 3, 2013

### arildno

A continuous distribution of micro-atoms??

4. Nov 3, 2013

### Gh778

I consider the pressure come from top, like an object is in water (but liquid can be another than water).

The red force come from left and right atoms that compress atom. An atom receive pressure from top (in a liquid under gravity like you put an object in a swimming pool). But atom receive pressure from another atoms in the same depth (black arrows) this give red force.

Boundary line is the surface. A circle is an atom.

http://imageshack.us/a/img31/3277/o3qc.jpg [Broken]

Last edited by a moderator: May 6, 2017
5. Nov 3, 2013

### jbriggs444

As I read the drawing, he has the atom on the lower left pushing upward and right on the atom in the middle. The force on the atom in the middle is represented by the upward-and-right pointing black arrow. Similarly, the atom on the upper right is pushing downward and left on the atom in the middle with its force represented by the downward-and-left pointing black arrow.

The two black forces are nearly equal and opposite. But not quite. Their vector sum is the outward-pointing red arrow.

As Arildno seems to be suggesting, perhaps this is a depiction of a cylindrical single-atom-thick shell under pressure.

6. Nov 3, 2013

### Gh778

gravity is perpendicular to the screen, like that it's easier to understand for me. If radius of atom is divided by 2, the angle decrease, the red force too but the number of red forces increase too.

Last edited: Nov 3, 2013
7. Nov 3, 2013

### CWatters

The pressure at a "point" in a liquid is the same in all directions. No?

8. Nov 3, 2013

### dauto

Pressure doesn't have a direction.

9. Nov 3, 2013

### dauto

Yes the force between neighbor atoms creates a surface tension and that tension creates a pressure difference between the inside and the outside of the droplet due to the curvature of the surface.

Temperature's got nothing to do with it.

Last edited: Nov 3, 2013
10. Nov 3, 2013

### Gh778

1/ This would say the pressure on a dam http://en.wikipedia.org/wiki/Dam is bigger due to these forces ?

2/ If I place a tube full of liquid like the image show:

http://imageshack.us/a/img801/3471/3btm.jpg [Broken]

Red forces in the curvature are canceled by axis. But like sum of forces of all the tube is always zero, there are Fa and Fb forces. How the sum of torque can be to zero ?

Last edited by a moderator: May 6, 2017
11. Nov 3, 2013

### dauto

The curvature of the dam is too large to have an effect. This effect is noticeable in cloud droplets which have microscopic curvature radii.

The forces in the curved part of the tube will not be exactly axial

Last edited by a moderator: May 6, 2017
12. Nov 3, 2013

### Gh778

It's because the pressure of layer just above is lower ?

And what's cancel torque from Fb/Fa ?

Maybe it's easier with a square section.

Last edited: Nov 3, 2013
13. Nov 3, 2013

### Staff: Mentor

The pressure is the pressure. It is what it is and isn't two different pressures at once. It does not get concentrated or focused by varying the shape of the container. This is a basic principle of how pressure works. And I'm quite sure we've explained this to you before.