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King E

Archimedes principle in the molecular world.

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Posted (edited)
If all the molecules at the same height in a container feel the same pressure then how does it remain true if the shape of the container is changed? Where are the vertical reaction vectors coming from on the right side, given that the container is a single rigid body and is thus not being pushed down onto the liquid molecules by gravity?

 

 

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Edited by King E

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Posted (edited)

I recommend you ditch that model and its source pronto.

What is says is only incorrect in a very subtle point, but is is woefully incomplete.

It is incorrect because molecules do not feel pressure.
They are just not big enough.

Pressure is a macroscopic phenomenon on fluid 'particles' or fluid elements, which are much bigger than molecules.

So taking those balls as fluid elements they experience pressure as shown including the missing 'reaction' from he bottom of the container in the second diagram.
I can just see the tops of the hidden arrows under the bottom rows of balls.

However pressure is isotropic which means that at any point in the fluid it is the same in all directions.
Pressure is a scalar not a vector.

So the diagram should have included horizontal force vectors and reaction vectors from the sidewalls and the underside of the 'toe' in the second diagram.
(This last reaction answers your question)

Also the fluid elements would have been better drawn as little bricks or cubes.
Balls leave spaces between the points where they touch, but there are no spaces in fluids.

 

I will take this opportunity to off a tip , which is what I thought the thread was going to be about when I read the title.

There are often several forces active but one may be so dominant that others can be ignored.

This is true of molecules in fluids.
They do not feel the pressure force, but they are susceptible to surface forces, including surface tension.
Chemically and Biologically this has great significance for life, chemical reactions, cells and so on.
In Physics the balance of presssure, surface and gravitational forces determine the type and transmission of waves.

Similarly we ignore gravitational forces in the atom, which are many orders of magnitude weaker than the electrostatic ones.
Yet the electrostatic ones are outmatched by the strong nuclear force at very short range.

Edited by studiot

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