How does pressure act in all directions if gravity is downward only

[iVenky]

I have a fundamental question about the pressure and I am not sure if my reasoning is correct.

Let's take an example of 7 stones shown below. They don't fall because the center of mass of each stone is falling right under the stone below. If I replace the same thing with a liquid, and we know that the molecules are free to move in a liquid, they all spread out and become flat until the net potential energy is minimum. My fundamental question is what triggered this horizontal force in the first place if gravity is a downward (vertical force). There needs to be some horizontal force to make the molecules move in the horizontal direction. Is it because there is some horizontal force present already in a liquid even in the absence of gravity that makes the molecules move away from each other due to the pressure difference between air (more gaps) and liquid? And it's the same in all directions because only then there is no net force present on the molecule itself that stops it from creating gaps in the liquid?

If that's the case, even solids experience this pressure difference or net force acting on it due to the atmosphere but it doesn't move like liquid because it requires more force to break the force between the molecules. Is this intuition of pressure right?

 

[etotheipi]

There needs to be some horizontal force to make the molecules move in the horizontal direction.

I think you are getting at the right idea. You can try and analyse what's happening by considering a small parcel of water, which will be acted upon by pressure forces from all the surrounding water (surface forces) in addition to its weight (body force). Even in a direction orthogonal to the gravitational field, in a non-equilibrium situation the pressure forces on either side of the parcel may differ in magnitude which will result in the parcel accelerating horizontally too.

 

[hutchphd]

And don't forget that at any finite Temperature everything is constantly wiggling.

 

[Lnewqban]

... My fundamental question is what triggered this horizontal force in the first place if gravity is a downward (vertical force). There needs to be some horizontal force to make the molecules move in the horizontal direction.

That vertical arrangement is impossible to achieve (for more than few seconds perhaps) if you replace the stones, which enjoy some superficial friction, with steel ball bearings or magnets.
Even having a vertical direction, gravity will be the motor to take each piece to the lowest possible level.

 

[Dale]

My fundamental question is what triggered this horizontal force in the first place if gravity is a downward (vertical force). There needs to be some horizontal force to make the molecules move in the horizontal direction.

Gravity is not the only force acting on the molecules. There is also the internal stress from the other water molecules. This is an internal force so it cannot cause an acceleration of the center of mass, but it certainly can make the liquid spread horizontally.

Fluids deform under shear stress. In a “pile” the stress is primarily axial compression. This means that the principal stresses are unequal, and any time that is the case you will have shear stresses at 45 degrees. So the fluid will “slump” down until the principal stresses are equal and you are left with only an isotropic pressure and no shear stresses.