We say pressure is isotropic. At any point in a fluid pressure acts equally in all directions, but pressure is perpendicular force per unit area so how can pressure be there in all directions as pressure will only exist if fluid molecules apply force perpendicular on other molecules.
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Force perpendicular on other molecules is meaningless. Molecules have positions. What does it mean to be perpendicular to a set of positions? – GiorgioP-DoomsdayClockIsAt-90 Jan 05 '19 at 18:30
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1Moreover, pressure, even if is equal to the ratio between normal force and area of a surface, is not a force. A better definition requires the introduction of the stress tensor. Some good answers related to your question can be found at https://physics.stackexchange.com/q/429998/ or https://physics.stackexchange.com/q/142660/ – GiorgioP-DoomsdayClockIsAt-90 Jan 05 '19 at 18:35
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OK..After reading those answers. Am I correct to say that when force is applied perpendicular to surface pressure results but that pressure is in all directions (as pressure itself has no direction) it is not necessary everywhere force is perpendicular as at any point once pressure applied, it is transmitted in all direction. @GiorgioP – Muhammad Ali Jan 06 '19 at 14:53
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yeah, that's right. It is a convenient description of the tensor behavior underlying pressure. – GiorgioP-DoomsdayClockIsAt-90 Jan 06 '19 at 15:28
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Yes, pressure is the force perpendicular to a surface. Isotropic in that case simply means that the force will be the same regardless of the orientation of that surface.
Dale
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Fluid molecules apply force perpendicular on other molecules.
This does happen.
But because many molecules surround a single molecule, the net force cancels out and so there is net effect of this force.
Harshit Joshi
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Can you elaborate more. If net force cancels out how pressure is in all directions then ? – Muhammad Ali Jan 05 '19 at 17:59
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Pressure exists because the molecules collide with the wall and not due to molecule-molecule collision. – Harshit Joshi Jan 05 '19 at 18:00
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So you mean pressure applied by molecules exist in all directions because of collision with wall or object not because with each other ? – Muhammad Ali Jan 05 '19 at 18:09
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Yes exactly. Molecule-Molecule collision does not create pressure. – Harshit Joshi Jan 05 '19 at 18:10
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This answer is misleading. If the net force cancels why the pressure can be different from zero? – GiorgioP-DoomsdayClockIsAt-90 Jan 05 '19 at 18:37
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@GiorgioP The net force between the molecules cancel out. The net force between the molecule and wall does not cancel out. The molecules of gas exert a net force on the container wall. – Harshit Joshi Jan 05 '19 at 18:40
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@harshit54 you are wrong. If the average force that the molecules in a small volume exert on those in a volume close to the wall would be zero, tere would be no possibility of nechanical equilibrium. Pressure is an intensive quantity which has the same value everywhere in a fluid not subject to external fields. – GiorgioP-DoomsdayClockIsAt-90 Jan 06 '19 at 01:04
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@GiorgioP Do you agree that the internal molecule-molecule cohesive forces cancel out due to one molecule being surrounded by many others? – Harshit Joshi Jan 06 '19 at 01:50
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@GiorgioP Also do you agree that due to the weight of the fluid above it, the fluid would like to spread out of the container thereby exerting a net force outwards. – Harshit Joshi Jan 06 '19 at 01:53
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@GiorgioP In the comments I accidentally wrote collision instead of force. Sorry for that. – Harshit Joshi Jan 06 '19 at 01:54