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.

  • $\begingroup$ Force perpendicular on other molecules is meaningless. Molecules have positions. What does it mean to be perpendicular to a set of positions? $\endgroup$ – GiorgioP Jan 5 '19 at 18:30
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    $\begingroup$ Moreover, 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 physics.stackexchange.com/q/429998 or physics.stackexchange.com/q/142660 $\endgroup$ – GiorgioP Jan 5 '19 at 18:35
  • $\begingroup$ 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 $\endgroup$ – Muhammad Ali Jan 6 '19 at 14:53
  • $\begingroup$ yeah, that's right. It is a convenient description of the tensor behavior underlying pressure. $\endgroup$ – GiorgioP Jan 6 '19 at 15:28

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.

  • $\begingroup$ Can you elaborate more. If net force cancels out how pressure is in all directions then ? $\endgroup$ – Muhammad Ali Jan 5 '19 at 17:59
  • $\begingroup$ Pressure exists because the molecules collide with the wall and not due to molecule-molecule collision. $\endgroup$ – Harshit Joshi Jan 5 '19 at 18:00
  • $\begingroup$ So you mean pressure applied by molecules exist in all directions because of collision with wall or object not because with each other ? $\endgroup$ – Muhammad Ali Jan 5 '19 at 18:09
  • $\begingroup$ Yes exactly. Molecule-Molecule collision does not create pressure. $\endgroup$ – Harshit Joshi Jan 5 '19 at 18:10
  • $\begingroup$ This answer is misleading. If the net force cancels why the pressure can be different from zero? $\endgroup$ – GiorgioP Jan 5 '19 at 18:37

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.


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