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I was learning about pressure due to fluids when I encountered the lines such as 'liquid layers exert perpendicular force on each other, fluid exert force on a body perpendicular to it in all directions, gas exerts pressure on a wall or anything only perpendicularly etc. Why all fluids exert only perpendicular force. Why not parallel?

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Why all fluids exert only perpendicular force. Why not parallel?

Fluids certainly can and do exert parallel forces. Such forces are called shear, and the defining feature of a fluid is that it continually deforms under shear. In other words, a fluid flows in response to shear forces.

Conversely a fluid which is not flowing is not undergoing any shear deformation and therefore must not be experiencing any shear forces. Such situations are called hydrostatic. Therefore, it is only hydrostatic fluids that do not exert parallel forces.

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Answers are different for liquids and gases.

In liquids, there are forces parallel to surface. This happens when liquid flows alongside a surface. This force is called viscous force (or sometimes shear force).

It is slightly complicated compared to force due to pressure. So many books teach it at later stage.

For gases, force is always perpendicular to surface. The reason is easy to understand.

Gas consists of large number of very small 'balls' (atoms or molecules). These balls are moving randomly due to thermal agitation. They are colliding elastically with each other and with walls.

Walls experience a force when these balls collide with it. Think of walls receiving large number of very small punches. Can you see that force will be perpendicular to surface!

EDIT: As pointed by "Dale" in comments, gases have viscosity too. So my answer is not entirely correct. This viscosity causes air drag on moving objects. Viscosity for gases is much less compared to liquids.

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    $\begingroup$ Gasses have viscosity also. It is just smaller than liquids. So gasses have shear forces just like liquids. This is one of the things that causes drag on an airplane $\endgroup$
    – Dale
    May 15 at 14:02

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