Well it is well known that there are primarily three states of matter, namely solid, liquid and gas. Plasma is often included as a fourth state of matter. Here we will deal with fluids, which basically cover both liquids and gases. Fluids differ from solids in the fact that they cannot withstand any shear stress.
Any amount of shear stress, however small, will cause a fluid to deform continuously.
As such for any fluid at rest or in the statics conditions, the net force acting on any part of the fluid is perpendicular to the surface. Because if it is not so, the force will have a tangential component along the surface i.e. a shear stress, causing the fluid to deform or move (no more statics!).
Thus, in any reference frame, if a fluid body is in statics condition, the net force acting on any surface will be perpendicular to…
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