Show that the force exerted by a fluid at rest is perpendicular to its contact surface.

(N/A) Consider an object submerged in a fluid at rest. The fluid exerts a force on the surface of the object. This force must be normal (perpendicular) to the surface.
To prove this by contradiction,assume that the force exerted by the fluid has a component parallel to the surface. According to $Newton's$ third law,the object would exert an equal and opposite force on the fluid parallel to the surface.
Since the fluid is at rest,it cannot sustain a shear stress (a force parallel to the surface). If such a parallel force existed,it would cause the fluid particles to flow along the surface. However,since the fluid is in a state of rest,no such flow occurs.
Therefore,the force exerted by the fluid at rest must be strictly perpendicular to the surface in contact with it.