Question

stress at a point in fluid at rest and in motion derivation​

Answer 1

"Concept:

A solid can resist shear stress by a static deformation; a fluid cannot

Any shear stress applied to a fluid, no matter how small, will result in motion of that fluid

The fluid moves and deforms continuously as long as the shear stress is applied

So, a fluid at rest must be in a state of zero shear stress"