We conclude

• Fluid continues to deform (or move) under the application of a shear force.
• Fluid at rest cannot sustain a shear stress.

Fluid resists deformation by attaining an equilibrium rate of deformation.
Viscosity is a property of the fluid which relates its resistance to the applied shear force.

Newton's law of viscosity

A class of fluids called Newtonian fluids such as water and air follow the behavior: “applied shear stress varies linearly with the rate of deformation”. The mathematical representation is