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# Newton's Law of Viscosity, Newtonian and Non-Newtonian Fluids

## Synonyms

## Definition

Viscosity is the physical property that characterizes the flow resistance of simple fluids. Newton's law of viscosity defines the relationship between the shear stress and shear rate of a fluid subjected to a mechanical stress. The ratio of shear stress to shear rate is a constant, for a given temperature and pressure, and is defined as the viscosity or coefficient of viscosity. Newtonian fluids obey Newton's law of viscosity. The viscosity is independent of the shear rate.

Non-Newtonian fluids do not follow Newton's law and, thus, their viscosity (ratio of shear stress to shear rate) is not constant and is dependent on the shear rate.

Dynamic viscosity is the coefficient of viscosity as defined in Newton's law of viscosity. Kinematic viscosity is the dynamic viscosity divided by the density.