Newton,s Law of Viscosity:

According to Newton,s law of viscosity, the shear stress on a layer of a fluid is directly proportional to the rate of shear strain.

Another way it can say, for a given shear stress acting on a fluid element, the rate at which the fluid deforms is inversely proportional to the viscosity.

This implies that for a constant shear stress, the rate at which deformation takes place is larger for fluids of low viscosity.

For solids the resistance to shear deformation  is due to modulus of elasticity whereas for fluids, the resistance to rate of shear deformation is on account of the viscosity. The hook,s law for solids analogous to the Newtons Law of Viscosity.

As per Newtons law of viscosity the following important points may be noted for viscous flow:

(1) A fluid which has no viscosity is known as an Ideal fluid.

(2) A fluid which has viscosity is known as real fluid.

Newtonian Fluid: A fluid whose viscosity does not change with the rate of deformation or shear strain is known as Newtonian fluid.

Or, a fluid which obeys Newton,s law of viscosity is known as Newtoniun fluid.

Example of Newtonian fluid: Air, water etc are example of Newtoniun fluid.

newtoniun and non newtoniun fluid 

Non Newtoniun Fluid: A fluid whose viscosity change with the rate of deformation or shear strain is known as Non-Newtoniun fluid.

Example of non-Newtoniun fluid: Tooth Paste, honey, blood are example of Non-newtoniun fluid.

Laminar flow:  A flow in which viscosity of fluid dominating over the inertia forces, is called laminar flow. It takes place at very low velocities.

Turbulant flow: A flow in which inertia force dominating over the viscosity is called, turbulent flow.

Critical velocity: the velocity at which the flow changes from laminar to turbulent flow is called critical velocity. It is of two types,i.e lower critical velocity and higher critical velocity. The velocity at which laminar flow stop is called Lower critical velocity, while the velocity at which turbulent flow starts is known as turbulent flow.

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