Viscosity

 The force which opposes the relative motion between different layer of liquid or gases is called viscous force.

The viscosity of liquid is due to cohesive force between its molecules.

The viscosity of gas is due to diffusion of its molecules from one layer to another layer.

Viscosity of gasses is much less than that of liquids. there is no viscosity in solids, viscosity of an ideal liquid is zero.

With rise in temperature, viscosity of liquid decreases and that for gas increases.

SI unit is decapoise or pascal per second.

Bernoulli's theorem:

Streamline flow of incompressible and non viscous fluid  through a tube, total energy(sum of pressure energy, potential energy and kinetic energy) per unit volume of fluid is same at all point.

** Critical velocity: the maximum velocity up to which fluid motion is streamline is called critical velocity.

If the velocity of flow les than critical velocity then flow of fluid basically depends upon viscosity of fluid.

if the velocity of fluid id more than critical velocity, the rate of flow depends on the density of fluid.