Explain viscosity. Give some examples. Where is its part being played in nature?

Explanation

Viscosity of a liquid is the property of the liquid by virtue of which it opposes the relative motion amongst its differnet layers.
Consider a liquid layer moving over another layer which is at rest. Due to force of cohesion, the upper layer has tendency to move the lower layer along with it and lower layer tends to stop teh motion of the upper layer. As a result, and internal frictional force called 'viscous drag' comes into plays , which opposes the relative motion amongst its different layer. this internal frictional force in a liquid which oppose the relative motion amongst its different layers is called viscosity.
Some of application related with the steady of viscosity are:
(i) Chemists use this study to determine molecular weights and shape of large organic molecules such as proteins and cellulose.
(ii) In industry, the measurement of viscosity and its variation with temperature are useful in judging whether a given lubricant oil is useful for a machine or not.
(iii) In railway terminals, the liquids of high viscosity are used as buffers.