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give an explanation about viscosity
Let us pour equal amounts of water and castor oil in two identical funnels.It is observed that water flows out of the funnel very quickly whereas the flow of castor oil is very slow.This is because of the frictional force acting with in the liquid.This force offered by the adjacent liquid layers is known as viscous force and the phenomenon is called viscosity.

Viscosity is the property of the fluid by virtue of which it opposes relative motion between its layers .Both liquids and gases exhibit viscosity but liquids are much more viscous than gases.
Expression for viscous force
Newton showed that the tangential  viscous force is directly proportional to (i) the surface area of the layers and 
(ii) the velocity gradient.
i.e; F ? A, and F ? (dv/dx) 
Therefore F ? A (dv/dx) 
F = ? A  (dv /dx)
where ? is a constant known as coefficient of viscosity of the liquid.


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