Define viscosity. State the factors on which the viscous force depends.

Solution

Viscosity is the property of fluid by virtue of which an internal resistance comes into play when fluid is in motion and disturbs its motion.

Let a liquid is flowing on a horizontal surface.
Consider the two layers of liquid each of area A having a velocity gradient

dv over dx

as shown in the figure.
According to Newton. the viscous force is,
i) proportional to area of contact of two layers.
That is,

straight F space proportional to space straight A

ii) proportional to the velocity gradient between two layers.
i.e.,

straight F space proportional to space dv over dx space Now comma space combining space both space the space factors comma space we space have space space space space space space space space straight F space proportional to space straight A space space dv over dx space rightwards double arrow space straight F space equals space straight eta space straight A space dv over dx space where comma space straight eta space is space called space coefficient space of space viscosity. space

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