From where viscous force come about in moving liquids. Discuss the factors on which viscous drag of liquid depends.

Explanation

When a liquid is in motion, its different layers move with different velocities. Due to force of cohesion between liquid molecules, a fast moving layer will tend to increase the velocity of the adjacent slow moving layer and a slow moving layer will tends to decrease the velocity of the adjacent fast moving layer. As a result of it, an internal frictional force (called viscous drag) comes into play within the liquid which opposes the relative motion amongest its different layers.
A viscous drag (F) acting between the two layers of liquid in motion is related with the following factors.
(i) `F prop A` (A= Area of each layer)
(ii) `F prop d upsilon//dx` ,`dupsilon//dx`= velocity gradient between the layers)
Thus, `F prop A d upsilon//dx or F = - eta A d upsilon//dx`
Where `eta` is a constant of proportionality and is called coeff. of viscosity of liquid. The negative sign shows that the direction of F is opposite to the direction of velocity.