Question

according to stuck's law,the viscous force 'F' acting on a body depends upon the radius 'r' and velocity 'v' of the body and coefficient of viscosity 'n' of fluid.Derive a formula for viscous force.

Answer 1

This is about Stoke's law for drag force on a body particle traveling in a viscous fluid.
F = 6 pi eta r v

Derivation.

Force required is to overcome friction exerted by fluid on the surface of the body.
Force is directly proportional to the radius as bigger the body larger the force required.
Force required is also more if the body travels faster. As collisions are more at higher velocities.
Also if eta is more then force required is more.

So force F = k * eta * r * v
k = constant of proportionality.