Stoke's law : 

when an spherical object moves through a viscous medium (at rest), it creates a relative motion between the different layer of liquid. As a result of which a viscous force comes into action which opposes the motion of falling object. This viscous force is given by ------

                                 F = 6𝛑𝛈rv

Terminal velocity : 

If a spherical object of radius r dropped in a liquid, then it will first accelerates and then its acceleration becomes zero and it attains a  constant velocity. The maximum constant velocity acquired by the object falling through a viscous medium is called terminal velocity.

In equilibrium-

Fv + Fup = W      {Fv = Viscous force & Fup = upthrust force}
Fv = W – Fup                     {m'= mass of liquid displaced}
6πηrV = mg – m’g              {m=mass of object}

6πηrV = volume(ρsg) – volume(ρl g)     {ρs= density of solid}

 6πηrV = volume g(ρs - ρl )                    {ρl= density of liquid}
 6πηrV = 4/3πr3 g (ρs - ρl )           {η= coefficient of viscosity}

V = 2r2 g(ρs - ρl ) / 9η                            { V= terminal velocity}

                                       

                                    V = 2r2 g(ρs - ρl ) / 9η
                                   https://youtu.be/UCHIvvAvekA