Application of Stokes' Law to the motion of a bubble of a gas through a liquid

If a body is allowed to fall freely from rest in air, under the influence of gravity alone, it falls with an acceleration of 981 cm(per second)2. The velocity which it has at any particular time is a function of the time during which it has been falling and is given by v = gt; where v = velocity; g = Acceleration due to gravity; t = time during which it has been falling.;The original mass of the particle = 4/3 pi A3sigma.;But if a small spherical particle falls from rest, in a medium as a No.1 shot in a liquid or as an oil drop in air, it increases in velocity until a certain maximum velocity is reached and continues with this velocity until it reaches the end of its path. According to Stokes' Law the resultant force F acting on the particle is: F = 6 pi ANV (1); where A = radius of the sphere; N = Coefficient of viscosity of the medium; V = terminal velocity; pi = 3.1416.