Stokes’ Law and its formula: Eminent physicist Sir Stokes investigated fluid dynamics and came up with an equation for the viscous drag (F) on a small sphere at low speeds. This formula is now called Stokes’ Law.
The Stokes’ Law formula is represented in this way: F = 6 πrȠV ………… (1)
where r is the radius of the sphere (with mass m), v is the velocity of the sphere (m/s) and Ƞ is the coefficient of viscosity of the fluid (Pa s).
Thus in such a simple situation, the viscous drag force is directly proportional to the radius of the sphere and directly proportional to the velocity. This drag force is also directly proportional to the coefficient of viscosity of the fluid.
Also, read how using Stoke’s law the equation of terminal velocity can be derived.