Stokes' law is the equation for the viscous drag F on a small sphere at low speeds.The drag force is directly proportional to the radius of the sphere and the velocity of the sphere. F = 6 π r η v
r = radius of sphere in metresv = velocity in m s-1η = coefficient of viscosity of the fluid in Pa s
r = radius of sphere in metresv = velocity in m s-1η = coefficient of viscosity of the fluid in Pa s