Stokes' Formula: When a spherical object falls by the fluid, layer in immediate touch with it, sticks to it and goes along with the object with the same speed. The layer next to it has lower speed, the next still lesser and so on, whereas the part at a considerable distance from the object remains at rest. As a result of this relative rotation, a backward dragging force is set up, which repels the motion of the object. This backward dragging tension, known as the viscous force. As the object falls, its speed goes on enhancing and likewise the viscous force on the object becomes similar to the weight of the object. Then, the object goes with a constant speed, known as terminal speed. Stokes found that when the object attains terminal speed, the viscous force on the object is shown by,

Here n, r and v respectively represents the coefficient of viscosity of the fluid, radius of the spherical object and the terminal velocity attained by the object. The relation (i) is known as Stokes' law.