That formula determines the resisting force influencing a spherical body which moves uniformly in a given medium. The force R is equal:
where n - viscosity of medium, r - radius of a drop, v - velocity of a drop.
In the gravitational field, in the air the spherical droplets after a while begin moving uniformly - the downward force is equal:
The mass of a single drop is equal:
where f - density of the drop.
That force is equal the upward resisting force. Equating the two forces and transforming the formula we get the velocity of a drop equal: