When small spherical bodies move through a viscous medium, the bodies drag the layers of the medium that are in contact with them. This dragging results in relative motion between different layers, which are away from the body. Therefore, a viscous drag comes into play, opposing the motion of the body. It is found that this backward force or viscous drag, increases with increase in velocity of the body.
According to Stoke, the viscous drag 'f'' depends on-
1. the coefficient of viscosity 'h' of the medium,
2. the velocity (v) of the body and
3. radius (r) of the spherical body
By methods of dimensions, the values of a, b and c are 1,1,1 respectively. Therefore,
F drctly prtnl to v.n.r ie,
F= 6 pie .v.n.r