Define terminal velocity.

Explanation

Stoke.s law: When a body falls through a highly viscous liquid, it drags the layer of the liquid immediately in contact with it. This results in a relative motion between the different layers of the liquid. As a result of this, the falling body experiences a viscous force F. Stoke performed many experiments on the motion of small spherical bodies in different fluids and concluded that the viscous force F acting on the spherical body depends on
(i) Coefficient of viscosity `eta` of the liquid
(ii) Radius a of the sphere and
(iii) Velocity v of the spherical body.
Dimensionally it can be proved that F = k `etaav`
Experimentally Stoke found that k = `6pi :. F = 6pi etaav`
This is Stoke.s law.
Terminal velocity: Terminal velocity of a body is defined as the constant velocity acquired by a body while falling through a viscous liquid.