The viscous force on a solid moving through a fluid is proportional to its velocity. When a solid is dropped in a fluid, the forces acting on it are,
(a) Weight W acting vertically downward.
(b) The viscous force F acting vertically upward.
(c) The buoyancy force B acting vertically upward.
The weight W and buoyancy B are constant but the force F is proportional to the velocity v.
Initially, the velocity and the viscous force F is zero and the solid is accelerated due to the force W-B. Because of the acceleration the velocity increases. Accordingly, the viscous force also increases. At a certain instant, the viscous force becomes equal to W-B. The net force becomes zero and the solid falls with constant velocity. This constant velocity is known as the terminal velocity.