Gravitational force remains constant on the falling spherical ball. It is represented by straight line $\mathrm{P}$. The viscous force $(\mathrm{F}=6\mathrm{\pi \eta rv})$ increases as the velocity increases with time. Hence, it is represented by curve $\mathrm{Q}$.

Net force = gravitational force $–$ viscous force

As viscous force increases, net force decreases and finally becomes zero. Then the body falls with a constant terminal velocity. It is thus represented by curve $\mathrm{R}$.