**Stokes’ law : **If a fluid flows past a sphere or a sphere moves through a fluid, for small enough

∴ Viscous force = gravitational force-buoyant force

= mg – mLg

where m = mass of the sphere = \(\frac43\)\(\pi\)r^{3}ρ and m_{L</sub}. = mass of the liquid displaced = \(\frac43\)\(\pi\)r^{3}ρL.

At its terminal speed v_{t}, the magnitude of the viscous force by Stokes’ law is

[**Note : **Theoretically, v → v_{t} as time t → ∞. In practice, if η is appreciable, then v_{t} tends to v in a very small time interval.]

[Data: g = 9.8 m/s^{2}]