The terminal velocity of a copper ball of radius $5\,\text{mm}$ falling through a tank of oil at room temperature is $10\,\text{cm}\,\text{s}^{-1}$. If the viscosity of oil at room temperature is $0.9\,\text{kg}\,\text{m}^{-1}\,\text{s}^{-1}$, the viscous drag force is: