The terminal velocity of a copper ball of radius 2.0 mm falling through a tank of oil at `20^o`C is 6.5 cm `s^(-1)`. Compute the viscosity of the oil at `20^o`C. Density of oil is `1.5 ×10^3` kg `m^(-3)`, density of copper is `8.9 × 10^3` kg `m^(-3)`.