Physics · MECHANICAL PROPERTIES OF FLUIDS

A copper ball of radius \( 2.0\text{ mm} \) falls through a large column of oil at \( 20^\circ\text{C} \) with a terminal speed of \( 6.5\text{ cm/s} \). The density of copper is \( 8.9 \times 10^3\text{ kg/m}^3 \) and that of oil is \( 1.5 \times 10^3\text{ kg/m}^3 \). Taking \( g = 9.8\text{ m/s}^2 \), the viscous drag force acting on the ball at terminal speed is approximately:

1. A
   \( 4.8 \times 10^{-3}\text{ N} \)
2. B
   \( 1.2 \times 10^{-3}\text{ N} \)
3. C
   \( 2.4 \times 10^{-3}\text{ N} \)
4. D
   \( 3.6 \times 10^{-3}\text{ N} \)