A steel ball with a specific gravity of 7.86 g cm^-3 and a diameter of 0.2 cm falls through a thick liquid along 10 cm for 25 s. The density of the liquid is 1.5 g cm-3. What is the radius of the tube when the viscosity of the liquid is 3.46 Pa s.(1 Pa = 1 Nm^-2 = 1 kg m^-1s^-2; g = 9.8 m s^-2).