A spherical ball of iron of radius 2 mm is falling through a column of glycerine. If densities of glycerine and iron are respectively `1.3 xx 103 "kg/m"^(3)" and" 8 xx 103 "kg/m"^(3)`. `eta` for glycerine = `0.83 "Nm"^(–2)` sec, then the terminal velocity