In a plant, solid particles $($i$.$e$.$ ash$)$ are carried away by the stack gas and auses air poliution. What should be diameter of the stack to prevent pollution if the fiow rate of the gas is $10\frac{m^3}{s}?$ Take the stack gas as air. The density and diameter of the solid particles are $680\frac{Kg}{m^3}$ and $1\mathrm{.}25mm,$ respectively.