Typical silt (hard mud) particle of radius $\mathrm{20 }\mathrm{\mu }\mathrm{m}$ is on the top of lake water, its density is $2000 \mathrm{k}\mathrm{g}\mathrm{ }{\mathrm{m}}^{-3}$ and the viscosity of lake water is $1.0 \mathrm{mPa}$, density is $1000 \mathrm{k}\mathrm{g}\mathrm{ }{\mathrm{m}}^{-3}$. If the lake is still(has no internal fluid motion). The terminal speed with which the particle hits the bottom of the lake is .......... ${\mathrm{m}\mathrm{m }\mathrm{s}}^{-1}$