A ball rolls off the top of a stairway with horizontal velocity $u$. The steps are $0.1 \mathrm{~m}$ high and $0.1 \mathrm{~m}$ wide. The minimum velocity $u$ with which that ball just hits the step 5 of the stairway will be $\sqrt{x} \mathrm{~ms}^{-1}$ where $x=$ __________ [use $\mathrm{g}=10 \mathrm{~m} / \mathrm{s}^2$ ].