\( \text { Initial energy }=\mathrm{mg} \ell \)
\( \text { So, } \frac{9}{10} \mathrm{mg} \ell=\frac{1}{2} \mathrm{mv}^2 \)
\( \Rightarrow \frac{9}{10} \times 10 \times 10=\frac{1}{2} \mathrm{v}^2 \)
\( \mathrm{v}^2=180 \)
\( \mathrm{v}=\sqrt{180}=6 \sqrt{5} \mathrm{~m} / \mathrm{s}\)
($g = \pi ^2$ )