A stone of mass 20 g is projected from a rubber catapult of length 0.1 m and area of cross section $10^{-6} \mathrm{~m}^2$ stretched by an amount 0.04 m . The velocity of the projected stone is $\_\_\_\_$ $\mathrm{m} / \mathrm{s}$.
(Young's modulus of rubber $=0.5 \times 10^9 \mathrm{~N} / \mathrm{m}^2$ )