Two identical uniform discs roll without slipping on two different surfaces $A B$ and $C D$ (see figure) starting at $A$ and C with linear speeds $\mathrm{v}_1$ and $\mathrm{v}_2$, respectively, and always remain in contact with ths surfaces. If they reach $B$ and $D$ with the same linear speed and $v_1=3 \mathrm{~m} / \mathrm{s}$, then $v_2$ in $\mathrm{m} / \mathrm{s}$ is $\left(g=10 \mathrm{~m} / \mathrm{s}^2\right)$