Two identical uniform discs roll without slipping on two different surfaces AB and CD (see figure) starting at A and $C$ with linear speeds $v_1$ and $v_2$, 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 $\left(g=10 \mathrm{~m} / \mathrm{s}^2\right)$