In the figure shown the velocity of lift is $2 \mathrm{m} {\mathrm{s}}^{-1}$ upwards while the string is winding on the motor shaft with velocity $2 \mathrm{m} {\mathrm{s}}^{-1}$ and block $\mathrm{A}$ is moving downwards with a velocity of $2 \mathrm{m} {\mathrm{s}}^{-1}$, then find out the velocity of block $\mathrm{B}$.