A bullet of $10 \mathrm{g}$, moving with velocity $v$, collides head-on with the stationary bob of a pendulum and recoils with velocity $100 \mathrm{m} {\mathrm{s}}^{-1}$. The length of the pendulum is $0.5 \mathrm{m}$ and mass of the bob is $1 \mathrm{kg}$. The minimum value of $v \text{in} \mathrm{m} {\mathrm{s}}^{-1}$, so that the pendulum describes a circle. (Assume the string to be inextensible and $\mathrm{g}=10 \mathrm{m} {\mathrm{s}}^{-2}$ )

Hence, the correct answer is 400.