[JEE 2012]

Solution

(i) When insect is moving with respect to the rod

(L_Rod is constant since ω is constant and hence its derivative is 0.)

(ii) Once insect reaches the end and stops with respect to the rod, the angular momentum of the system of rod + insect no longer changes (earlier it was changing due to the movement of the insect). Hence, torque becomes 0.

Answer: B