Question

Which of the following quantities is zero if torque on a particle is zero?

(a) Angular momentum of the particle

(b) Angular speed of the particle

(c) Change in kinetic energy of the particle

(d) Rate of change of angular momentum

This question was addressed to me at a job interview.

The above asked question is from Angular Momentum in case of Rotations about a Fixed Axis in section System of Particles and Rotational Motion of Physics – Class 11

Answer 1

Right option is (d) Rate of change of angular momentum

Explanation: Torque is defined as the rate of change of angular momentum. So, if torque is zero, the rate of change of angular momentum will be zero. An important thing to note is that one may think how kinetic energy can change if torque is zero. For this consider the example of a ballet dancer, when she/he pulls in her hands while rotating she/he becomes faster but angular momentum remains constant since net torque is zero. But her kinetic energy = 1/2 (I) will increase as  has increased.