Question

What is the rate of change of angular momentum?

(a) Force

(b) Torque

(c) Work

(d) Angular velocity

This question was posed to me during an interview.

The doubt is from Angular Momentum in case of Rotations about a Fixed Axis in chapter System of Particles and Rotational Motion of Physics – Class 11

Answer 1

Correct choice is (b) Torque

The explanation: Let L, r & p be the angular momentum, radius & momentum vectors respectively.              Angular momentum of a body is ∑(r X p). On differentiating w.r.t time,                                                                  dL/dt = ∑{dr/dt X p + r X dp/dt} = ∑{v X mv + r X F} = ∑{r X F} = total torque ‘τ’.

Here, F is the total force vector. And v X mv is zero because the cross product of two vectors in the same direction is zero.