Question

When a rigid body rotates about an axis and the external torque is zero, then for that body which of the following is a costant

Answer 1

If the net torque is not zero, there will be angular acceleration. It implies the angular velocity will be continuously changing.

Conversely, if angular velocity is not changing, that is , if angular velocity is constant, angular acceleration is zero and there is no torque.

The body is either at rest or is rotating with constant angular speed.

Consider a ceiling fan rotating with constant speed.

It should imply net torque is zero.

But, in fact, the current produces a torque.

Similarly there is an opposing torque due to friction.

When these two torques balance each other , net torque is zero and the fan rotates with constant angular speed.

If not balanced, it rotates with increasing or decreasing speed.