$$\tau = I\alpha$$
where:
* τ is torque
* I is the moment of inertia
* α is angular acceleration
If the angular momentum (L) of the system is constant, then the angular acceleration (α) must be zero. This is because angular momentum is defined as the product of the moment of inertia and angular velocity:
$$L = I\omega$$
where:
* L is angular momentum
* I is the moment of inertia
* ω is angular velocity
If L is constant, then I and ω must be constant. This means that the system is not accelerating, and therefore the torque acting on the system is zero.
In summary, when the angular momentum of a system is constant, the torque acting on the system is zero.
