The angular momentum of a system of particles is not conserved

Angular momentum is a characteristic of rotating systems, and under certain conditions, it is conserved. The conservation of angular momentum is dependent on the fact that there is no net external torque applied to the system. Torque is a force that produces rotation and is a direct factor in the quantity of angular momentum. When there is no net external torque, the angular momentum will remain constant regardless of other external forces.

However, when some other torque applies on the system, it ruins this balance and leaves angular momentum to change. This principle can be noticed in everyday examples, such as figure skaters spinning faster once they pull their arms inside: no external torque is involved in the scenario. On the other hand, an external torque, such as friction or a push, applied to a rotating wheel changes its angular momentum.

External forces or impulses, per se, don’t change the angular momentum of a system unless they create torque. For example, a tangential force might change the linear motion of a system but wouldn’t change its angular momentum. Thus, whether or not external torque is applied to a system is the sole determining factor regarding whether its angular momentum is conserved. This is the most important concept in rotational mechanics and explains many of the phenomena occurring in the physical world.

Click here:
https://www.tiwariacademy.com/ncert-solutions/class-11/physics/chapter-6/