Question

Conservation of plain old momentum, p, can be thought of as the greatly expanded and modified descendant of Galileo’s original principle of inertia.

No force is required to keep an object in motion.

The principle of inertia is counterintuitive, and there are many situations in which it appears superficially that a force is needed to maintain motion, as maintained by Aristotle.

Think of a situation in which conservation of angular momentum, L, also seems to be violated, making it seem incorrectly that something external must act on a closed system to keep its angular momentum from “running down.”

Answer 1

A rotating shaft is a currect example of situation, where conservation of angular momentum seems to be violated.

We have keep supplying fuel for its running. Why does this appear so? It is simply due to some dissipative forces in the system. For example, here frictional force is acting as damping force where decreases the angular momentum over time.

This is true for every conservation law, if there is dissipative force in the system then it will not be conserved.