Your friend says that the law of moment um conservation is violated when a ball rolls down a hill and gains momentum

 

Answer

The Law of Momentu m Conservation states that if no external forces act on a system,then the system’s momentum will not change. If you treat the ball as your system, it is clear that the gravitational force is acting on the ball, provides an impulse, and therefore changes the ball’s momentum. But this is not a violation of the Conservation of Momentum because there is an external force acting