Question

Your friend says that the law of momentum conservation is violated when a ball rolls down a hill and gains momentum. What do you say?

Answer 1

Answer:

The friend is wrong: the law of conservation of momentum is not violated

Explanation:

The law of conservation of momentum states that:

"For a system of an object, if the net external force acting on the system is zero, then the total momentum of the system is conserved"

In our problem, the system is the ball rolling down the hill. The ball gains speed as it rolls down, so its momentum increases, according to the equation:

$p=mv$

where m is the mass of the ball and v its velocity. This may appear as a violation of the law, however it is not. In fact, the net external force acting on the ball is not zero, since there is the force of gravity acting on the ball. As a result, the force of gravity gives an acceleration to the ball, increasing its momentum: therefore, the law is not violated.