Question

The momentum of a system is conserved:

I. when no net external force act on the system

II. when no internal forces act on the system

III. always

Answer 1

I think the answer is I.

Answer 2

I. when no net external force act on the system

Step-by-step explanation:

Suppose you are exerting an external force F on an object, it's velocity keeps increasing. Which means its momentum = velocity * mass is also increasing. In this case the initial momentum of that object is not preserved (but increased).