Question

1. how do internal and external forces affect the law of conservation of momentum?

only internal forces can act on a system for the law of conservation of momentum to occur.
external forces can act on a system, but the internal forces must double to counteract the external factors for the law of conservation of momentum to be observed
only external forces can act on a system for the law of conservation of momentum to occur
both external and internal forces must be present for the law of conservation of momentum to occur.

Explanation:

The law of conservation of momentum states that the total momentum of a closed system (a system with no net external force acting on it) remains constant. External forces would change the total momentum of the system, so for conservation to hold, only internal forces (which cancel out in pairs within the system) can act on the system.

Answer:

A. Only internal forces can act on a system for the law of conservation of momentum to occur.