Conservation of Momentum (Generalized Question)

[NY152]

Homework Statement

Will momentum not be conserved in the case of a free fall because gravity is considered an external force? Is this because the system is not isolated? If this is the case, I understand the concept now, but could someone give me another example of an external force that will impact momentum in a way in which it wouldn't be conserved?

Homework Equations

The Attempt at a Solution

 

[PeroK]

Any ball sport. E.g. a system of two tennis players, their rackets and the ball. Perhaps consider only horizontal momentum to take out the effect of gravity.