You've read that the net force acting on an systems will change the system's momentum, but until now you haven't considered any particular forces. The first force that you will consider is one that results from the interaction between objects with mass: the gravitational force.

For the time being, you will consider only the motion of systems near the surface of the Earth and, hence, it is an observational fact that the gravitational force is a constant vector. Later, you will find that the gravitational force near the surface of the Earth is an approximation to the more general description of the gravitational force between objects.