In an inelastic collision where mechanical energy is not conserved, momentum is still conserved. Should the Earth or the ground be considered as part of the system in order to account for the conservation of momentum? Since there is friction in a collision, is it appropriate to include the ground as part of the system to analyze the conservation of momentum?
Are air particles considered to be a part of this isolated system?
To clarify I am specifically talking about two vertically-moving objects colliding.