The density of a system could be non-uniform. Just imagine a baseball with feathers glued to its surface. If someone throws that baseball at you, getting grazed by the feathers is a fundamentally different experience than getting hit with the ball itself. This seems trivial and obvious, but it implies something interesting, which is that the density of a system could easily be non-uniform, and as a consequence, its momentum would in that case also be non-uniform. This is why getting grazed by the feathers is preferable, simply because they have less momentum than the body of the ball itself, despite the system moving as a whole, with a single velocity. As a general matter, this observation implies that interactions between materials, including mediums like air, could be locally heterogenous, despite the appearance of uniformity.

A second and not entirely related note is that all materials make use of free energy from fields. This must be the case, for otherwise gravity would cause everything to collapse to a single point. This does not happen because of intramolecular forces, that are the result of free energy from fields. This is again an obvious-in-hindsight observation, but it’s quite deep, for it implies that the structure of our Universe is due to the free energy of fields. That there’s a constant tension between the force of gravity and the intramolecular and atomic forces at work in literally every mass.