Does Newton's first law apply also to non-newtonian physics?

Question

I think I understand than an example of Newton's first law (intertial frames) would be a single asteroid in vaccum with no other bodies around, a comet in such free space or likewise. Then I think the asteroid or comet would continue just moving in the direction of the tangent vector from the original force if no other force acts. But would that theory also apply to non-newtonian gas or a non-newtonian fluid in the same situation? Or considering a particle small enough that quatum mechanics should apply instead of newtonian mechanics, would it still be apprioriate to presume that newton's first law will hold for microscopic particle in a vacuum?

Or would non-newtonian bodies, fluids and gases move randomly instead and we should believe that newton's first principle only holds for objects affected only or mostly by gravity?

Answer 1

Newton's first law does not apply to objects, but to observers. If you are an inertial observer, then you will see everything that is not acted upon by a force travelling in a straight line. There's no qualifier on the everything here - if it is not travelling in a straight line, it has a force acting upon it.

Non-Newtonian fluids derive their name almost tautologically from that they are not Newtonian fluids, which are fluids where "the tensors that describe the viscous stress and the strain rate are related by a constant viscosity tensor that does not depend on the stress state and velocity of the flow". Their nomenclature has nothing to do with the validity of Newton's laws for them, and they indeed obey these laws.

Now, a quantum particle is something else - it has (not counting the de Broglie-Bohm interpretation of QM) no trajectory to speak of, and so it does not make sense to ask whether it is travelling in a straight line or not. Yet, in the path integral approach, it is the classical path that contributes the most to the path integral, and that path would, for particle on which no forces act, be indeed a straight line. Again somewhat tautological, "in the classical limit", all quantum objects obey the classical (Newtonian) laws.