In classical mechanics, I can measure the inertial mass of a particle by measuring force and acceleration: m=F/a. In QM, this equation only holds for expectation values <F> and <a>. Does this lead to the fact that inertial mass is not an observable? Is there a deeper underlying principle which determines if a classical observable has a quantum mechanical analogue? Like, it must be obtainable by some kind of "direct" measurement? For example, I'd guess that moment of inertia is not an observable in QM since mathematically, it is very similar to inertial mass. Also, time isn't an observable, but this is probably a different case (see the other thread).