Classical mechanics is for 'everyday stuff'
Relativistic mechanics is for things going very very fast
Quantum mechanics is for very small things---it's not too tough... just the same seven postulates repeated endlessly
For spectroscopic work the relativistic and quantum mechanics are both important...and that's a very nasty topic:
Einstein's time dilation theory was proven using an two atomic clocks and a DC-8...the moving one slowed a bit---
according to the correspondence principle QM will converge to suit the classical case
All measuring devices introduce a perturbation in the system... a thermometer or electric metering transformer,..flowmeter.... for example...thereby altering the system and creates a definite inaccuracy...albeit infinitesimal is most cases...an NOTHING CAN BE DONE ABOUT IT
Lastly wrt to momentum and position there is no limit, to accuracy of measurement only if one of those variables is zero [i.e not moving],..if not, then it is impossible to measure either quantity with an accuracy greater than h/2pi...now ..
and lastly
they have some guys already working on the final unknowns in theoretical and experimental physics...I know one...
