What is an example of a closed system in classical mechanics? From what I understand, a closed system does not exchange matter nor is subjected to external forces, but could such a system actually exist on earth? Afterall, wouldn't gravity be an external force affecting any object on any system on the surface of earth, thus preventing any system to be considered closed? Or, do we consider the force of gravity apart of the closed system? In either case, how would we determine if a given force is to be deemed external or internal?

Question

anonymous · Answer

No, it's not possible for a closed system to exist if you're being strict.  However, many many physical systems can be considered closed to an exceedingly good approximation.