I'm trying to grasp basic concepts of energy and I have a question regarding conservation of energy.
According to Feynman's book and Wikipedia, the law of conservation of energy states that isolated physical systems (physical systems with no resulting force being exerted on them, only internal forces) have their energy conserved.
But then I picture some object at some point above the surface of earth (disregarding air resistance). It is clearly not an isolated physical system (gravity is being exerted on it) but somehow, while it's freefalling, its energy is being conserved (its potential energy, based on its position relative to earth, is being transfered to its kinetic energy).
I know the earth-object system can be thought of as an isolated system, but what I'm curious is that for some non-isolated systems (the mass above the earth), energy is being conserved.
So, it seems like the conservation law of energy sometimes works only for isolated systems, but sometimes for non-isolated systems.
So, to what kinds of system does the law of conservation of energy really apply? Or equivalently, what are "isolated physical systems", really?