I understand that: 1) For work to be done there must be a force (or a component of it) exerted upon an object, causing it to move. Work done on a system puts energy into it. 2) If the force is applied in the opposite direction to that of the object’s displacement, then the work done by that force removes energy from it. 3) If work is done by a system, energy is removed from it as well. But here is where I get confused: If I put a box on a table, it gains PE. If I let it fall from that height, it will lose its PE. My question is, when falling what force in the upward direction is being applied to the box, removing energy from it? On what is my system (the box) doing work? My textbook asks for giving an example where the work done by the system removes energy from it. I've been thinking in a car toy after having pushed it. It’s easy to think about it as friction doing work on the car with a direction opposite to its motion, and hence removing KE from it; but if my system is the car toy, on what is this doing work? On the ground? And ultimately (and my today’s silly question), Can a system do work on itself? I guess it can’t, because Newton’s Third Law would be violated. If this conclusion isn’t wrong, then what about the act of eating? A person eating is putting energy on themselves, but how is this interpreted in the context of work? Thanks !!