Okay, so I've reached the Work Chapters in my textbook, and I've noticed some contradictions, especially in what consists (and what doesn't) an Isolated System, plus the external and/or applied forces. For example, in one of the "Speed Questions" it categorizes a single cube as a non-isolated system, the surface as a non-isolated system, and the cube/surface (there is friction) as an isolated system. Later, in an exercise, it describes a cube/surface system with friction as a non-isolated system. The problem is, it's got a ton of formulas that he constructs, reconstructs, renames and whatnot, and it's confused the hell out of me. From what I gathered, an isolated system is something that each force exists within (eg a cube, the surface and the earth), and a non-isolated one is something that each (or some) force(s) is/are external (eg just a cube, where the force that the earth exerts onto it is external). Can anybody plainly explain to me the nature of those systems, external/applied forces, and how work fits in all this? I mean, I've got about 50 or so equations and formulas at this point, with most of them lumped together and reconstructed in every page. I've memorized most of the formulas and mostly know how to use them in the various exercises, but I'm kinda lost as to why I'm using them. Things were pretty clear fro my High School studies, and I never had any particular trouble with Work and the like, but this book (Physics for Engineers and Scientists, 8th Edition)has confused me a bit. I'd really appreciate it if someone could go over the basics briefly.