What counts as an “isolated” system ?

1. APPROXIMATELY isolated:

external interactions are very weak (e.g. deep space)

2. FUNCTIONALLY isolated:

external interactions are not weak, but they cancel eachother (e.g. gravitational interaction and “normal” interaction on system on an air track)

3. COLLISIONS

- look at system “just before” and “just after” collision. External interactions do not have enough time to transfer much momentum to system