In classical mechanics (also in QM I guess), the fundamental laws of conservation (energy, momentum and angular momentum. tell me if I forget something) are valid only if the system is closed and the net external force is zero. WE define what the system is. We can include or exclude things. These things could exert (or not) forces of different nature (gravitational, electric,magnetic, etc...) on the objects that are part of the system. We call the system closed if no energy is coming in or out. The net external force being zero is the necessary condition for the conservation laws. My dilemma: Can we really decide, arbitrarily, that all those external forces sum up to zero when they are much smaller (how much smaller?) than all the forces exerted by the internal objects of the system? Do we include to be part of the system ONLY those objects that we think could have a role in the situation under study? Any example? My example is this: a collision between two cars. Total Momentum is conserved. This implies and assumes that the "net external force is zero (however there must be some other things in the world beside those 2 cars, that exert forces on those two cars too. I guess those forces are negligible and taken to be zero when compared to the mutual, action reaction forces between the two cars). Can anyone validate?fix my thinking? thanks for the help.