Main Question or Discussion Point
Do you ever lie awake at night worrying about natural laws?
If a system does not interact with its environment in any way, then certain mechanical properties of the system cannot change. They are sometimes called "constants of the motion". These quantities are said to be "conserved" and the conservation laws which result can be considered to be the most fundamental principles of mechanics. In mechanics, examples of conserved quantities are energy, momentum, and angular momentum. The conservation laws are exact for an isolated system.
How can we 'focus on basic principles', or indeed 'clarify the nature of physical laws' by using a model that doesn't exist in nature?Isolated Systems
An isolated system implies a collection of matter which does not interact with the rest of the universe at all - and as far as we know there are really no such systems. There is no shield against gravity, and the electromagnetic force is infinite in range. But in order to focus on basic principles, it is useful to postulate such a system to clarify the nature of physical laws. In particular, the conservation laws can be presumed to be exact when referring to an isolated system:
Conservation of Energy: the total energy of the system is constant.
Conservation of Momentum: the mass times the velocity of the center of mass is constant.
Conservation of Angular Momentum: The total angular momentum of the system is constant.
Newton's Third Law: No net force can be generated within the system since all internal forces occur in opposing pairs. The acceleration of the center of mass is zero.