Physical system

Physical system consists of two or more physical elements.

Example:
A table and a glass of water put on it forms a physical system, a train, a railroad tracks, and the station forms another physical system etc.

No physical system is isolated from surrounding physical systems, which means they interact thermodynamicly with neighbors or in other words, it is not adiabatic.

There are an infinite number of physical systems. All are moving relative to each other. Why relative? Because every body or physical system, at the same time is both, state of rest and state of motion, according to the inertial referential to which it relates. Moreover, the movement itself may be uniform or accelerated according to the inertial referential. Why inertial? Because, a non-inertial referential doesn’t exist. But why there isn’t a non-inertial referential? Because the universe is made of matter which is in constant motion. Everything is connected and interdependent. Quat erat demonstrandum!

Heat spread through contact when, for example, a heated metal bar touches a cold metal bar and stay touched for a while. Entropy law follows its course and the two metal bars will have the same temperature: the warmed one will get colder a little bit and the cold one will get warmer a little bit, until they reach thermodynamic equilibrium, given that the system is not adiabatic.