StellarSystems
A system refers to a specific portion of the physical universe chosen for analysis, often to study the interactions within or across its boundaries. A system can be as simple as a single particle or as complex as the entire cosmos, depending on the scope of the study. How you define your system in a problem can change the math in a problem, but will not change what is physically happening in the problem. Everything outside the system is referred to as the environment or surroundings. Depending on how a system interacts with its environment, it can be classified as either an open or closed system.
Open systems can interact with their surroundings. This can be seen through a boiling pot of water. Steam releases from the pot, therefore the system interacts with the environment by losing mass. Closed systems have no interaction with the surroundings. In the real world, this is impossible; however, in AP Physics 1 systems can be considered closed.