A system, as it is defined in physics or chemistry, is nothing more than a collection of objects (or smaller systems) that can be identified. Usually, the word system refers to a collection that makes thinking about a problem more convenient. Often, the most convenient system is an isolated system, one where outside influences can be ignored (either because they cancel out or because outside influences are negligible). For example, two balls rolling across a smooth surface to strike each other can be considered an isolated system, while two balls rolling across a gravel path or thick carpet are influenced by friction originating from outside the system and therefore are not an isolated system.
While anything can be a system, much of the study of physics and chemistry is about figuring out which systems are useful to consider in studying a particular property or relationship of items within the system.
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