System

The modern concept of energy emerged during the nineteenth century as scientists and engineers tried to understand the work output of steam engines. It was later generalised to other heat engines, chemical reactions and biological systems. The same accounting idea connects all these situations: energy may be stored in different ways and transferred by different processes, but it is not created or destroyed.

It applies when the system is closed, meaning that external forces have no significant effect during the collision. External forces such as friction may exist, but their impulse is often negligible over the very short collision time.

Gas particles spread out and may become invisible, but they still have mass. This is why the boundary of the system matters. If liquid evaporates from an uncovered beaker, the balance reading falls because vapour has left the system being weighed—not because mass has been destroyed.

A system is the object or group of objects being studied. Choosing its boundary matters: energy can cross the boundary of an open system, but no energy enters or leaves a closed system.