A system within its surroundings.

As physical systems undergo change, one question always on the short-list is: What is happening with energy?

As the system changes configuration, what happens to the energy within the system and between the system and its surroundings? In thermodynamics, we would call dissipation of energy from the system heat flow into the surroundings.

Almost any real world transformation of potential energy into kinetic energy coincides with an exchange of energy between a system and its environment, ie. emission of light, sound, or thermal energy through conduction. The friction on a block sliding down an inclined plane leads to energy loss to the surroundings through heat flow.