It also has to do with the "heat engine," and particularly the Stirling engine, and the related carnot cycle. An engine transforms heat to extract work. A rock

It also has to do with the "heat engine," and particularly the Stirling engine, and the related carnot cycle. An engine transforms heat to extract work. A rocket motor has the expansion part of a heat engine in an obvious way. An electric motor produces motion, but isn't an engine.