What is the definition of an engine?

The definition of an engine as a machine designed to convert energy into useful mechanical motion captures the fundamental purpose of engines across various applications. Engines operate by transforming different forms of energy—such as chemical energy from fuel, thermal energy from heat, or even potential energy from gravitational forces—into mechanical energy, which can then perform work.

For example, in a car engine, the combustion of fuel generates heat, which creates pressure that moves pistons, ultimately producing motion that drives the vehicle. This definition emphasizes the efficiency and function of engines in translating energy into movement, distinguishing them from other devices that do not directly perform this transformation.

In contrast, devices that store mechanical energy, as mentioned in another choice, do not actively convert energy; rather, they retain energy for later use. Tools for cutting materials serve a different purpose altogether, as they facilitate the manipulation of materials but do not convert energy into motion consistently like an engine. Lastly, systems for generating electrical power focus on electricity production rather than mechanical motion, which further sets them apart from the definition of an engine.