Schematic diagram of atomic spontaneous emission

In systems defined by gravitational, electrical, or contact forces, energy interconverts between many forms and can be transferred among components of the system or between the system and its surroundings. The movement of the mass-spring involves the inter-conversion of potential and kinetic energy, for example, as does the movement of the pendulum.

As an object falls from a height and strikes the ground, energy is transformed from gravitational potential energy to kinetic energy and finally to sound (sound energy involves propagating mechanical compressions in a medium).

A similar process involves an electron falling from an excited energy state to a lower energy level, with an overall energy conversion from electrical potential energy to light.