ABSTRACT
Study of the interactions of material bodies led to the notion of a ‘potential energy’, whose spatial gradient is associated with a force. Such ideas arose from the context of motion under gravity; for example, with planetary bodies orbiting a massive central object. When further out from the central object, a body could be considered to have more potential energy. If falling to a position closer in, while following an eccentric orbit, say, a force must have acted, whose result is the conversion of some of the potential into ‘kinetic energy’: the work done by the attractive force corresponding to the increase of kinetic energy. Conversely, the extra kinetic energy supplied to the body that would move it back to its original more distant position accounts for the work done against the attractive force on the return journey.
