Law of conservation of energy

The law of conservation of energy states that energy can neither be created nor destroyed; it can only be transformed from one form to another, and the total energy of an isolated system stays constant.

For a mechanical system acted on only by conservative forces, this becomes: the total mechanical energy, $E = KE + U$, remains constant throughout the motion, even as $KE$ and $U$ individually change. A falling body illustrates this perfectly: released from rest at height $h$, its energy starts out entirely potential ($U=mgh$, $KE=0$); partway down, at some intermediate height, both $KE$ and $U$ are nonzero but their sum is still the same constant $E$; and just as it reaches the ground, its energy is entirely kinetic ($KE=mgh$, $U=0$). At every single instant during the fall, potential energy is being converted into kinetic energy at exactly the rate needed to keep their sum unchanged. …