If a person such as Mendel uses a lever to move a heavy box across a room, he is illustrating the point that you cannot get 'something for nothing'. At one end of the lever he puts in a certain amount of work, and gets out exactly the same amount of work at the other end. The total amount of 'work' in the universe has not changed, only moved around a bit. Call this the law of 'conservation of work'.
But where does the work come from? At rest, when Mendel is sitting in a chair for example, he isn't doing any work, but a moment later, when he is moving the box, work is being done all over the place. If work is conserved, and not being suddenly created out of nothing, that work had to already exist in Mendel's body. But work is expressed as motion (force moving through a distance), if no motion is involved (Mendel just sitting in a chair) then the 'stored-work' must be in a different form.
Gottfried Wilhelm Leibnitz (1646 - 1716) thought that living things carried around 'stored-work' in their bodies in the form of vis viva, or 'living force'. But this ignored the fact that non-living objects such as the wind, or rivers, can turn windmills or turn waterwheels, and thus also contain 'stores-of-work'. Work was, therefore, a general property of both living and non-living systems and in 1807 the English physician Thomas Young proposed a scientific name for this 'work-store'; energy, meaning 'work-within'.
It was immediately apparent that there were at least two forms of energy; that associated with motion, and that associated with 'storing'.
The term kinetic energy was chosen by the English physicist Lord Kelvin for the type of energy associated with motion. (The word 'kinetic' is from the Greek word meaning 'motion'). A large, heavy stone dropped off a shelf has motion and thus kinetic energy as it plunges to the floor and an awaiting toe.
But while it was on the shelf that stone still had 'stored-energy', and it had the potential to convert that 'stored-energy' into kinetic energy the moment it began to fall. The Scottish engineer William J. M. Rankine suggested in 1853 that the 'stored-energy' be called potential energy, a suggestion that was just becoming popular by Mendel's time.
When a heavy object, like a stone, is lifted onto a shelf, the person lifting the rock applies a force equal to the weight of the rock and moves it though a distance. This kinetic energy is converted into potential energy, which is stored in the rock on the shelf. When the rock falls, all that potential energy is once again converted into kinetic energy. These two forms of energy are thus interconvertable.