How did it came into being? Did it all strated when someone (Huygens?) discovered that the quantity 1/2mv^2 was (almost) conserved in some collision experiments? Or was it discovered purely analytically from the laws of motion that when a force acted on a particle on a certain distance, it produced a variation of this bizzare 1/2mv^2 quantity. Or was it discovered "backward"? I.e. through the need for the "potential energy" of an object in a gravitationnal field (pendulum)?