Energy and Action

It is necessary from time to time either to coin a new word to convey a new meaning, or to use an old word to serve a fresh purpose or idea. The precise meaning given to “energy” by Rankine is now well appreciated, with the further qualification that it may be either due to motion (kinetic), or due to position or shape (potential). All moving masses—air, rivers, projectiles and fly-wheels—have this capacity to do work in virtue of their motion, and the modern term kinetic energy replaces the historically interesting “vis viva”—the living force! The vis viva of a moving body was measured by its mass (or weight) multiplied by the square of its velocity, and kinetic energy is one-half of this quantity. There is nothing mystical about the one-half, it is merely a matter of mathematical convenience. It makes not much difference whether a man buys wine by the pint or by the quart, unless it makes a difference in the amount that he consumes.