Newton's original work "Principles" was written in Latin. An English version translated two years
after his death changed the first law from "in directum" to "in a right line." Later generations think that
only static or uniform straight line motion is inertia, while excluding rotational motion. This article
proposes that the gravitational potential energy of a planet's revolution is equal to its centrifugal kinetic
energy (mυ2). The work done by an object is a process quantity, but kinetic energy is a state variable
that can be divided into instantaneous kinetic energy and average kinetic energy. Uniform motion does
not do work, and instantaneous kinetic energy in variable speed motion does not also do work, only
the average kinetic energy of variable speed motion does work. Only the energy between work and the
average kinetic energy of variable speed motion can be converted into each other. We have clarified the
relationship between work and kinetic energy, completed the transition of the kinetic energy formula
at low and high speeds, achieved the unification of the kinetic energy formula and the mass energy
relationship formula, and explained the √2 factor in the escape velocity.
