This page is from a section entitled “Equations of Motion of the Material Point.” Einstein shows how the most basic freshman-physics equations must change to fit the new world.
In the bottom paragraph, m is an object’s mass q; its speed, and c is the speed of light in the vacuum. The text, continuing on the next page, reads: “The expression within the brackets on the right plays the role of the energy E of the moving mass point. This expression: [formula 28] grows to infinity when q approaches the value c; thus, it would require an infinite expenditure of energy to impart the velocity c to the body.
That is why even today’s highest-energy particle accelerators can only push particles to approach—but never equal or surpass—the speed of light.
The old Newtonian formula, gives zero if q is zero. Here, for q=0 the formula reduces to a special case, called rest energy:E=mc2.