Energy Conservation as the Basis of Relativistic Mechanics, R. Penrose and W. Rindler, Am. J. Phys. 33, 55-59, 1965
The relativistic theory of collisions of macroscopic particles is developed from the two axioms of energy conservation and relativity, by use of a standard relativistic kinematics (without, of course, assuming the mathematical expression for relativistic energy). We derive, in turn, the equivalence of rest-mass and rest-energy, the usual mathematical expression for the total energy in terms of rest-energy, the conservation of 3-momentum, and the equivalence of total energy and inertial mass. We incidentally find all possible conservation laws of quantities described by the product of a scalar with a function of speed.