1. C. Møller:The Theory of Relativity, Sect.38 (Oxford, 1960), p. 105;J. L. Synge:Relativity, The Special Theory, Chap. VI, Sect.3 and4 (Amsterdam, 1956), p. 167;J. Aharoni:The Special Theory of Relativity, Chap. V, Sect.3 (Oxford, 1959), p. 143;M. Von Laue,La théorie de la Relativité, vol I, Chap. VII, Sect.29, IV Ed. (Paris, 1942), p. 249;R. C. Tolman:Relativity, Thermodynamics and Cosmology (London, 1934), p. 46.
2. T. Levi-Civita:Rend. Lincei,8, 329, 621 (1928);A. Sommerfeld:Mechanics, Lectures on Theoretical Physics, vol. I (New York, 1952), p. 28;S. L. Loney:Dynamics of a Particle and of Rigid Bodies (University Press, reprint of 1960), p. 124;K. B. pomeranz:Amer. Journ Phys.,32, 386 (1964);J. A. Van den Akker:Amer. Journ. Phys.,32, 387 (1964).
3. K. B. Pomeranz:Amer. Journ. Phys.,32, 955 (1964); also34, 565 (1966). The usual «rocket» equation (see for exampleR. L. Haflman:Dynamics (Reading, Mass., 1962), p. 542, is a particular case of Pomeranz’ equation.
4. The «renormalization» factorz is the same that appears in the relativistic equations of motion with resistance forces of Newtonian type, proposed by one of us in a preceding paper (G. Cavalleri:Amer. Journ. Phys.,34, 901, (1966)). This factor has been overlooked byM. Pellegrini:Istituto Lombardo (Rend. Sc.),99, 243 (1965), and taken into account only byG. Carini:Istituto Lombardo (Rend. Sc.), A101, 491 (1967), however, after one of us (G. Cavalleri:Boll. SIF,50, 102, 8c 3 (Oct. 1966) presented eq. (3) at the Italian Physics Meeting of 1966.
5. Classically a momentum may be equalized to an arbitrary mass multiplied by a velocity parallel to the momentum. This does not occur in relativity because the four-momentum includes both classical momentum and energy. Notice moreover that thevelocity of the mass centre is unique, whereas itsposition is not unique (see for exampleMøller in ref. (1)64).