A purely mechanical version is given of Einstein’s 1905 argument that the mass of a body depends on its energy content. The Gedankenexperiment described here is the same as Einstein’s, except that the body loses energy not through electromagnetic radiation, but through the emission of massive particles. The concept of mass is not assumed, but is extracted as one of two constants of integration. Viewing mass in this way emphasizes a sometimes obscured distinction between mass as a proportionality constant in the kinetic energy (for which E=mc2 has profound physical content), and mass as it appears in the conventional relativistic rest energy (for which E=mc2 is merely a convenient convention).

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