 Purely dynamical derivations of \(E_o = mc^2\) typically proceed by analyzing an inelastic collision from the point of view of two inertial frames in a state of relative motion (the centre-of-mass frame, and an inertial frame moving with a relative velocity \(v)\). One of the first papers to appear following this approach is Perrin’s (1932). According to Rindler and Penrose (1965), Perrin’s derivation was based largely on Langevin’s “elegant” lectures, which were delivered at the Collège de France in Zurich around 1922. Einstein himself gave a purely dynamical derivation (Einstein, 1935), though he nowhere mentions either Langevin or Perrin. The most comprehensive derivation of this sort was given by Ehlers, Rindler and Penrose (1965). More recently, a purely dynamical version of Einstein’s original (1905b) thought experiment, where the particles that are emitted are not photons, has been given by Mermin and Feigenbaum (1990) and Mermin (2005).