 However, it is important before going much further to note that in Einstein’s equation \(E_o = mc^2\), the symbol \(m\) is the mass of an object as measured in the inertial frame in which that object is at rest. Physicists also call this mass \(m\) the “rest-mass” of the object. The rest-mass of an object is numerically equal to its Newtonian inertial mass, though arguably the symbol \(m\) (or the corresponding term “mass”) has different meanings in Newtonian and relativistic physics (see, e.g., Kuhn 1962, p. 101 ff. and Torretti 1990, p. 65 ff.). 