What if a fundamental particle like a resting electron is composed of a circling photon-like object with energy Eo and vector momentum p = Eo/c where c is the speed of light? If we start with Newton's second law of motion F = dp/dt = MA where dp/dt is the time rate of change of the circling vector momentum p = Eo/c, M is the inertial mass of the circling photon-like object, and A is the centripetal acceleration c^2/R of the circling photon-like object (where R is the radius of its circle), we find with very easy math (and using the circling vector relation dp/dt = pc/R) that the inertial mass M = (dp/dt)/A = (pc/R)/(c^2/R) = p/c = (Eo/c)/c = Eo/c^2. That is, the inertial mass M of an electron (if it is composed of a circling photon-like object) is derived from the circling photon-like object's energy Eo and its circling vector momentum Eo/c to be M = Eo/c^2 or Eo = Mc^2 , which is Einstein's equation for the energy content Eo of a resting electron of inertial mass M. This result is also published at https://www.academia.edu/29799123/Inertia_Explained . This derivation of the relation of the energy content of a resting fundamental particle to its inertial mass is done without using Einstein's special theory of relativity. Note: Einstein's 1905 article in which he first derived m = E/c^2 or E = mc^2 for a resting object by using his special theory of relativity is titled "Does the inertia of a body depend on its energy-content?"