The physical origin of the equivalence between inertial and gravitational mass remains one of the fundamental open questions in classical physics. This paper proposes a heuristic mechanical model wherein elementary particles are conceptualized not as dimensionless points, but as spatially extended structures composed of concentric, vibrating spherical shells arisingfrom a nonlinear scalar field. Within this framework, inertia is derived as the active structural resistance of these shells to deformation during acceleration. Conversely, gravitation emerges as a passive topological tendency of overlapping shell systems to restore asymptotic concentricity to minimize energy. We demonstrate that Newton’s inverse-square law (1/r²) arises naturally as a geometric consequence of the radial dilution of shell density. Furthermore, the model addresses historical optical paradoxes by postulating a selective drag mechanism: the spherical geometry implies that while radial interactions are significant, tangential drag is negligible. This constraint offers a consistent interpretation of both the null result of the Michelson-Morley experiment and the observation of Stellar Aberration, without invoking an absolute static ether. Although the model is a classical approximation, it provides a coherent derivation of the Equivalence Principle, suggesting that inertia andgravity are dual manifestations of a single underlying structural dynamic. Keywords: Inertia, Gravity, Shell Model, Classical Field Theory, Equivalence Principle.
Ernesto De Luca (Sun,) studied this question.