Thermodynamic Origin of Inertia: Unruh Radiation as Substratum Friction and the Mechanics of Variable Mass

The phenomenon of inertia — the property of matter to resist changes in its state of motion — is treated as a fundamental axiom in classical physics. This paper proposes a hydrodynamic interpretation of the origin of inertia within the framework of Substratum Theory (Papers III, IV, VI). We treat mass not as an intrinsic constant of matter, but as a result of electromagnetic and hydrodynamic interactions with the vacuum medium. We demonstrate that the Unruh Effect represents the thermal signature of "hydrodynamic friction" experienced by an accelerated body within the Substratum. By applying principles of quantum resonance, we derive a formula for variable inertia that theoretically allows for Inertia Dampening. This approach aligns with high-energy experimental data (Higgs mechanism, QCD) and provides a theoretical foundation for propellantless, inertia-free propulsion systems.