This paper aims to present a new theory that explains the mechanism of inertia by providing a satisfying explanation for the yet unknown mechanism for inertia. By considering the vacuum as a liquid with a measurable density, hydrodynamics laws are used to describe the behavior of the vacuum when it is dragged by a moving body. The inertia is the result of the initial resistance between the moving bodies against the static vacuum. The moving body drags the resisting vacuum during acceleration, till the point that the vacuum travels with the moving body and has the same velocity. When the body decelerates, the vacuum continues to flow and to push the body at the same direction of the original flow till its complete stop. Formulations based on Planck theory derived to prove its equivalence to Newton inertia law.
Formulation based on hydrodynamics is derived to confirm the theory that the force exerted by the vacuum on a static body in gravity and on a moving body in inertia is equivalent to Newton law. The strong equivalence principle is reaffirmed and, consequently, Einstein’s equations are preserved.