Inertial & Gravitational Mass


Chapter 36: Einstein’s Legacy – Albert Einstein (1879 – 1955)

“He also published an analysis indicating the equivalence of gravitational and inertial mass is not a mere accident of nature, but the basis of a profound physical principle that leads to a new theory of gravity.”

NOTE: Inertial mass is a mass parameter giving the inertial resistance to acceleration of the body when responding to all types of force. Gravitational mass is determined by the strength of the gravitational force experienced by the body when in the gravitational field g.


To me, inertial mass balances the intrinsic motion. Light has very large but finite velocity because it has an “inertial mass” restraining infinite motion. As inertial mass increases the intrinsic velocity decreases. A body of infinite inertial mass may be postulated to be at absolute rest. Different inertial masses mean different intrinsic velocities. Thus, relative velocities may be understood in terms of differentials of inertial mass.

From this point of view, when a body is accelerating in a gravitational field, it means that the inertial mass of the body is somehow reduced by that gravitational field. Since this reduction is extremely small, the “gravitational mass” appears to be the same as the “inertial mass”.


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