The Equivalence of Gravitational and Inertial Mass
A unique feature of the gravitational force is that all massive objects accelerate in the same manner in a gravitational field. This is often expressed as "The gravitational mass is equal to the inertial mass." This allows us to think of gravity as a curvature of spacetime. Objects move in spacetime along geodesics, the paths which yield the shortest distance between two points in spacetime.
Read more about this topic: Newtonian Motivations For General Relativity
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