What principle states that gravitational mass and inertial mass are equivalent?

The principle stating that gravitational mass and inertial mass are equivalent is the equivalence principle. It says, in its basic form, that the mass determining how strongly gravity acts on an object is equal to the mass determining how that object accelerates under a force.

This idea explains why objects in the same gravitational field fall with the same acceleration when air resistance is negligible. A heavier object has more gravitational “pull,” but it also has proportionally more resistance to acceleration, so the two effects cancel.

Galileo’s falling-body investigations anticipated the principle, and Newtonian physics treated the equality as an observed fact. Albert Einstein transformed it into a foundation of general relativity: locally, a uniformly accelerating laboratory can be physically indistinguishable from a laboratory at rest in a uniform gravitational field.

The principle is not the same as Mach’s principle or the principle of relativity. Modern experiments test increasingly precise versions, including possible violations involving composition-dependent gravity.