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Fundamental principle of physics that in its weak form states that gravitational (seegravitation) and inertial (seeinertia) masses are the same. Albert Einstein's stronger version states that gravitation and acceleration are indistinguishable. It implies that the effect of gravity is removed in a suitably accelerated reference frame, such as an elevator with its cable cut, in which a person would experience free fall.
This entry comes from Encyclopædia Britannica Concise. For the full entry on equivalence principle, visit Britannica.com.