In modern physics the equivalence principle states that an observer obtains the same measurement for the mass of an object whether such calculation of mass is derived from the inertia of the object or is measured by the force of gravity on the object.

The equivalence principle is applied to two related concepts of gravitational theory. The weak equivalence principle relates to Galileo's famous experiment that demonstrated, by dropping objects of various composition or rolling them down inclined planes to determine if they fall at the same rate, that all bodies fall at an equal rate regardless of their mass or chemical nature. Isaac Newton's theory of gravity advanced the weak equivalence principle in a slightly different form in its assumption that inertial mass and gravitational mass are equal in a given body. This equivalence, which Newton also tested by comparing the motion of pendula of...