When you stop to think about it, this equality is a remarkable fact. The
property of an object that determines how strongly it is pulled on by the
force of gravity is equal to the property that determines its resistance to
acceleration by any force, not just the force of gravity. The equality of
gravitational mass and inertial mass is called the equivalence principle, and it
is the equivalence principle which led Einstein to devise his theory of general
relativity.
If the equivalence principle did not hold, then the gravitational accelera-
tion of an object toward a mass M g would be (combining equations 3.1 and
3.2)
a = −
GM g
r 2 µ m g
m i ¶