““In general the fact that gravitational and inertial forces are both proportional to the mass of the body on which they act but depend on no other property of the body makes it possible at any point in any gravitational field to identify a "freely falling frame of reference" in which neither gravitational nor inertial forces are felt because they are in perfect balance for all bodies. When we do feel gravitational or inertial forces it is because we are not in a freely falling frame. For example, on the earth's surface freely falling bodies accelerate toward the center of the earth at 32 feet per second per second, and we feel a gravitational force unless we happen to be accelerating downward at the same rate. Einstein made a logical jump and guessed that gravitational and inertial forces were at bottom the same thing. He called this the principle of equivalence of gravitation and inertia, or the equivalence principle for short. According to this principle, any gravitational field is completely described by telling which frame of reference is freely falling at each point in space.””
