“Whether it is a falling man or an orbiting satellite, the effect of inertia is to create an apparent upward force that depends on the mass of the object. It is the same force we feel when riding in a car that goes around a tight curve. This inertial effect is equal and opposite to gravity and therefore cancels the pull of gravity. In physicist language, the "gravitational mass" and "inertial mass" are equal. This is not a tautology, as Ambrose Bierce thought, but a recognition that the pull of gravity is proportional to the inertia of the object being pulled. Einstein called this the "Principle of Equivalence", and it became the basis for his new theory of gravity that he called the general theory of relativity.”