© Blayne Heckel.
In a uniform gravitational field (constant g), objects experience a force given by mgg where mg is the gravitational mass of the object. If the object is free to fall, it will acquire an acceleration, a, given by F=mia where i is the inertial mass. Because g is constant and experiments find that a is the same for all objects, it follows that mg and mi are proportional to one another. In a nonuniform gravitational field, such as at the surface of the Earth, objects of different shapes need not experience the same acceleration. Identical horizontal and vertical rods whose centers of mass are at the same height will have different accelerations: the vertical rod will fall faster because g changes in magnitude in the vertical direction and changes direction in the horizontal plane. (Unit: 3)