answer:In classical physics, the two were theoretically different because Newton’s formulas were really only approximations for large bodies. Inertial and gravitational mass are experientially the same, but Newton doesn’t require them to be the same. This was reconciled in special relativity, however. In fact, the equivalency of inertial and gravitational mass is one of the main underpinnings of general relativity. Einstein argued that the differences between the two are based on frames of reference and a misunderstanding of gravity in classical physics and that, in fact, the two have to be the same. If you want a more detail understanding, the Wikipedia article on the equivalence principle is a pretty good overview.