 One of the motivations for the equivalence principle was the long-standing problem with Isaac Newton's classical physics when it comes to mass. Newton defined two kinds. One was inertial mass. Inertial mass was defined by how much force it took to accelerate an object. It is described by the force equals mass times acceleration formula. The other was gravitational mass. Inertial mass was defined by how strong an attractive force it exerted on other objects. It is described by Newton's universal gravitation formula. These are two very different definitions for mass. But ever since Galileo's experiment with falling objects, we have known that masses do accelerate at the same rate no matter how massive they are. That objects with different inertial masses fall at the same speed in a gravitational field can only happen if the inertial mass and the gravitational mass for any object are equal. Measurements to this day show that these two kinds of mass are indeed equal. The data led Newton to declare them equal, but he could not explain why they were equal. Einstein felt that before you can declare two things equal, you need to demonstrate an equality in the real nature of the two concepts. In other words, we can only say they're equal after their real nature is found to be equal. His equivalence principle does just that. Acceleration and gravitation are the same, and therefore the mass associated with acceleration and the mass associated with gravitation will naturally be the same. Some solved, but a new set of non-intuitive consequences followed.