 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. For these two to be compatible, the inertial mass and the gravitational mass for any object would have to be equal. Typically, inertial mass is measured by using a spring with a known spring constant. The mass is attached to the end of the spring and the spring is stretched, released, and allowed to oscillate freely. Counting the angular frequency, we can calculate the inertial mass. To measure an object's gravitational mass, we use a balance. Since gravitational fields apply the same gravitational force to equal masses, known masses can be added to the scale until the balance is balanced. Examples 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. Problem solved. But a new set of non-intuitive consequences followed.