First, I would like to say that this is my first post in this forum and that my knowledge of GR is weak. So I am hoping that my question can be answered in layman terms. If I hold a 1kg object in my hand, I can easily feel it's inertia if I move it from side to side. I could also measure it's inertia by attaching it to a spring and timing the period of oscillation, or frequency. However, if I wanted to measure the strength of it's gravitational force (active gravitational mass), then I would have a hard time because the forces would be extremely weak. My question is, how can two quantities or properties that are so different be declared equal? If you were to tell me that the two quantities will always have the same ratio, then that would make sense. For example, I could double the inertial mass of the object and the gravitational mass (force) would also double. But to the best of my knowledge, the equivalence principle is not a ratio, it is an equality. In what way is inertial mass and active gravitational mass equal? By the way, I think this is called the strong equivalence principle (SEP). But please correct me if I'm wrong. edit: Just to clarify, I am questioning the equivalence of the "inertial mass" and "active gravitational mass" of the 1kg object only. No other other objects, not even the earth, are involved.