gravity. In that law there appears another quantity which determines the amount of gravitational pull an object exerts, and the amount of gravitational pull it feels when in the presence of another object. That quantity is also called the mass. The two definitions of mass play quite different roles, but they are both called mass for good reason: it turns out they are one and the same.
Why should they be equivalent? That question, plus Einstein's brilliant logic, led to his realization that the scaffolding of space and time reacts to the presence of matter and energy.#565•