If there are two objects of unknown mass. To determine the ratio of their weights, in nature, all we need is a weightless rod and a fulcrum. If we place two objects on the rod and fulcrum at the balance point, then the balance point becomes the barycenter. Measuring the distance between them from the barycenter yields the relationship m_1 r_1= m_2 r_2. Now, differentiating each distance with respect to time yields p= m_1 v_1= m_2 v_2, which defines Newton's first law of motion, momentum. Therefore, interacting between two objects has two opposing momenta. Newton's second law is the law of acceleration. Differentiating one momentum of the two momenta with respect to time gives F=ma. Since interaction is mutually attractive forces between two objects, it is the sum of the two forces between two objects. In other words, 2F=2ma. Newton's third law, the law of action and reaction, is two forces with opposite directions repel each other, but the result is the same as interaction. In Newton's law of universal gravitation, gravity also applies equally to the interaction between two objects. The gravitational force is 2F=2G Mm/r^2 . In the former case, kinetic energy is E_k= mv^2, and gravitational energy is E_g= 2GMm/r. If the two energies are equal, the relation between the two becomes v^2= 2GM/r. This is the same equation given in the Einstein field equation. When an object moves at the speed of light, all of the object's mass convert to energy, which is Einstein's energy-mass equivalence principle, E=mc^2. Furthermore, if we extend the Newton's law of universal gravitation, the law of universal gravitation between three bodies becomes 2F= 2G((m_1 m_2)/〖r_12〗^2 + (m_1 m_3)/〖r_13〗^2 + (m_2 m_3)/〖r_23〗^2 ).