Gravitational Force

One of the four fundamental forces, gravitation is the force of mutual attraction that is exerted between massive bodies and between particles that have mass. Although gravitation is far weaker than the other three fundamental forces over short ranges, it is the dominant force on large scales because its range of influence is far greater than that of the nuclear forces and because, unlike electrical charges, which can be positive or negative, all mass is mutually attractive. Gravitation alone determines motions of, and mutual interactions between, planets, stars and galaxies, and dominates the overall dynamics of the universe.
Newton’s law of universal gravitation works in most situations, but is inadequate where gravity is very strong (e.g., in the vicinity of a black hole, or when dealing with the structure and dynamics of the universe as a whole). The best present theory of gravitation is Einstein’s general theory of relativity, where mass distorts the geometry of 4-dimensional space-time. In this theory, paths followed by material particles, or rays of light, near massive bodies are determined by local distortions of space.
For two bodies, with masses m1 and m2, separated by distance, r, the mutual force of attraction, F, predicted by Newtonian theory is:

where, G = gravitational constant. An equal and opposite force acts on each mass, but the force acting on m1 is in the opposite direction of that on m2.

Some or all content above used with permission from J. H. Wittke.