Before Newton, many scientists have speculated about the existence of gravity.From the outset it was clear that the attraction between any two bodies should be independent of their mass and diminish with distance.Johannes Kepler, the first to describe the elliptical orbits of the planets of the solar system, believed that the sun attracts the planets with a force inversely proportional to the distance.
Kepler Newton corrected the error, he came to the conclusion that the strength of the mutual attraction of bodies is inversely proportional to the square of the distance between them and is directly proportional to their masses.
Finally, the law of gravity is formulated as follows: any two bodies with
mass are mutually attracted, and their drawing power is

F = G * ((m1 * m2) / R ^ 2),

where m1 and m2 - body weight, R - distance between bodies, G - the gravitational constant.
gravitational constant equal to 6.6725 * 10 ^ (- 11) m ^ 3 / (kg * c ^ 2).This is an extremely low number, so that the force of gravity - one of the weakest force in the universe.Nevertheless, it was she who holds the stars and planets on orbits and generally forms the shape of the universe.
If the body is involved in gravity, has an approximately spherical shape, then the distance R should not measure from the surface and from the center of mass.A material point with the same mass, located exactly in the center, gives rise to exactly the same gravity.

In particular, this means that, for example, when calculating the force with which the earth attracts person standing on it, the distance R is not zero, and the radius of the Earth.In fact, it is the distance between the center of the Earth and the center of gravity of a person, but this difference can be neglected without loss of accuracy.
gravitational pull is always reciprocal: not only the earth attracts man, but man, in turn, attracts the Earth.Because of the vast difference between the mass and the mass of the planet's human it discreetly.Similarly, when calculating the trajectories of spacecraft usually neglected the fact that the device attracts the planets and comets.

However, if the mass of the interacting objects are comparable, their mutual attraction becomes visible to all participants.For example, from the point of view of physics it is not quite true to say that the moon revolves around the Earth.In fact, the Moon and the Earth revolve around a common center of mass.Since our planet is much larger than its natural satellite, this center is inside it, but still the center of the Earth is not the same.