If the main task of mechanics is to determine the coordinates of a body of known mass and its velocity at any given time by using known forces acting on the body and given initial conditions (a direct problem of mechanics), then when solving the inverse problem, it is necessary to determine the forces acting on the body, if it is known how it moves.
The solution of this problem led Newton to discover the law of universal gravitation: "All bodies are attracted to each other with a force directly proportional to their masses and inversely proportional to the square of the distance between them." Like all physical laws, it takes the form of a mathematical equation.
The proportionality coefficient G is the same for all bodies in nature. It is called the gravitational constant
G = 6.67·10-11 N * m2/km2 (SI)
There are several important points to be made about this law.
First, it explicitly applies to all physical material bodies in the Universe, without exception. In particular, for example, you and a book experience forces of equal magnitude and opposite directions of mutual gravitational attraction. Of course, these forces are so small that even the most accurate modern instruments will not detect them — but they are real, and they can be calculated.
Similarly, you experience mutual attraction with a distant quasar, tens of billions of light-years away. Again, the forces of this attraction are too small to be recorded and measured instrumentally.
The second point is that the force of attraction of the Earth at its surface equally affects all material bodies located anywhere on the globe. Right now, the earth's gravitational force, calculated using the above formula, is acting on us, and we actually feel it as our weight. If we drop something, it will continue to accelerate toward the earth under the same force.