How do you calculate grativity?

Answer 1

Gravity is not technically a constant force. Without going into deep detail, Einstein's theories on general relativity predict that the "Gravitational Constant" is not truly a constant, and recent orbital experiments have proved him correct.

That all said, we still live in a practical world; Newtonian Physics still is what we use for "normal" calculations, despite the proof that Einsteinian Physics is actually correct.

OK, enough deep philosophizing. In Newtonian physics, "gravity" describes the force two objects with mass exert on each other. For any two object with mass, the force of gravity (i.e. attractive force) is described by this equations:

F = G (m1 * m2) / r^2

That is, in English: Attractive force is equal to the product of the Gravitational Constant with the product of the masses of the two objects, divided by the square of the distance between the center of mass of each of the two objects.

The Gravitational Constant is an empirical value (not like PI, which is merely an Irrational Numbers), whose actual value has been determined by experimentation and observation. Currently, G is calculated to be

6.67300 × 10-11 m3 kg-1 s-2