How do you calculate grativity?
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