Newtons_Laws_of_Gravity

aka Newton's Law of Universal Gravitation.

"The gravitational attraction (a force) exerted by two bodies is on one another is proportional to the product of their masses and inversely proportional to the square of the distance between their centres of gravity. The force is directed along the line joining the centres of gravity."

i.e. if the bodies have masses m₁ and m₂, and are a distance r apart

::Force = G m₁ m₂ / r²

If the units are SI units, G = 6.673 * 10^-11 N m²kg^-2