F12 is the magnitude of the gravitational force between the two point masses,
G is the gravitational constant,
|m1| >0 is the (negative) mass of the first point mass (the minus is put out to show negative force; m1 is actually <0),
m2 >0 is the mass of the second point mass,
r is the distance between the two point masses.
Stuff like that to make a game realistic! Adding new and advanced equations for a game that involves true physics! How to start?