Newton's Law of Universal Gravitation states that the magnitude of the gravitational force of attraction Fg between any two objects of mass m1 and m2 is  proportional to both the masses and inversely proportional to the square of the distance r between them:

        Fg = G * m1 * m2 / r^2

This law applies when r is the distance between point particles. Extended bodies may be modelled as point particles if they are very small with respect to the distance between them. Newton also proved that for two spheres of uniform mass distribution, this formula gives the correct force if r is taken to be the distance between the centers of the two spheres. Large extended bodies like the Earth and the Moon should be approximated as such uniform spheres in Andes problems. The force of gravity will therefore point along a line between their centers.

The constant of proportionality G is known as the gravitational constant; its value has been measured experimentally. In Andes problems you should enter the value of G as:

            G = 6.67 E-11 N*m^2/kg^2

Note  there is a negative sign after the "E". The value above is written using scientific notation for 6.67 times 10 to the minus 11th power, or
            0.0000000000667
So G has a very small value, which accounts for the fact that we do not notice any gravitational attraction between ordinary-sized objects in our daily experience.