Weightlessness/Talk

I thought that when using SI units it was unnecesary to put in these constants as the system is designed around them --sodium

SI doesn't eliminate the constant in this case. It does for F=ma, for instance (this is F=kma in other unit systems) but not for F=k/r^2. In this case k=-G M1 M2, where G is 'big G', the gravitational constant, and M1 and M2 are the masses of the gravitating bodies. -- DrBob