1. The problem statement, all variables and given/known data does the moon orbit the earth or the Center of mass of the earth-moon system. 2. Relevant ideas well, i think that it orbits the COM of the earth-moon system. If this is the case then is the radius of the moon from the earth always constant. ie, if the moon orbits the COM of this system then the earth must also orbit the COM of the system, hence the distance of earth from the moon would always be changing and therefore the force applied would be changing too.