"Sam Wormley" <email@example.com> wrote in message news:GpednRUGXv3pVuvPnZ2dnUVZ_o2dnZ2d@giganews.com... > On 11/3/13, 2:31 PM, Tom Potter wrote: >> Two bodies interact about a common point in a common time. The common >> point is the center of mass of the system and the common time is the >> period of the system. > > Potter, in the case of the earth and moon, the center of mass of the > system is about a quarter of the radius of the earth beneath its > surface. Just for reference.
The URL below models a water molecule being affected by a field that can be moved about.