From considerations of translation and Galilean invariances it is shown that the motion of a nucleon in the relative frame of reference fixed to the center of mass of an A-particle target may be described by a Schrödinger equation which, essentially, differs in one point only from the Hartree-Fock equation determining the independent motion of the target nucleons relative to the lab. frame — the kinetic energy term contains the reduced mass m(1 + A −1) −1 instead of the nucleon mass m.