code: def com(self, center1_x, center1_y, center2_x, center2_y, Fm):
          
          com_x = (Fm * center1_x + center2_x)/(Fm + 1.)
          com_y = (Fm * center1_y + center2_y)/(Fm + 1.)
          return com_x, com_y
docs: :return: center of mass