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