Let us consider an inertial frame F . The motion of a free particle follows the law of inertia relative to the inertial frame F. Further, let us consider a frame F' which is accelerating relative to the inertial frame F . The free particle will be accelerating relative to the frame F'. Is the force acting on the free particle relative to the frame F', is called as the inertial force? I think that it is this force which is considered in the Einstein's thought experiment with additional requirement that the force is constant, for the proposition of the principle of equivalence. Besides this force whether, any other force may exists in the frame F' by giving it a suitable amount of acceleration to the frame F' relative to the inertial frame F ? Also, what would be the geometrical structure of the frame F' ?