That is not how either relativity or momentum work. The cube is still inert and the portal’s movement would have no effect on it unless it has some acting force that pushes it when passing through. Even if the exit portal was moving, the cube would remain stationary unless it had nothing to rest upon and gravity pulled it down.
Of course it has momentum relative to the portal.
That is not how either relativity or momentum work. The cube is still inert and the portal’s movement would have no effect on it unless it has some acting force that pushes it when passing through. Even if the exit portal was moving, the cube would remain stationary unless it had nothing to rest upon and gravity pulled it down.