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@NEETzsche @marine
Wrong, since momentum is relative towards the frame of reference.

Immagine, that instead of portal, it is a car window, and the box is a person standing still.

While outside of the window, the person is still, but once he is met with it, he can be seen flying through the car :D

Also, you can imagine looking through the blue portal, you will see the box flying towards you at great speed as it enters the portal, and once it passes, it has no reason to stop.