If you have a portal moving towards you, at say 2 MPH and you are holding your arm out, From the other side of the portal, they would see your arm coming out at 2mph. Any other result would involve compressing or stretching your arm.

Lets say further here. a man dangling his legs off the back of a moving cart. Man is moving 20mph forward, portal trolley is chasing him at 22MPH. As his feet pass through the portal, they would start coming out the other end of the portal at 2mph.

As a result I concur that the only logical exiting of a portal. |travler velocity - Entry portal velocity| + exit portal velocity = objects exit velocity.

Now the real physics debate would be what happens if 2 portals were moving forward and someone was in the path of one. The only logical conclusion that fits my mind there, is instant compression

[–] Natanael@slrpnk.net 1 points 1 year ago

Yeah, the imparted momentum has to be something like the averaged vector between the two, which does cause compression if you enter a pair of portals both moving your direction.