On the other hand, if you're looking at the object from the exit portal then you expect its trajectory to be smooth.
I'm confused. Both trajectories are smooth.
More formally, the portal can be implemented by making space into an appropriate Riemannian manifold (with singularity at portal edges) and mechanics with the usual Lagrangian leads to B.
Well, yes, but I think the question is concerned with the peculiar implementation of portals in the game, which apparently conserves momentum.
I'm confused. Both trajectories are smooth.
If you're looking from the exit portal you see the object coming towards you. Therefore, when it crosses the portal you expect it to keep moving with the same velocity rather than suddenly stopping (I assume that the portal is a smooth geometric surgery of space and nothing singular is happening at the boundary except at the edge).
Well, yes, but I think the question is concerned with the peculiar implementation of portals in the game, which apparently conserves momentum.
I have no idea about the game (never played it). I'm just saying what seems more natural from the POV of physics.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one.
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.