I'm always fascinated at the ginormous arguments that this picture is guaranteed to cause, and I wonder at what kind of experiments you could do with it to investigate people's different intuitions of physics.
I think the answer B makes more sense. There's no reason this portal configuration should conserve momentum since it breaks space translation symmetry. On the other hand, if you're looking at the object from the exit portal then you expect its trajectory to be 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.
2gwern
(It's A, right?)
[...]
To some extent, 'folk physics' has already been studied a fair bit. For example, see the links at the end of http://lesswrong.com/lw/khd/confound_it_correlation_is_usually_not_causation/ about a quiz designed to measure how well people understand Newtonian mechanics and to what extent they succumb to incorrect folk physics beliefs.
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