>>719011896
Whether velocity changes by direction or magnitude is entirely down to what reference frame you are using. If you're looking at the reference frame where the portals are stationary the velocity will only change by direction.

But in the A or B problem that reference frame doesn't exist.

>>718893368
The claim made was that portals don't impart momentum. Even when they aren't moving they don't conserve momentum.

And yes, the movement of the portals also matters. It can be claimed that the cube's speed will be conserved as long as the two portals are not moving. Which means it matters whether the portals move or not. See how in the reference frame where the portals are moving the cube's speed is suddenly observed to change in this gif.

>>716506098
>You wanna take that again?
Acceleration is change in velocity over time. If no time passes, there is no acceleration.

>But the cube at rest will suddenly accelerate away without an impulse?
It will move away from the platform without a force or acceleration. Same as in this gif, which you could also apply to portals on the wall when the whole Earth is the moving train. This is what happens in game there and also for the moon scene.

>The question is about the portal and you cannot ignore it.
Why Not? What would change if you close the portal after the cube makes it thought? Also going back to the gif and assuming the answer were A and there were some mechanic that could explain the movement of the cube, what would happen if you closed the portal? In B it doesn't matter, but that may not be the case for A.

>>713545372
>Portals are functionally holes in reality creating impossible spaces by linking two areas in a non-euclidian manner.
Yes they bend reality to enable two seperate doorways to behave as a singular passage.

However the result of this is that the position and momentum of an object as judged by space unconnected by the portal will be subject to change on passing through the portal. You seem to have struggled with grasping that second part.