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B. Velocity is relative.
but relative to what? assuming portals work similarly to windows, if I take a hoop/window and place it quickly over an object, that object won't launch in the opposite direction
if I take a hoop/window and place it quickly over an object
Then the velocity of the object relative to the "exit" of the hoop would be the same as the velocity of the object relative to the "entrance" of the hoop, which is option B.
In your analogy, option A would mean the object has a relative velocity of entering the hoop but suddenly no relative velocity exiting it, so the object magically starts following the hoop.
that's true relative to the hoop, but relative to the ground the velocity would stay zero. otherwise, relative to the ground, the object would gain velocity without any force being applied to it.
Technically, relative to the ground the object becomes moving infinitely fast as soon as it enters the portal. I think a more intuitive answer can be found by replacing a nice discrete object like a box or group of people with a long pole that enters the portal lengthwise. Obviously, it's going to have to be exiting the other portal at whatever speed the first portal is moving. The out speed should always be the same as the relative speed of the object to the entrance portal, it's the only thing that makes sense, and also the only way to appease conservation of momentum.
Yeah, but that's because both sides of the window are traveling at the same speed. If the blue portal was on the other end of the tram, they'd plop.
If you strap a camera to the window, it will appear as if the object launches from the camera's perspective.
"Velocity is relative" doesn't really apply here. The question is momentum, the pedestrians have none, the portal will pass right around them. Imagine the exit portal is on the back of the train, it would be as if a large hollow tube was passing around the pedestrians, they would still be laying there stationary.
Momentum is relative too, since it's equal to mass times velocity (in classical mechanics, of course)