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Combining two different internet debates
(sopuli.xyz)
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this post was submitted on 18 Aug 2023
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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
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.