But reference frame changes as the cube passes through the portal. The piston would feel resistance as its momentum is transferred to the cube to conserve it.
And if the blue portal is at a different angle than the orange portal, then… well, how fast is the milky way traveling through the galaxy relative to the universe?
Speedy thing goes in, speedy thing comes out.
Cube had no speed going in, therefore no speed going out. The portal speed is irrelevant
But reference frame changes as the cube passes through the portal. The piston would feel resistance as its momentum is transferred to the cube to conserve it.
The cube has a speed of about 30km/s around the sun. You want to measure speed relative to the blue portal, but why that one and not the orange one?
And if the blue portal is at a different angle than the orange portal, then… well, how fast is the milky way traveling through the galaxy relative to the universe?