>>13763503B.
Break it down second by second, assuming an l=1 m cube and the yellow portal moving at 100 m/s. At t=0 the edge of the cube is at the boundary of the portal. At t=0.001 a slice x=0.1m into the cube is at the boundary of the portal. For the cube to retain its shape, the front of the cube must be 0.1 m away from the blue portal, meaning it will have traveled 0.1 m in 0.001 seconds, so it as traveling at 100 m/s. When the back end of the cube passes through the portal it will still be traveling at 100 m/s.
Also, considering that this is a question of whether portals look at absolute velocity or relative velocity, and they're already moving in Earth's orbit/rotation (hell, even flat earthers claim that Earth is accelerating upward at 9.8 m/s/s), it's fairly obvious that the answer should account for relative motion. Further, the claim that it violates the laws of conservation of momentum or energy are stupid because momentum is a vector and changing its direction is already changing it, and even stationary portals can allow for perpetual motion, if two portals face each other on the floor and ceiling.
