>>1455031Well, let's say half the mass of the cube went into the portal.
If you decelerate the portal, you also decelerate the relative velocity between the cube and the room on the other side of the portal.
Let's say you stopped the portal instantaneously once half the mass of the cube passed through the portal. This means half the mass of the cube would have passed through the portal with a certain velocity. But once the room decelerated, the other half of the cube would not have this velocity. Therefore the front half of the cube would pull on the back half to accellerate it up to speed.
Depending on the strength of the cube, this would either tear it in two, or result in a final velocity half what it would have been had you not stopped the portal.
Since F = ma, forces emerge in the system whenever any accelleration is applied. By changing the speed of the portal, you are accelerating the cube, therefore creating a force.
