After playing Portal 2 co-operative through, a friend and I were chatting and realized a bit of an issue with the physics in Portal, and neither of us were sure if this is something Valve even thought of when producing the games.
The graphic to the left1 shows a situation where in Portal 1 and 22, you could put a portal on the ceiling, and one on the floor, and basically fall “forever”. But, for those that have done it, you’ll remember that there was a terminal velocity that was reached where you couldn’t fall any faster. Terminal velocity being the speed at which the air resistance of the falling object is great enough to counteract the gravity that’s pulling the object down. And that rate changes based on the shape of the object (so a person falling with hands at his/her side, legs together, pointed down, will fall faster than someone falling spread out, chest down).