I think you said at one point that you plan on implementing the ability to perform actions whilst falling in adventure mode. If my adventurer were in the process of falling down a deep vertical shaft and I removed a heavy object from my backpack and threw it, would the conservation of horizontal momentum be respected? In other words, would my falling adventurer be propelled in the opposite direction?
I think that would be ridiculously difficult to do on a set of tiles like DF does, not to mention that the effect would be imperceptibly small anyway.
It wouldn't be imperceptibly small generally; if you throw a heavy backpack while falling, you'd noticeably move in the opposite horizontal direction. However, there's not usually enough time during a fall to do that kind of thing. Currently we can try to grab tree branches and cliff sides during a fall, which is more practical.
Keep in mind that realistically you shouldn't be able to throw heavy enough objects to make a noticeable difference. At the very most (realistically) it would have the same effect as you would notice from pushing off a wall , as that's the most force your arms could exert. Thinking you could throw a dragon-loaded backpack in real life would be ridiculous, and although if you could, it would propel you in the opposite direction, realistically you would just be pushing off of it.
Also realistically, if you fall 20 z's and somehow manage to grab onto a cliff face, it would rip your arm off. 
But I consider that an acceptable break from reality, if it means I live a little longer.
Did I use "realistically" enough in this post? 
It won't make a difference in slowing your fall, but throwing a 60 pound backpack in mid aid should make a difference in where you land. So doesn't need to be unrealistically heavy. It's not really a useful thing to do though. Firing a bow should have a comparable effect, since your arms are working just as hard, but again, firing a bow while falling or being thrown isn't practical. Firing a preloaded crossbow could be slightly more realistic, and would push you more.
It would be cool to model this accurately, but it doesn't really add value to the game. Especially when there are more pressing ways to make falling better, such as having some notion of landing on your feet.
