There are many out there who take issue with the Fall of Gandalf bit from Lord of the Rings: The Fellowship of the Rings. They argue that the Balrog's whip is too long, Gandalf couldn't have fallen the distance, etc. Read the link and you'll see their methodology. The math is fine, but the assumptions are not. Let's go over this with a different set of assumptions and see what it nets us. Did someone say math + LOTR? Let's do this!
So, we know that Gandalf fell for 17 seconds, and that he was head down (which is more aerodynamic than the spread eagle most skydivers perform, which is what the nerds used for the initial equations). This gives a terminal velocity of ~200 mph, or ~90 m/s. We'll find out some stuff with just this.
So, we have a terminal velocity equation for Gandalf of:
This gives us a coefficient of drag of 0.2847 for Gandalf. It will take him 9.2 seconds to reach this velocity. That means, in 17 seconds, we have a distance of:
So Gandalf has fallen less than a kilometer to catch up to the Balrog. This also tells us something about the Balrog's decent, since we know that, adding up the numbers for how long he fell (12 s until the whip, 30 s until Gandalf's fall, 17 s until Gandalf catches up), the Balrog fell 832.5m in 47 s.
Almost there . . . a little more math to go.
I'm going with 20 m/s because of the wings (feel free to do the math for any of the other possibilities). This also makes sense as the Balrog is made of fire, so he probably doesn't weight that much (otherwise he would essentially crush Gandalf when they sword fight). It could be even less while he's falling initially since he could open up his wings and drift like a parachute; I'll cover that in a sec.
So, assuming a terminal velocity of 20 m/s it would take the Balrog 5.4 s to get there. That makes the distance equation for falling for the initial 12 s:
This would put the Balrog's whip at about 1.5 football fields, which is still ridiculously long; even accounting for some magic stretching which we might expect.
It also means that Gandalf slams into the Balrog with a closing speed of about 150 mph, which would be fatal (or, at least, catastrophic). But these are fairly simplistic calculations for bodies falling. We need to take some other things into account.
The Balrog has wings. While he obviously can't use them to fly, it's silly to assume they do nothing. If he could fly, he would have flown back up and incinerated Gandalf, the Hobbits, et al in a barrage of fiery doom then gone on to find the descendant of Smaug and had an all out, aerial death battle. Now I wish that happened.
I digress. The Balrog had wings, and we can't assume they're useless. If they were to, say, act like a parachute, he would have an almost immediate terminal velocity of ~5 m/s, or about 11 mph. If you watch the clip again, you can see that the Balrog initially falls slowly, then falls rather quickly once he hits Gandalf. It's reasonable to assume that the Balrog flared his wings out in order to get one, last ditch shot at his foe. 5 m/s for 12s is 60m, but that's the distance his feet travelled. He looks to be some 4 m high with arms that are about 2 m, so that gives us 6 m less, give or take. So we're talking a whip length of about 54 m.
The actual fall time is more like 8s, because the initial 4s is more the bridge crumbling (again, watch the clip). So a spread-wing fall could be more on the order of 40m, giving a whip length of some 34 m give or take. Now, this may still be longer than it has a right to be, but the whip did appear out of seemingly nothing, so there's obviously some magic involved. It doesn't take that much suspension of belief to say that the whip could expand from, say, an original length of some 20 m to a bit over 30 m.
Also, once the Balrog saw Gandalf in pursuit, he could have furled his wings for a more aerodynamic profile and thus sped away quicker. Gandalf as well could have splayed out before getting to the Balrog to slow his descent.
I'm not going to do the calculations for all that, but it's entirely reasonable that the scene went down just as we saw it. To say it's physically impossible is to assume, I think, more than one has a right to.