brad_d,
Yeah, I have a comment. I’m no scientist, so I would have been interested in entry speed calcs for a heighth of 102 ft vs 224 ft.
This seems like what I was talking about. Stats say it’s barely survivable, real life (for those who WANT to live, know what they are doing and are sober) suggests to me it’s easily survivable. I’ve made a hell of a lot of jumps from bridges, cliffs, and hotels with no injuries other than sore balls.
By the way , I can attest from first hand knowledge (jumping into pools from hotel roofs into 8’ of water), that there is some “wicked deceleration” (i.e. I’ve never hit the bottom, granted this requires a more refined technique) but it doesn’t feel “wicked” and IT DOESN’T HURT. I know - I’ve done it! PERIOD.
Is it just me, or is surface tension very important when dealing with water? It does have an awful lot of it… 
All that it’s really saying, to boil it down, is this:
We don’t need to worry about pretending that the surface of the water has to have any special magical wall-like properties. If you’re moving through air at 100+ mph, and the air magically became water at some instant (no surface to pierce), you’d still slow down extremely fast - perhaps lethally so.
Air resists your motion through it to some degree - the existence of a “terminal velocity” indicates that, at some speed, this resistance (“drag”) is so great that gravity can’t push you any faster.
Were you to suddenly find yourself in water at the same speed you’re moving in the air, the force resisting your motion (drag) would instantly jump by a factor of 1000. I’m thinking that this alone may account for, ah, deceleration sickness.
Yeah, I hear what you’re saying, warmgun. I was trying to decide if the results I got are completely reasonable, and I’m not 100% convinced yet.
This site gives some experimentally determined human deceleration tolerances, and as I suspected both magnitude and duration are critical. If I interpret their terminology correctly, entering water feet-first is an “eyeballs down” mode, and they quote a tolerance there of 25 G over 0.1 seconds.
They sort of make it sound like you should be fine under their “limit”. Twice the limit (50 G) will be almost certainly fatal, while it becomes iffier at 1.5 times (37.5 G) the limit. 1.25 times the limit (31.25 G) or less can be fatal, depending on a host of other factors.
If I neglect air resistance on the jump, I get an entry velocity of 36.6 m/s (close to terminal velocity) on a jump from 224 feet. I also get an average deceleration over the first 0.1 seconds of 30.6 G’s.
Doing the same thing from 102 feet, I get an entry velocity of 24.7 m/s, and an average decel. over the first 0.1 seconds of 19.0 G’s.
Say I plug in 15 foot jump (known to be easily tolerated): Entry velocity of 9.5 m/s; 5.2 G’s over first 0.1 sec.
Well…I suppose this might jibe well with reality. Certainly would imply that 102 feet is survivable, while 224 would be a problem. It’s not really what I’d call a slam-dunk, though - even if my thinking and method is more or less correct, I’m not silly enough to think that it’s that precise.
There is, of course, the problem of the diver’s body being about 2 meters long, and most of the action that I’m dealing with take place in the first 2 meters. My guess is that this will make the deceleration more gradual, but I don’t really know by how much. I suspect that, at higher speeds, the effect will be less because the amount of time that you’re “half in/half out” will drop with increasing speed.
Basically, I don’t really know. I still think that the numbers make this approach seem more or less valid, but it can’t be too accurate.