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DMark
07-01-2002, 08:35 PM
I've seen pictures of guys jumping off cliffs in Mexico, and I have seen high dives at the Olympics, and I remember the Harrison Ford jump in The Fugitive - but what is the absolute highest altitude one could safely jump/dive into water without killing yourself from the force of hitting the water?
(Would it depend on your weight and height, or would that be a moot point after a certain speed of free fall and then hitting the ocean blue?)

fallom
07-01-2002, 10:39 PM
I'd say you could fall from pretty high up, if you just point your toes to break the surface of the water. It's hard to say, since (to my knowledge) this kind of thing hasnt been tested.

Telemark
07-01-2002, 10:48 PM
Have you checked the Guiness Book of World Records? The record for high dive used to be in there. It's a very dangerous activity, because any slight break in form can kill you. There is famous footage of someone jumping from 125 ft (dim memory) and entering the water at a 4 degree angle instead of vertical. The blow knocked him cold.

Quick google search reveals the following:

Outdoor high dive record: 177 ft
Indoor high dive record: 110 ft

http://www.peterjohnson.co.uk/ArenaText/olivers.htm

China Guy
07-01-2002, 11:08 PM
If you're jumping, try to wear shoes. Make sure your feet are together or it will feel like a hammer hits your crotch. That's just from about 50 feet

Fleetwood
07-01-2002, 11:14 PM
When I was a kid, we used to jump in the local quarries. I never jumped higher than 45' or so, but ther were a number of jumps over 100'. The guys that did those used to throw rocks over the side to break the water.

peepthis
07-02-2002, 06:08 AM
"Street magician" David Blaine was talking about plans to jump from the Brooklyn Bridge for a future stunt -- how high is that? Apparently the trick is to have something hit the water under you just before you hit the water, to break the surface for you. With that sort of "assistance", what becomes the upper limit on height?

Mangetout
07-02-2002, 08:57 AM
I'm skeptical about this idea of 'breaking the surface' - it seems to be a commonly held view that the water's surface tension would be what does you the damage, but if that were the case, hitting a soap bubble at the same velocity would be just as dangerous.

The danger, when entering water at speed, is that the water has to be quickly displaced in order for your body to enter; water has mass and so the quicker you want to displace it, the more force you need (the force is supplied by your body.

Breaking the surface with another object might splash some water out of the way, but I can't see how this would be a significant advantage, as your body is still going to have to displace it's own volume in a very short time at some point.

muttrox
07-02-2002, 09:09 AM
What about plane accidents? IIRC, in other threads, survivors have been mentioned who fell out over the water from very high up. Certainly, survival is against the odds, but...

Telemark
07-02-2002, 09:11 AM
High divers usually have something splash the water so they can see the surface easier. A sheet of flat water is hard to see and get a range on, this allows them to pick out the water and get lined up correctly.

I don't know, and kind of doubt, that it would have any affect on the impact.

tomndebb
07-02-2002, 11:34 AM
There was a Navy Pilot in the late 1950s who lost hydraulics in his Crusader and had to bail at some incredible height. (I want to say 35,000, but it has been years since I read his biography. It was high enough that when he blew the cockpit hood, the difference in air pressure caused him a lot of pain where various body cavities suddenly distended.)

He wound up surviving with just broken legs, I think.
Don't try this at home. He was a trained pilot in excellent physical condition who had a lot of luck.

One thing to remember about long falls is that the human body has a fairly low terminal velocity. It is certainly fast enough to kill most people, but there is a speed beyond which the body stops accelerating as drag from the air fights with gravity. If you calculate the height from which you can rach terminal velocity, then if you can survive that fall, you can survive from 10 feet higher or 100 feet higher.
(Few of us, obviously, could control our descent and be in sufficiently good condition to survive splatting the water at terminal velocity, but it has been done.)

DMark
07-02-2002, 05:51 PM
I had never heard of the "breaking surface" theory, but the idea of using it to calculate ETA for entry into water sounds good (the first time I jumped off a high dive as a kid, I held my breath at the top, and was about out of air by the time I hit water).

tomndebb - I am both dubious and intrigued at the idea of anyone surviving anything close to a 35,000 foot drop...do you know anything more about this story? Was there no parachute?
The guy just did a big ol' swan dive?

bryanmcc
07-02-2002, 06:47 PM
DMark, like tomndebb said, once the body has reached terminal velocity, it doesn't matter if you're falling from 20 feet or 200,000, you'll hit the water at the same speed.

I believe I've heard quoted before that terminal velocity for a human is about 120 mph (disreguarding the position of the body here). Acceleration due to gravity is 9.8 m/s^2. Converting that to English (and hoping I don't screw up my units), I get about 21 mph/s. So a jumper reaches terminal velocity in about six seconds. Distance covered = .5at^2, so in six seconds a person falling from rest travels 180 meters, or about 600 feet. So, if you can survive a jump from 600 feet, you can survive a fall from thousands. Ignoring such things as reduced air pressure, etc., of course. Also assuming I didn't goof the math there somewhere.

Now, whether someone could survive a fall from 600 feet is a different kettle of fish. :)

-b

BobT
07-02-2002, 06:53 PM
According to http://www.greatbuildings.com/buildings/Brooklyn_Bridge.html

The Brooklyn Bridge's roadway is 119 feet above the water at the towers. It is 276.5 feet above the water from the tops of the towers.

Green Fool
07-02-2002, 07:55 PM
.......I did a belly flop off a 30 foot high dive when I was 9.

I didn't understand why it hurt so much.... It was just water!



Someday I want to go swimming in my Halloween costume. :D

Fern Forest
07-02-2002, 08:45 PM
I agree with Telemark. Maybe somebody could squeeze out a few more feet but that's about it. We have a waterfall (Wailua) here that's 200ft and people foolishly jump off into the water because the height is deceptive and it looks less. They jump, point there toes, lock there body and almost all die. The few lucky ones that survive are quite messeed up. And I suppose don't qualify for Guiness.

This is one of those things we have debated long and hard about. There are two general camps. Those that believe that you can use terminal velocity to save you and those (including me, maybe only me) who believe that at those speeds even if you lock all your joints into a perfectly straight line (and do it so quickly that you don't let your t.v. increase significantly) that there is no way your joints would be able to resist the forces and they would bend and break and kill you.

Sadly since Cecil has yet to give us his take we're in an stalemate.

This thread (http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=74052) probably goes into way more detail then you'd ever want. And gives all theories into this debate the best defence.

tomndebb
07-02-2002, 09:21 PM
tomndebb - I am both dubious and intrigued at the idea of anyone surviving anything close to a 35,000 foot drop... I don't blame you. That's the number that sticks in my head, but I would never defend it. I used to have a copy of his autobiography, but it disappeared years ago.

The facts that I will attest to, (even though based on memory):
- The F8U lost hydraulics
- He blew the canopy and ejected, suffering a great deal of pain as his stomach, lungs, eardrums, and a few other body parts underwent immediate decompression (indicating a fairly high altitude)
- His chute(s) failed to deploy
- He survived the fall with broken bones and some (unremembered by me) ill effects from the decompression
- He ended the book by making a point that he was often asked why he was not wearing a g-suit and replied that they were bulky and hot and that it was worth it to fly unencumbered against the one-in-a-million chance that he would have to bail out at that altitude again.

I don't know at what altitude g-suits become recommended (or were recommended in the late 50s). And my 35,000 feet figure is almost certainly wrong, because the record for a (non-fatal) fall without a chute is 33,330 (http://www.terra.com/specials/skydiving/records.htm).

ski
07-03-2002, 12:23 AM
G-suits are not used to combat high altitudes, they are used to delay the onset of "G-induced loss of consciousness", or G-LOC. G-LOC is caused by hard turns and such that force all the blood to your toes, in other words, away from your brain. If you undergo too many G's, you pass out. The G-suit has air bladders that wrap around your legs and midsection that fill with air under high g-loads, helping to press the blood back up to your brain.

It has nothing to do with altitude. So I'm not sure what this pilot means in the last statement you mention.

Shalmanese
07-03-2002, 12:43 AM
Well, considering that people have survived falls from terminal velocity onto LAND before, I dont see anything that would make water more lethal that solid dirt. The trick is to use the 120mph you have going to try to land as SIDEWAYS as possible, not straight down.

tomndebb
07-03-2002, 12:46 AM
G-suits are not used to combat high altitudes You're right. I grabbed the wrong term (and cannot, now, remember the correct one). The author described a suit, much like a space suit, that was available for high altitude flights that he chose not to wear for the reasons given.

Fern Forest
07-03-2002, 01:04 AM
Originally posted by Shalmanese
Well, considering that people have survived falls from terminal velocity onto LAND before, I dont see anything that would make water more lethal that solid dirt. The trick is to use the 120mph you have going to try to land as SIDEWAYS as possible, not straight down. I don't think anyone has survived when they hit solid dirt from any significant height. Those very rare cases where people survive have happened over wetlands where the combination of mud, water, gas and plants combine to produce a sort of foamy consistancy for the ground. These foamy surfaces provide a lot of give and extend the time between 1st contact and full stop. That's the key number, how long you have to decelerate. Water is a very dense form of matter that deforms only with great difficulty leaving very little time to decelerate.

Finagle
07-03-2002, 01:20 AM
I'm wondering if the "throw something in the water just before you go in" trick works not because of the surface tension but because of cavitation -- that is, it essentially adds a lot of bubbles to the local area, reducing the density and increasing the compressibility of the water.

Lance Turbo
07-03-2002, 01:40 AM
Don't get me started.

The thread Osiris linked to has links to all other threads on this in which I have participated.

GusNSpot
07-03-2002, 01:56 AM
When high speed water skiing, it is wise to never put your arms out in reaction to falling which is very easy to do, speed skiing is like trying to balance on oiled bouncing glass. I have only gone down at 70+ MPH and did remember to tuck into a tight ball, made six skips and slid across the surface a loooong way. Another skier went down the same day and stuck his elbow out only a little bit. Took his shoulder out of socket.

Do a frog with a webbed or wings jump suit and T.V. is a good bit less and horizontal movement is much greater, might get lucky........ Some Frenchy was gonna try to hit a grassy slope of the right degree. I have not heard if he ever really tried, but he was practicing with the webbed jump suit.

Mirage
07-03-2002, 02:05 AM
Finagle's right, the point of throwing something heavy into the water right before jumping is to create alot of bubbles in the water where you will hit. The air in the bubbles is very compressable and allows the water enough room to displace as you enter it. Water isn't compressable and so jumping into a calm pool requires the water to move out of the way, which it can do only so fast and under much duress.

TheLoadedDog
07-03-2002, 04:19 AM
I'm having trouble finding a cite for this, but during the construction of the Sydney Harbour Bridge (opened 1932), there were quite a few workmen who lost their lives in falls. The only one to survive was a guy whose toolbox hit the water a moment before he did. He suffered fractures, and IIRC, his boots somehow "welded" themselves to his feet.

Boo Boo Foo
07-03-2002, 07:39 AM
Guys, when I was a kid I was given a "Guiness Book of Records" as a Cristmas present. And being the lover of useless trivia that I am, I memorised literally hundreds of facts.

One in particular was a Russian pilot who bailed out of his prop driven fighter near the end of WW2 from about 29,000 ft - with no parachute! And he lived!

The article quoted that the pilot bailed out over incredibly forest which was covered by powder snow and he hit the powder snow on one the tall pine trees in his landing zone - he bounced thru about 80 ft of branches and came to a relatively soft landing believe it or not and survived to tell the tale.

I must say I like the idea of the cavitation bubbles. In theory, a huge bubble making device straight under your dive zone could make a huge difference.

Another bizarre question is this... if you were in a specially designed steel diving javelin, how high a dive could you survive from? Gotta be a few miles surely! Imagine the terminal velocity on something like THAT! More importantly, how deep would one of those suckers go before trying to resurface!

sailor
07-03-2002, 08:15 AM
>> They jump, point there toes, lock there body and almost all die

That's neither here nor there, is it?

Fern Forest
07-03-2002, 08:32 AM
Well if at 200ft you have a very high mortality rate then I believe we have a very good upper limit for the OP. Most definately at 200ft you can not safely jump into water.

And if you're an average Joe then I would strongly urge you not to attempt to claim those world records Telemark mentioned. I'm sure those record holders were not only fit but had trained quite a bit to make those records.

rivit
07-03-2002, 10:39 AM
I remember seeing something before the Winter Olympics about how ski-jumpers were able to practice without snow by jumping into pools. The only catch was that the pools had to have air pumped up from the bottom of the pool so that the skis wouldn't break. So I think that supports the idea of cavitation increasing the compressability of the water.

Amfet
07-03-2002, 12:05 PM
Originally posted by Shalmanese
Well, considering that people have survived falls from terminal velocity onto LAND before, I dont see anything that would make water more lethal that solid dirt. The trick is to use the 120mph you have going to try to land as SIDEWAYS as possible, not straight down.

Water is not compressible, soil is...

d12
07-03-2002, 01:35 PM
Water is not compressible, soil is...
Water is. How do you think firehoses work?

Amfet
07-03-2002, 02:51 PM
You're right water is compressible, but not a whole lot...

from

http://www.sbu.ac.uk/water/explan.html#comp

It may be thought that water should have a high compressibility (kT = -[V/P]T/V) as the large cavities in liquid water allows plenty of scope for the water structure to collapse under pressure without water molecules approaching close enough to repel each other. The deformation causes the growth in the radial distribution function peak at about 3.5 with increasing or pressure (and temperature), due to the collapsing structure. The low compressibility of water is due to water's high density, again due to the cohesive nature of the extensive hydrogen bonding holding. This reduces the free space (compared with other liquids) to a greater extent than the contained cavities increase it. At low temperatures D2O has a higher compressibility than H2O (e.g. 4% higher at 10C but only 2% higher at 40C) due its stronger hydrogen bonding producing an ESCS equilibrium shifted towards the more-open ES structure

Rocketeer
07-03-2002, 02:57 PM
Water isn't compressible to any great extent. Bulk modulus for Skydrol hydraulic fluid is 200,000 psi, and although I don't have a number for water, I'm guessing that it's pretty close to that since the densities are similar.

However, a little bit of dissolved air makes a big difference; in our hydraulic systems, the effective bulk modulus is only about 80,000 psi, almost entirely because of dissolved air.

Firehoses work because you have a pump pushing the water out the open end of the hose. Compressibility doesn't enter into it at all.