That’s how kayakers are able to run 100+ foot falls.
Cite?
I would argue it depends on the volume of air being injected at the landing site. But that brings its own set of problems.
My very 1st job involved working around a sewage treatment plant. The final treatment involved aerobic aeration of the effluent before being injected into the ocean. We were warned that if we fell in the water, we would drown because the bubbles would not permit enough buoyancy to stay afloat.
I doubt that this is equivalent to a situation typically found in nature.
The resistance of the water has nothing to do with surface tension.
Cite that this effect is specifically due to cushioning by air bubbles in the water?
True, but I assumed we are talking about natural conditions.
Again, I suspect this is much greater than found in normal turbulence.
Darn. Well I may be incorrect that the benefit the bubbles have is ‘breaking surface tension’ but that is how the theory is explained by many, including cliff divers who are landing in natural waters. But it does appear that the bubbles do in some way lessen the overall impact the diver’s body experiences upon landing and they exploit choppy waters to get a safer dive.
From How cliff-diving works - cited earlier:
And under “diving facts” at swimming.org:
I have to wonder - what was the plan there? Grab the guy in mid air and then you both flap your arms and fly to safety?
I thought the air bubbles gave the water somewhere to go to get out of the way of the falling object, giving the water some form of compressibility.
Mythbusters did a segment on whether bubbly water will cause a person to sink faster or not. Unfortunately, I don’t remember what the outcome was. I do recall that they had a tough time trying to replicate the situation with aerating a tank of water without causing a current that had more effect than the bubbles.
I think last time we talked about this I had asked a semi-serious question: what if you dove head-first while holding onto a large depleted uranium shell in your hands (below your head)? Could the DU shell shove the water aside to allow you to penetrate safely? I assume that, unlike a concrete block, the shell would be streamlined enough to keep going into the water faster than you do, so that you don’t knock your head against it.
Regarding the actual entry into the water - how much control would you have over that? The cliff divers, I assume, have the ability to position themselves for the smoothest entry; but the average (suicidal) Joe, it seems to me, would be flailing about all the way down and his ‘landing’ position would likely be random.
mmm
The surface tension notion only persists through widespread repetition of a plausible-sounding, but incorrect fact.
Surface tension is sufficiently weak to permit a single drop of water to detach from a dripping tap.
In theory, it’s possible if you managed to propel the thing ahead of yourself, so it entered the water a short time before you, creating a column of bubbles, although I think there are probably far too many variables to make it reliably workable in reality.
If you’re talking about just holding onto it with straightened arms, then it won’t work. The shell will decelerate when it hits the water - even if it’s streamlined (it’s pushing through water, you’re still falling through air). Your arms break or buckle, your head hits the shell and/or water, pretty much as if nothing had slowed you. This scenario is really just another way of asking if you can arrest your own fall using your outstretched arms, in a distance not much greater than their length.
What about the dive off the dam made by Dr Richard Kimble in** The Fugitive** and the dives in** Butch Cassidy** when Butch and Sundance jumped off the cliff into that little river? Are those 3 dives not possible?
There was a show on superhuman abilities recently. One such one was a shallow water high diver. Something like 12 ft into a kiddie wadding pool. They found that he was not doing something superhuman but just a very good way of doing it.
By letting his arms enter the water first he allowed some deceleration and also created a voids in the water, so when his belly flop entered the pool it hit much more gradually with the water then if the surface was a solid flat plane.
This would also seem to allow a place for the water to go, which the flat surface didn’t allow, and compressible air bubbles though they were not mentioned.
In the Navy, they taught us to jump off the edge of a sinking ship with legs straight down, one leg crossed over the other (to keep you from doing the splits on impact), and your arms tightly crossed across your chest, with your hands on your shoulders.
It was pointed out that the flight deck of an aircraft carrier is some 50 feet or so above the surface, and could be much higher if the ship were sinking and at an angle.
I have often wondered if this Naval jump technique would help at all for higher jumps.
I saw a program where some fellow was discussing jumping off the Golden Gate and he mentioned how deeply under water he went. That would probably be just as bad a shock to suddenly be dozens of feet under icy water with no breath (because it was slammed out of you).
And for the poster who mentioned depleted uranium: The Mythbusters episode folks have alluded to included precisely the scenario you describe, where a person drops a heavy hammer moments before impact, so the hammer breaks the surface tension.
It is in season 1, episode 5. The myth was busted.
I used to work offshore. During our safety training we were tought firstly not to jump unless as a last resort - the air gap depending on which deck you are on varies from around 100’ to 175’ at the helideck. No regard was taken as to whether one deck was better than the other for jumping - although it seems from this that it might be - as jumping as the very last resort.
You supposedly had a much better chance holding your nerve waiting for liftboats to be loaded and lowered, even with flames licking around you, than entering the water. That said I mainly worked in the North Sea where the cold would kill you anyway even if your survived the jump.
If you have to jump it was exactly how **minor7flat5 **describes. I guess getting that correct was deemed a better chance than getting a total rookie to execute the perfect dive.
I went looking for a cite to counter this. FINA FR 5.3.10 (emphasis mine):
Usually it’s only there to help with depth perception (it’s hard to judge distance if you can’t see the surface). But the second part seems to hint that high pressure bubbles have some other effect, so you may well be right.
**user_hostile **is correct. my boss fell in one of the aeration tanks at the treatment plant i used to work at. the aeration process keeps the sludge moving, which means the top and bottom of the 14 foot deep tanks are constantly swirling around and around. anything that goes in goes straight to the bottom, which means a person would be unable - or find it damn difficult - to get back to the surface before running out of air. the guys dropped a lot of phones and pagers into those tanks over the years. they stay there until the tank gets emptied for a cleaning.
bossman was working on top of the tank canopy and was tied off with a rope (with five very hefty men holding the other end of it, thank god) and was wearing a dry suit at the time when the canopy failed and in he went.
the rope saved his life. he was maybe 160 pounds soaking wet and about 5’10" tall. it took *all five *(two were ex-SEALS) men to haul him out and he still had to go to the hospital of course (OSHA requirement). nasty doesn’t begin to cover his experience. 
It’s baffling to me that surface tension keeps being cited as any kind of factor in discussions of water entry.
Surface tension of common liquids such as water is typically tens of milliNewtons per meter. A milliNewton is about 0.0002 pounds of force. If a human being hits water at 120 MPH, the impact force will be measured in thousands of pounds. In the scheme of such an event, surface tension effects are vanishingly small, neglible.
Most of the drag on a falling human being is form drag, and is proportional to the density of the fluid through which the object is moving. So if you’re falling at terminal velocity (~120 MPH) through the air, you’re experiencing about 175 pounds of drag force. Water is about 800 times as dense as air, so when you hit the water at that speed, you can expect to immediately experience about 140,000 pounds of drag force, and whole-body deceleration of about 800 g’s.
The ram pressure experienced by a flat surface impacting the water is calculable: at 120 MPH falling into fresh water, it’s 208 psi. Your face is, what, 6x9 inches = 54 square inches? So that’s 11,200 pounds of force, just on your face. Your sinuses will implode, your lower jaw will probably break, and I imagine even your front teeth will be shattered out of their sockets. Imagine similarly horrible things for the rest of your body.
For a given impact speed (and a given body shape/orientation), the only way to decrease this force is to decrease the effective density of the fluid medium through which you’re falling. You can entrain air bubbles in the water, but it would take a LOT of air to reduce the density to the point where the impact forces aren’t fatal. Want a 20-g deceleration event (think “fighter plane ejection seat,” which still hurts)? OK, your fluid density needs to be 1/40th that of water. At 120 MPH, decelerating at 20 g’s, you’ll need to sustain that deceleration for about a 1/4-second to bring your speed to zero; during that time you’ll penetrate the water to a depth of 24 feet, so that’s how deep your incredibly fluffy bubble-generating machinery will need to function.
Note that “1/40 the density of water” means a very, very high fraction of air. If you watch the pool at diving competitions, the bubble generators are not putting out anywhere near the required quantity of air to make a useful difference.
I think you may be referring to the infamous Waterskier’s Enema. :eek:
(The link above should lead to the full, free-text, article. Apologies if not; I tried)
The Mythbusters segment called the “myth” that you can’t stay afloat in bubbly water busted. Adam was able to stay afloat and attempt to swim through the bubbles they created, but he got very tired very quickly because of the turbulence. He was basically swimming in place while doing the crawl forward as fast as he could. That isn’t to say the situation wasn’t dangerous, but they seemed to think the turbulence was what made bubbly water dangerous, and not a lack of expected buoyancy.
Scubaqueen’s story (yuck!) seems to suggest maybe they didn’t go far enough in the test though.