Question about falling into water from a great hight

[RedSwinglineOne]

@Gyrate

. ISTR a worker falling off the Golden Gate (or was it the Brooklyn Bridge?) and surviving because his toolbox hit the water first and disturbed it. So there’s an unspecified degree of merit to the idea.

This is in fact the exact myth the mythbusters were testing. Whether a worker could save himself from death falling from a bridge by throwing his hammer into the water ahead of him to break the surface tension of the water.

[jjakucyk]

@RedSwinglineOne

This is in fact the exact myth the mythbusters were testing. Whether a worker could save himself from death falling from a bridge by throwing his hammer into the water ahead of him to break the surface tension of the water.

Exactly, and it didn’t help, because surface tension isn’t the issue.

[Timmy1]

@jjakucyk

Pretty sure this is the one: http://www.discovery.com/tv-shows/mythbusters/videos/hammer-jump-minimyth/ Though when they re-edited it for the website they cut out a lot of the background and mishaps.

Pretty cool. So with all this is it likely that ones limbs would be torn off with water impact?

[Gyrate]

@jjakucyk

Exactly, and it didn’t help, because surface tension isn’t the issue.

I didn’t think it was - I thought the point was to aerate the water as much as possible. Throwing a hammer won’t do much at all. A whole toolbox will do somewhat more, but I wouldn’t rely on it. Of course, if you’re already falling you might as well try it, as long as you don’t then land on the object you’ve just thrown.

[Machine_Elf]

@engineer_comp_geek

Fizzing up the water is just silly.

AIUI, the continuous trickling of water into the pool during high-dive events disturbs the surface so as to increase its visual conspicuity, helping divers keep track of how far they are from it. There’s a common myth that this does something to break the surface tension of the water for the protection of the divers, but that’s not true.

OTOH, the aeration system Troutman linked to appears to release massive amounts of air into the water from far beneath the surface. It seems plausible that this could substantially reduce the density encountered by the diver, and also introduce a significant degree of compressibility.

Whether it actually makes a meaningful difference in diver decal is unclear to me, but it seems like it ought to be easily testable by dropping a projectile fitted with an accelerometer into unaerated water and then into aerated water.

[Telemark]

@Machine_Elf

AIUI, the continuous trickling of water into the pool during high-dive events disturbs the surface so as to increase its visual conspicuity, helping divers keep track of how far they are from it. There’s a common myth that this does something to break the surface tension of the water for the protection of the divers, but that’s not true.

This is correct, spinning and twisting divers need to have some visual clues to where the surface of the water is. That’s the only purpose of what you see in competition.

The Mythbusters did a segment on trying to swim through bubbly water and results were inconclusive. http://www.discovery.com/tv-shows/mythbusters/videos/toaster-turn-on/

[Darren_Garrison]

@md2000

Water (all liquids) don’t compress significantly. It’s the defining characteristic of a liquid. So when you hit water after a fall, you have to push it out of the way, overcome its inertia.

It’s funny–go fast, and hitting liquid behaves kind of like hitting solid rock. Go even faster, and hitting air behaves kind of like hitting solid rock. Go fast enough, and hitting solid rock behaves kind of like hitting a liquid.

[Machine_Elf]

@Darren_Garrison

Go fast enough, and hitting solid rock behaves kind of like hitting a liquid.

That’s one of the concepts associated with kinetic penetrators used against tanks: if your projectile is traveling fast enough, then the density of the target matters far more than its mechanical strength.

[jjakucyk]

@Telemark

This is correct, spinning and twisting divers need to have some visual clues to where the surface of the water is. That’s the only purpose of what you see in competition.

I always thought the water jet was to give them a visual clue as to their alignment left-to-right from the diving board. Do they do that, or is it not really a factor since the judges sit pretty well to their side and wouldn’t really notice if they drifted a bit?

[Telemark]

@jjakucyk

I always thought the water jet was to give them a visual clue as to their alignment left-to-right from the diving board.

Once you leave the platform or board, there’s not too much you can do to move from side to side.

[TroutMan]

@Machine_Elf

OTOH, the aeration system Troutman linked to appears to release massive amounts of air into the water from far beneath the surface. It seems plausible that this could substantially reduce the density encountered by the diver, and also introduce a significant degree of compressibility.

Whether it actually makes a meaningful difference in diver decal is unclear to me, but it seems like it ought to be easily testable by dropping a projectile fitted with an accelerometer into unaerated water and then into aerated water.

Here’s a study of a flat plate slamming into pure and aerated water. They found a 50% reduction in forces on the plate with 1.6% aeration, so it certainly has a significant effect. On the other hand, if you’re falling at speeds sufficient to generate 10x the force that will kill you, a 50% reduction isn’t going to be the difference between death and survival. It just means a slightly less messy body.

[Machine_Elf]

@TroutMan

Here’s a study of a flat plate slamming into pure and aerated water. They found a 50% reduction in forces on the plate with 1.6% aeration, so it certainly has a significant effect. On the other hand, if you’re falling at speeds sufficient to generate 10x the force that will kill you, a 50% reduction isn’t going to be the difference between death and survival. It just means a slightly less messy body.

Thanks for the data. So it seems like impromptu aeration while falling at terminal velocity is unlikely to be helpful.

OTOH, the sort of diving pool aeration system linked to upthread might have saved this guy from some painful bruises (link starts at moment of dive; fast forward to 3:40 to see the massive bruising that developed soon afterward). :eek: