Okay…I was thinking about this as I was on an airplane earlier this week -
Let’s say you are in an airplane at 30,000 feet, over the ocean. You then jump from this plane, and hit the water with in a position offering as little resistance as possible. How deep would you go before your momentum would be stopped? Of course, assume that you would never hit the bottom.
Thanks,
Dirty Dingus McGhee
Height is irrelevant as you will hit the water at a terminal (get that? terminal! har har!) speed of about 125 MPH. I’ll let someone else figure how far into the water you’ll go but my answer is you won’t care or notice.
http://www.urbanlegends.com/
>> It is estimated that the human body reaches 99% of its low-level terminal velocity after falling 573m 1880ft which takes 13-14 sec. This is 117-125mph at normal atmospheric pressure and in a random posture
Yes, but if you “knife” straight vertical into the water, IIRC (no cite, so take it as you will) the terminal velocity is closer to 180-200 mph.
Now, if I had my fluids book I might be able to calculate how far you would go before the drag of the water would slow you down…oh, who am I kidding - I have no hope of calculating it. Its amazing how much you forget being out of grad school for a few years…
It would most likely be like hitting concrete at 125mph. The surface tension of the water would act as a solid surface that you would slam into. You wouldn’t be able to displace the water fast enough and end up, crunch, then slide into the water with your hips above your shoulders and your head between your ass.
Not something you would want to do.
PerfectDark
Thinking about the kind of mess that usually results from powerboat crashes, I’d say that hitting water at high speed isn’t the sort of thing you want to do, although in a boat crash, you wouldn’t ‘knife’ into the water.
Still, I can’t bring myself to believe that water is so unyielding that landing feet first, toes pointed, arms raised, you wouldn’t have at least some chance of survival, but I’ll be happy for someone to correct me.
no, concrete is an apt description. Part of the reason this happens is that water does not compress easily. Try taking a can of soda and squeezing it, you can’t, liquid not not compress easily, water itself is so dense that it expands when it begins to freeze. Add in the fact that your body is a very squishy and delicate instrument. I don’t mean to be disgusting but basically you’d splatter, maybe some of your bones might survive mostly intact but probably not. I remember hearing A LOT about this when a stewardess on that Hawaiian airlines flight got sucked out when the rook ripped off. shudder real nightmare stuff. Plus my mom’s boyfriend is a skydiver and he knows a lot of dead people and stuff like that just doesn’t happen over water.
If however you’re lucky enough to be flung into the air really high up over say, a peat bog, where the ground is very squishy and yielding, then there’s a chance you might survive. There are actually cases where this has happened.
Oh, here’s another thing. We got this waterfall here in Hawaii. Famous and pretty high. But it’s situated well enough that it looks jumpable. But most people who try die. And the knife technique only will save your sometimes. This last guy to try luckily survived but had severe injuries.
as a nitpik however, I’m not sure but at 30,000 feet I think you have to have some kind of life support it is so cold. Cold enough that if you open your eyes they’d freeze over. And if you’d breathe in there’d also be problems, because of the cold and lack of oxygen. Basically you wouldn’t be able to breathe until you fell low enough, which might take several minutes.
Hmmm, but a peat bog is only squishy because the soil is in water suspension , so what gives?
Would there be any hope of creating some sort of metal dart you could fit in and survive?
Something long and pointy to help you displace the water?
The difference is a body of water has a different density, viscosity, etc.
PerfectDark
The Navy SEALS tried something akin to this years ago near the Naval Test Center where I grew up. The SEALS often deploy into water from helicopters. Problem is, helicopters are slow and have to hover (or slow down a lot) to drop people. The SEALS wanted to eventually develop the means by which they could just roll out of the back of a C-130 or similar cargo craft and enter the water. As you would imagine, even with 60 lbs of gear, hitting the surface of the water at 130 knots tangent caused these creatures to skip over the water like big flat stones until they slowed down enough to sink. Lots of broken bones. Eventually, the hit upon the idea of a sled that would skip over the water safely until it sank. This worked better, but was hard to control, and they still had those “negative acceleration” problems.
Remeber kiddies, it ain’t the fall; it’s the sudden stop.
As far as falling into the water at TV, your best bet is to hit the water flat (belly flop but on your back) or in a tight ball. Hitting the water feet first (even with pointed toes) will snap the ball off of your femur and drive your legs straight up until they hit the pan of your hip and move outwards. At this point they often tend to rip off of your body. (there’s a tall bridge where I grew up with many jumpers.)
The tight ball, while not aerodynamic, will help to keep parts from moving too far away from you when they start snapping. I once had a discussion with my EMT dive buddy about the “proper” orientation to hit the water when balled up. We determined, after many beers, that there is no “optimal” orientation. Head first will cause almost certain Cranial Inversion (think Gallagher and watermelon), back will certainly break your back and you’ll drown, front will smash your legs and possibly cause fragments to go into your belly, or at least smash your face, and feet/butt first will cause one of the most explosive enemas you could ever imagine.
What if you could drop something ahead of you to break the surface tension of the water, like throwing down a shoe the instant before you hit? Would falling into a splash reduce the impact?
Isn’t it a little early to be drinking?
Actually, this is a good question.
I’ve always wondered about this, having made several trans-atlantic flights. Way I figured it was that I’d be better off pulling the emergancy door open, and jumping out as the plane got closer to the water. I mean, I don’t think I’ve ever heard of someone surviving a plane crash in water, but I know I’ve heard of skydivers that survived when their chute didn’t open. Or maybe I’m just thinking of bad 70’s movies plots.
So by all means, all you really smart people out there, please let me know where the better odds lie. Staying with the plane, surounded by several tons of metal that could shred me, or jumping out, and taking my chances hitting the water on my own?
Sofa, there is another, related, thread about this and breaking surface tension with sodium (creating foam). In short, no. You still have to move the water out of the way.
Hitting water is not like hitting concrete. Not ever. Check out some threads from the archives.
Why so many fatalities from jumping off bridges? - brad_d has some excellent posts in this thread.
Practical (not particle) Physics
The discerning reader might notice that this topic is a particular pet peeve of mine.
Hitting the water in a tight ball or flat on your back are really bad ideas. Decelleration is what kills you. Both these positions increase decelleration and would be more likely to kill you. Think about it. Landing flat on your back in water is painful from a height of about two feet.
Atrael,
in a smaller plane, such as a Cessna, jumping from an open door could truly save your life, since the top speed of propeller-driven craft is much slower. There is a chance, in a Cessna or a helicopter, unless the craft has lost all steering, that it could enter the water at a slower, by gliding it without power. Clearing yourself from the collision in these situations by jumping before impact would be ideal. Most falling helicopters, in autogyro(allowing the craft to fall freely without power, slowing descent by the automatic turning of the rotor), descend at around 30-60 mph, depending on how long autogyro was sustained.
If you were to jump from a jet plane traveling at 200 mph, provided you could even get the door open, you would leave the aircraft traveling at 200 mph and probably disintegrate upon impact with the water
You can’t cheat death. I know, I’ve tried.
Y’all are dead wrong about falling into water. I saw David Hasselhoff fall out of a plane with no chute, and he knifed right into the water, no problem.
Ain’t Baywatch great? 
The waterfall I was talking about was Wailua falls which is only 200 ft high.
The reason you may want to land flat on your back is the same reason wrestlers try and do that on falls and why people who lay on beds of nails never lay on just 1 nail, you spread out the pressure. If you knife in ALL the pressure is concentrated on that one little small area of your feet. However if you belly flop that pressure is spread out over that large surface area, reducing the pressure per square inch and increasing your likelyhood for survival. Still falling on water from an airplace in that position will not save you.
Peat bog senario: imagine water as a bunch of little tiny balls, all packed in together, there’s nowhere for them to go. But a peat bog is only partly water, there’s peat and a lot of gas, which bogs naturally produce. Gas, unlike liquid, is easily compressed. All that combines to allow a peat bog to act like a spring and increase the amount of time in which your speed is decreasing from terminal velocity to zero, increasing your likelyhood of survival. This isn’t to say you will alwys survive, it’s just you will be a lot more likely to survive falling on that then water. Although the toe first method might be best for the organs since there’s more cushioning that way and not towards the ribs.
water may not be like concrete but it does approach it as your speed goes up. Once you past about 200ft they’ll both kill you, water has a little more leeway but 200ft is a good approximation. The faster your speed when you hit the closer the apparant density of the two. At zero m/s concrete is 3 times water, but as you go faster they appear closer. And if you’re jumping from that high 30kft it’s a fine analogy.
Can someone please post the formula for apparent density with respect to velocity.
Pressure doesn’t kill you, rapid deceleration is what kills you. The rapid deceleration caused almost entirely by hydrodynamic drag. It would be best for you to be in the position that would cause the least hydrodynamic drag, and that position is feet first not flat on the back. brad_d an alleged fluid dynamicist agrees with me, and more importantly his calculations are supported by the real world statisitics of Golden Gate Bridge suicide jump survivors.
Nothing would please me more than to wipe out this little bit of ignorance that everbody knows is a fact. (That water is like concrete when struck at high speeds.)
Here’s a little experiment that you can do at home. Some people can slam a coffee mug down with a velocity that is a significant percentage of terminal velocity. (Around 70%) find one of those people, get some coffee mugs and head over to an inground pool with a concrete patio. Slam half the mugs down on the patio and slam the other half of the mugs down onto the surface of the pool. Which group did better? Do you still think that water somehow magically turns to concrete at some speed?
I might be wrong, but I think it’s misleading to be saying that the surface tension is going to have any influence on the outcome at all; consider:
Person falling at terminal velocity hits a large vat soap bubbles; surface tension of the bubbles isn’t going to matter a damn because the bubbles have very little mass and are easily displaced. (OK, there’s gas involved too, but you see what I mean)
Person falling at terminal velocity into water has to displace their own volume of water almost instantaneously, in other words they have to accelerate the volume of water from zero to terminal velocity (so that it can move out of the way) in a fraction of a second. Again,a vast oversimplification.
But what I’m saying is that I don’t think surface tension is going to be a considerable factor, it’s the fact that the water must be instantaneously displaced that causes the problem, so in a sense, it is like hitting a hard surface because you might as well be hitting the ground and trying to dig the soil out of the way with a spade in time.