Falling Great Distances

Factual Questions
21

In Minnesota, there is an amusement park where highdivers (ie. swimming board) jump from what seems to be 3 miles high and land into this little pool of water only 10 feet deep. Okay, I exaggerate because I don’t know the real measurements and I don’t know how they do it, but wouldn’t some of their methods at least be worth a try if you find yourself falling from an airplane (and you aren’t David Hasslehoff)?

How do they do it anyway? It’s pretty impressive…(or at least it was when I was a kid!) What % of terminal velocity do THEY reach?

22

Surface tension will have an effect here, but it’s because of displacement. Consider that you must move water molecules around when you “sink”. Consider that in moving the molecules, you are pulling, pushing, and sliding molecules past other molecules. The more force holding the molecules to each other, the harder it will be to move them. Because the molecules on the surface are experiencing more effect of hydrogen bonding than molecules below the surface, they will be harder to move. It is this that gives rise to the “hitting concrete” ideas.

How large is this “surface tension effect”? Well, compared to the force needed to move your volume of water, pretty darn small.

As Lance Turbo said, “Pressure doesn’t kill you, rapid deceleration is what kills you”. While this is true, one must keep in mind that it’s the force your body feels (and the effects thereof) that kills you. The problem with hitting the water at TV (or any high velocity) is that you don’t have much time to move the water out of the way. This results in a rapid change in momentum (damn fast to stopped in 1 second splat - er, flat). Recall from physics class that changing momentum requires an IMPULSE, which is the product of Force and Time. Since the momentum change occurs in a very small time, the force required to change the momentum is HUGE. This force acts on the body. This force breaks bones, smushes your guts, ruins the dental work and makes your whole day miserable. An attempt to dissipate this force over time would be welcomed, but can’t be done. The other possibility is to attempt to dissipate this force over area. Force/Area is pressure. So, yes, LT is correct; it isn’t the pressure. The result of the rapid deceleration is one big honkin’ force applied to your body.

23

As an aside to this thread, wouldn’t it be impossible to actually throw the shoe downwards at a speed above it’s terminal velocity? Perhaps for the begining of the decent you could, but if the shoe was less areodynamic than yourself you’d end up with a shoe in the face too! I mean if your going to die by smacking into the water, try and die with some dignity!

Nask

24

I read an interview with the guy who cleans up after bridge suicides. I belive it was at Salon.com. He said that most of them survive impact. What kills them is bones breaking, usually the ribs breaking and puncturing the lungs so they drown. He said the few who survive are usually the drunks, he theorized that their muscles were nice and relaxed so there wasn’t as much internal damage.

25

Thank you Muttrox!

Read the article at salon.com here.

However, a 240 foot fall onto concrete would definitely break bones regardless of body position. A 240 foot fall onto concrete is not survivable. And a lot less than one fourth of people who fall 240 feet onto concrete live long enough to breathe again.

26

I’ve probably lost track of tangents. The reason I borught it up was because a fall from a “350-foot-high” bridge is clearly survivable. I don’t know how well this compares (in terms of velocity reached) with the 1888 feet given earlier in this thread, but I think it at least gives some real world baseline.

27

ok I wish I caught this conversation earlier. I only have one thing to say and you can make your conclusions from this. I saw a diving coach shoot a 45 cal. Pistol into a bucket full of water. When he was done he stuck his hand in the bucket and pulled the bullet out. I would not have beleived it had I not seen it. so take what you will from that statement. and make your own conclusions about water displaced when you hit it going 188 mph. First I would say you would initially bounce, the bouncing would cause you to break some bones and as long as you didn’t break your neck, back, ribs etc…etc… you might survive…

28

I will ask some people about terminal velocity in different body positions this weekend. Belly to earth is about 120-125 mph for me, but I weigh 235 lbs and jump with about 40 lbs of gear. It is about 110-115 for lighter jumpers. These are measured numbers from digital altimeters that track exit altitude, freefall distance, time and opening altitude.

Re the OP, unless you are a very skilled skydiver you will land belly or back to earth, probably spinning. These are stable positions. Jumpers who can fall feet or head down are very skilled. It can take 50-100 practice jumps to get good at this. This assumes you have time to get stable; people will tumble for a while unless they make an effort to get belly to earth. I have some interesting videos of base jumpers going off New River Bridge in West Virginia demonstrating this.

You might consider landing on snow instead of water. Bailout, by Don Dwiggins, cites a well documented case of someone jumping out of a burning bomber in WWII. He fell 18,000 feet and survived with minor injuries. Seems that fir trees and heavy brush may have slowed him down somewhat before he landed in a snowbank. Another book, Parachuting’s Unforgetable Jumps, mentions a Russian experiment where airborne troops jumped out of a plane onto the snow. No info on speed or altitude so assume low and slow (20 feet and 60-70 mph). Story was there were no injuries, but they did not make this a standard procedure.

29

my problem with bard’s equations is it seems that he’s disregarding the effect the water might have on the shape of the body falling which would have a significant impact on the rate of deceleration. Because it seems to me that if you reach terminal velocity (which occurs just after falling 1/3 of a mile) the forces acting on your body at certain points of the 0.05 s entry time are absolutely killer and would seriously deform your body.

I did some calculation which now I’m not sure of, but here’s my logic. Estimate a volume for the 2 feet and a length. For simplicity sake assume it’s a nice tube shaped. So a simple V = 2(pi)r[sup]2[/sup] X h. This gets you a nice volume which you can then convert to the mass of the water those feet will push out of the way. Calculate the time the it would take for you falling at terminal velocity to fall h (the length of the feet). Now since when your feet enter the water (assuming no deformity yet) they have to push that water out of the way. I just calculated how much force it would take to accelerate that mass of water to the surface (no more no less) in that amount of time. And I got a HUGE force that would be acting on just the ankles. Something on par with 28 times the force a 100Kg man would put on them. Let me post my numbers now

volume of 2 feet = 0.002364 m[sup]3[/sup]
time = 0.005 s
h = 30.28 cm
x = h/2

I wasn’t sure about the raising of the water, whether it was linear or exponential. I thought it was linear so I used this equation x = 1/2 at[sup]2[/sup], placing all the matter at its center of mass. Thus getting an a which when plugged into F = ma gave me that large force.

So what do you think brad? Am I making any sense?

30

I’m going to bump this because I’d like to hear a response to my challenge to brad’s equations which it seems a lot of the anti-concrete ideas are based.

brad’s equations prove the concrete like idea wrong because one of his assumptions is that we are already wrong. He’s treating the human body like a piece of straw being driven into a tree trunk by a tornado. But our body is not anywhere near as durable or strong as a straw, we’re probably halfway between the straw and a wet noodle.

What I assume would happen as your body hits would be first your feet will experience a very swift deceleration as they attempt to push the water out of the way. Swift enough that the weakest points in your lower half would have no choice but to bend, either the right way or the wrong way. These weak points being your ankles, knees and hips. Probably your butt would actually hit your heels. I doubt there’s a person on the planet with the muscle power to successfully lock their joints against the magnitude of forces this unfortunate body would be undergoing. As your waist beings to undergo rapid deceleration your spine would probably get crunched and your head, which is already precariously balanced on your body will fall forward, very fast. Probably even snapping off.

Another thing that I feel would happen is that there would be a shock wave produced in your body, just like ripples in a pond. I can’t tell how fast these would go, maybe faster then the body is falling but maybe not. If it is faster then the body is falling then it will likely push all the fluids in your body before it, rupturing all your organs and blood vessels. Probably even your skin. And the ruptures in your skin would have your partially liquified guts spewing out your body. If not the this wave will stay just on the air side of water and still have the same effect.

Ok, so when you fall from the hights mentioned in the OP and you strike the water with such force the result on your body will be qualitatively similar to a body hitting concrete. Not quantitaively since the forces on that would be about 3 times larger, but the similarities between the 2 resulting messes would be very very similar. And there is absolutely no chance you would ever survive a fall from 30,000 into an ocean or sea out there. You would always die. A very very swift death.

I don’t think any person here has the ability to do the types of equations needed to find out what actually happens. I don’t see how there’s any way any person can do this without a super computer. Which is why I had to make so many simplifications in my previous post. Just to try and get a handle on the situation, and even then I could only got 5 thousandths of a second into the situation.

31

Remember, brother rat, when theory disagrees with experiment, it’s always, always, theory which gives way. All experimental data show that hitting water at any speed is not like hitting concrete. People jump off bridges and survive. People jump out of planes and survive. Coffee mugs hit water and survive. Bullets get deformed on hitting water, but they get even more deformed on hitting concrete. If your equations say otherwise, then there is something wrong with your equations.

32

I agree, real life always trumps theory. But in this case all we have is theory. No one has given any evidence as to what a living human body would experience when striking water at a perpindicular angle at speeds ranging from 60 m/s on up. 1) All evidence given from bridges is circumspect since the speed at which they strike are relatively slow. It takes roughly 430 meters to reach 95% of a body’s terminal velocity where as the highest heights from which any one who has jumped into water and survived was only 110 meters. And when one falls from that height he’s only going roughly 50% of his terminal velocity. By falling from heights at which a plane flies the speed with which the body strikes will be double and that double makes a lot of difference. Which is why no one has survived falling into water from any heights higher then that 110 meters. 2) brad’s theory is fine for a perfectly rigid body but our body is absolutely not rigid, our knees, ankles and hips will fold like cloth at those speeds. Our neck and spine will bend like reeds in the wind and snap. He may be a great fluid dynamicist but he’s not a physiologist. You absolutely must have a good understanding of the human body in order to predict how the body would act when suffering those forces. 3) The coffee mug falls to the same problem as brad’s equations. Those coffee mugs are real strong and unbendable. Just like the straw that gets nailed into a tree by a tornado. But you go try that experiment with water balloons and you’ll get a different result. We’re a lot more like a balloon then any coffee mug. Our outer skin is very weak. Especially at our bellies and especially if they have sharp bones poking through them from the inside. Maybe if we had a really tightly packed body lined with scales like a tuna we might survive but any human would have absolutely no chance.

Anybody who wants to get evidence on what really happens at TV will find themselves confronted with a big problem. Getting to the crash site even half a second late would result in the evidence being marred by sinking. If you were lucky (or unlucky) enough to have a high speed camera film the crash then you’d have some evidence.

33

I hadn’t thought of that poor Hawaiian flight attendant in years. What about flight 800, which had its whole nose ripped off from the front door and then flew miles more. Can you survive a 500 mile per hour wind?

34

I’d like to see this too Lance. Id love to see the experiment where the density of a fluid changes depending on whether some object approaching it speeds up or slows down!
Some of the pseudo-science and bullshit in this thread has really amazed me. I’m sure we’ll get some more coherent answers soon, but the suggestion that landing presenting the largest surface area is a good idea is plainly ridiculous. The length of time of the impact would be massively reduced (you’d descend a lower distance in the water), hence the decelleration would be enormous!!
And as for ‘The water has nowhere to go so it’s like hitiing concrete’…puhlease, have you ever seen a ship sail through concrete? Me neither, but I have seen them sail through water by displacing the water.
I’m not a physicist. I hope I’m not totally wrong.

35

A ship has a great deal more time to push the water out of the way. Ships are made to withstand the force of being pushed throught the water. A human being going well over a hundred miles an hour couldn’t possibly withstand an impact with water. The resistance of the water to “stay in the way” is greater than the structural integrity of a human being. It is as simple as that. Orientation of the doomed in relation to the water is inane. Haven’t any of you ever gone 150 on a bike? Just imagine hitting a wall of water. You’re splatter matter. End of story.
The viscous jelly of your brain compresses and then tears even if your head remains integrated.

36

I apologize for being so tardy in joining this thread. I just moved across town, and my presence here on the board has been nearly non-existent for more than a week.

Anyway, brother rat, I’m not totally sure just how to address you points, but I’ll try to bang out something here. :slight_smile:

When I sat down and derived those equations, I was making some pretty big assumptions. The primary one relating to your point is that I was totally ignoring the time during which the person was part in, part out of the water. As you point out, this may be a critical thing to overlook, but let me explain why I think there’s still some merit to it.

I was not trying to come up with the final word on this, by any means. In my opinion, people were getting so hung up on the surface properties of the water that I felt that something far more important (and simple) might be getting ignored - plain old drag. So, I forgot all about the interface, and looked at what would happen to a person who magically found himself totally submerged in water, moving downward (well, feet-ward) at 30+ meters/second.

I don’t really think I proved anything, but the numbers I got suggest (IMO) that hydrodynamic drag would slow this person down fast enough to kill him. I got my info on G-tolerances here. It sounds like their numbers are given for properly restrained subjects, which in our case is likely not true. Tolerances may well be lower for unrestrained individuals. They make reference to internal injuries suffered, including vertebral fractures, when limits are exceeded. (For example, at 45G of forward acceleration, the heart rotates in the thorax and causes tears in the aorta. Bad.)

You (quite correctly) point out that some external deformations of the body are likely to take place - probably before it’s completely submerged. That’s fine, but what is it that causes these deformations? Drag or surface effects, seems to be the unresolved question, right?

This is a very incomplete response, but I’ve got to get home tonight. Rather than save this, I’ll post it anyway to keep the thread alive and let you guys know that I’m here. I’ll post more tomorrow.

37

I posted this in another thread a while ago, but there is a documented case of a Russian Stewardess that got sucked out of a plane at 30000FT + and survived. She landed on the downward slope of a hill and rolled a whole hell of a lot and broke some bones (plus I think it was in the winter so the snow would have helped)… but she made it.

38

Good site, Glad you’re back brad. I doubt this is something we’ll ever really know. It’s not really something you can test in the lab and real life incidents are destroyed within seconds of the occurance. Plus just look at all the factors in the deceleration, all of which are variable in very complex ways.

I think the models needed to run an experiment would be extremely complex. Possibly covering several hundred to maybe even a million pieces. A good summer of work to write the program. Anyway maybe my position will be clearer if I give a prediction.

And that is this:
Assumptions: body of height H and velocity V which takes time T to travel a distance H.
If we were able to take a crystal clear picture of the body at time = T after first contact with the water I believe you’d find the body crunched into a pit in the water about 9 times the size of the crater of a similar crash into concrete.

hope that wasn’t too complicated. Sorry to be a fly in the ointment but it’s the rat in me :slight_smile:

39

So am I to take it from your reply that for a given height x which you know you are about to fall from (without, of course, being given the height beforehand), you would have no preference as to whether you fell onto concrete or water? And you’d have no preference as to whether you fell head first, horizontallly or feet first?
If not then the assertion that falling onto water from some great height is just like falling onto concrete is ridiculous, as is the suggestion that the position of the body on impact is unimportant.
Fact: when a given body (in the physics sense) collides with another body, the nature of either of the bodies is important to the outcome of the collision, be they solids or liquids.

Ultimately, it really would be interesting to find the answer to the question “At what height does it cease to be of any real importance, regarding survivability of a fall, for it to become unimportant whether one hits water or concrete?”

Any volunteers?

40

A more useful question would have used speed rather than height as I suppose the terminal velocity of a human body is reached pretty quickly :rolleyes: