# Falling from the sky

Maybe I am WAY off base with my understanding of this, but…

Say you are skydiving. Your parachute and backups fail to open. You are now falling from the sky and a fantastic rate towards a body of water. Now, if I understand this correctly, the surface tension of the water makes it act like concrete. For all intents and purposes, you die rather quickly.

Now, same scenario. Only this time, you have an economy size bottle of Sunlight Dish Soap that you picked up at Sam’s Club. Shortly before you hit the water, you dump the contents of the bottle out onto the spot where you will land. Since soap makes water “wetter”, would that be enough to allow you to break through the surface “safely” and survive your accident? Or once you pass the layer of soap, would it be flatsplatcity all over again?

You never know when this information could come in handy…

Duh! I just realized that it would be impossible to make the soap fall faster than you. But let’s ignore that for now please…

I don’t believe it is the surface tension of the water that kills you in this case since the surface tension of water isn’t great enough to support the weight of a human being. Rather, the issue relates to the relative non-compressibility of water. When you dive into water, you impart a force on the water beneath you. The water compresses slightly but largely resists compression. The water pushes back at you. Of course, the water, being a fluid, soon gets displaced by your body, allowing you to dive in and swim about.

Now, consider your case where your parachute fails to open. You hit the surface of the water at roughly your terminal velocity. This imparts a great force on the water below, but the water compresses only slighty and resists the compression. A large, sudden force is then imparted on your body, killing you. Your body displaces the water and you sink into a watery grave. Unless you course you have a high body fat %, in which case you’ll float on the water, dead as a doornail.

Also note that the soap’s terminal velocity is likely to be lower than your own (smaller objects have more surface area per mass, and hence more air resistance), so even if you did drop some soap in, you’d hit before the soap did.

Of course, if you’re gonna die anyway, you might as well be clean and dead…

This again. We discussed this in another thread. Basicly, water never acts like concrete, no matter how fast you hit it. The oft repeated “hitting water at high speed is like hitting concrete,” is hyperbole. There is evidence that it it possible for a human to hit water at terminal velocity and survive. I don’t think the same is true of concrete.

Chronos, the falling person could take a bottle of dish soap and squirt it down. This would send the soap to a lower altitude than the faller. If timed correctly, the soap would hit the water before the faller. Not that it would do any good.

I think a humans terminal velocity is somewhere in the ball park of 120mph (i could be WAY off, someone correct me). If you’ve ever been water skiing/tubing/kneeboarding…etc and you’ve hit the water going around 35-40mph then you know that it can be pretty solid and thats considering that you’re probably going in at a generous angle. While hitting water at 120mph may not be <i>exactly</i> like hitting concrete, i can imagine its pretty close. Anybody think it would make a difference if a skinny guy put his hands over his head and did a perfect swan dive though? I always wonder that when im flying over the ocean heh.

ahhh i see html doesnt work on this board. by the way, how do you edit posts for when stuff like that happens? i couldnt figure it out earlier.

Surface tension does play a part, as does the inertia of the water. Displacing water rapidly takes a lot of force, and that force is being transmitted through your body. But breaking the surface tension is also important, as anyone who has ever high-dived knows. When you dive into glassy water with your hands apart, your head will get a painful smack from the surface of the water. Break the water with your hands before you hit, and you won’t feel a thing.

It’s no coincidence that cliff divers always dive into choppy water. If the water were glassy they’d probably be injured by the impact.

Being the daredevil sort, I’ve jumped off of a lot of very high things. My personal measured best was 92 ft 7in (carefully measured with precise instruments) that I know of. I also jumped off the Freeport bridge over the intercoastal in Freeport ,TX - heighth unknown (given to tide fluctuations) and I have a photo to prove it!. But I do know that people commit suicide by jumping off this bridge occasionally. Point is through trial and error you can learn to ‘land’ just right and live just fine from great heights. Now I realise this wasn’t terminal velosity but I would wager I could make it ‘O.K.’ if I had to from TV. Maybe Chronos could figure my entry speed so far. There is a documented case of an Air force pilot who bailed out TWO MILES up and lived!
Tips for those interested :

1. Lake Powell is a great place for this.
2. Wear wet-suit or neoprene shorts or not only will you get your balls slapped up to your chin, you will get the mother of all enemas.
3. Cross your legs slightly for the same reasons.
4. Though the urge is overwhelming, NEVER look down. Keep your head looking forward.
5. Cover your face with your hands and keep your elbows IN. This keeps water out of your nose and the water surface from smacking you under the chin which could knock you out. Think upper-cut. Its actually happened in diving competitions.
6. Realize it willseem to take FOREVER to hit, so again don’t look down.
7. Most impotantly - just have fun.

Using these tequniques I’ve developed, I’ve never been seriously hurt. No need forsoap!

Geronimooooooooo!!!

dhanson wrote:

I don’t doubt that the surface of the water would give a good smack to your head, but I think this is not surface tension.

Don’t say impotant.

I’ve got another use for the soap bottle. Throw it straight down hard. If done right, you’ll be able to step lightly to the ground.

I think one of the main things that makes water a bad thing to land on is that it doesn’t exactly get out of your way quickly. As mentioned, it is incompressible, so it all has to move over to the side. Surface tension will slow it down even more.

If you do a belly-flop, you first have to break through a substantial amount of surface tension, then the water has to get shoved to the side very quickly to make way for you, assuming you are still in one piece. If you make yourself tall and skinny, you don’t have as much surface tension to deal with, and the water has more time to make way for you. Even if you could eliminate all surface tension with soap, you’re better off following darkcool’s advice instead.

You are not way off. My reference gives the figure as 130 mph, but there must be a at least some variation because of body shape, etc. By the way, it takes about a quarter mile for a falling person to reach 95% of the terminal speed. The terminal speed of a parachutist (with parachute deployed, of course) is only about 10 mph.

[QUOTE]
*Originally posted by Lance Turbo *
**

The air going past you at 100mph+ would probably blow the dishsoap back behind you. I’ve spit out my car sunroof and hit windshields behind my car. (I was going only about 75mph)

By the way, it takes about a quarter mile for a falling person to reach 95% of the terminal speed **
[/QUOTE]

Is this with the person spread out to increase wind resistance or in “diving” position? Disregarding wind resistance (which I know is silly), I make it to be about 600 feet.

Darkcool – don’t be too confident about surviving a terminal velocity impact. Your highest jump of 92 feet would only have resulted in a velocity of 52 mph, disregarding air resistance. Given that energy increases as the square of velocity, your impact would be 6.7 times nastier at terminal velocity.

Okay, I think the basic question has been answered, but I would like to carry it forward a bit and ask, suppose just for the sake of argument that as you fell, you happened to discover a large, folded patio-table/beach umbrella was falling near you, close enough that you could move over and grasp it. Assuming that it was really far too small to actually use as a pseudo-parachute, what would your best course of action be? Would it be to wait until just the last instant or so before impact, invert the umbrella while holding tightly to the pole, and allow it to ‘break’ the majority of your velocity before giving way? Or, would it be more advisable to arrange things so that the point of the folded umbrella hits the water first, opening a path for you, ‘standing’ with your feet against the opposite end of the pole?

It sometimes bothers me that I actually think about such things. But, it also bothers me sometimes to consider that my cat may have a much more satisfying fantasy life than I do…

According to the online version of the Encylopedia Britannica (as well as a few other web sources), the surface tension of water at 20 degrees C is 0.07275 Joules/square meter. This doesn’t sound to me like it would hurt anybody.

http://www.britannica.com/bcom/eb/article/3/0,5716,72273+1+70440,00.html

Now we’re getting into the compressibility and durability of patio furniture.

I’m not quite clear about your two scenarios, Dennis, but the end result would be that you’d still die. I believe what you’re trying to convey is that the you’d be attached to the pole of the umbrella in some fashion with the canopy between you and the water. The canopy would buckle under the force of impact without absorbing a great deal of the impact (umbrellas aren’t very massive, after all), and you’d still hit the water at a high rate of speed and die. Now, if you’re assuming an uber-umbrella that won’t buckle under any stress, if you were holding onto the pole of such an umbrella, the force of impact would be transmitted through this indestructable umbrella and to your body, again, killing you.

I suppose if you had such an indestructable umbrella, you could use it right-side-up, ala Mary Poppins, to brake your descent. Suddenly grabbing onto such a device, though, would probably slow your descent so rapidly that the jolt would again kill you.

I think what you’re trying to suggest is some kind of impact absorbing device, as in the crumple zone of a car. These work by absorbing large forces over (relatively) long distances, increasing the overall time required for the full force to be imparted on the object to be saved (such as passengers in a car). So, to save yourself from impact you’d have to increase the overall impact time. Perhaps hanging on to one end of a very large accordion would work by allowing the opposite end to hit the water first… hmmm…

Caldazar, I think you’re on the right track but have got my postulated situation wrong. In one, the umbrella is folded, canopy down, but the peak is pointed toward the water’s surface like an arrow. In the other, the umbrella is falling the same way, but at a point just before impact, you swing it around so the canopy snaps open in response to the air resistance, but almost immediately tears away after absorbing some/most of the kinetic energy of the fall. The question becomes, would it absorb enough that it would keep you from dying? And in the first instance, would it open a channel with it’s “bow wave” that would reduce the force of impact enough for you to survive as you followed it into the water – or would you simply end up with an umbrella pole up your hiney?

I think I see what you’re saying now.

For the first case, where you try to use the umbrella to generate a compression wave, as stated before, water displaces slowly when compared to times involved in the impact with the water. My guess is that the force required to generate a compression wave large enough to displace enough water beneath you to is on an order of magnitutde equal to the force required for your body to displace that amount of water. Since the force required to create a compression wave would create an equal and opposite force on the pole, assuming the pole doesn’t collapse, the force would be transmitted to your body, killing you.

As for the second case, it can be answered, as with most physics questions, by making wild assumptions. Taking the information given in this post as true, a person falling at a terminal velocity of 54 meters/second (~120 MPH) would need to slow to 22 m/s (~50 MPH) by the time he hit the water. Assume that what you see in the movies is true, and that from the film “Armageddon,” a human being can withstand an acceleration of +/- 8G’s without falling unconscious. Further, make the gross assume that your speed would decrease linearly. Doing a quick calculation, you find that you’d need to open the umbrella about 15.5 meters above the surface of the water to hit the water at 22 m/s. The time it would take to decelerate over 15.5 meters would be a shade under half a second. Can an umbrella withstand 8G’s for half a second before collapsing? I highly doubt it.

Also consider that you don’t decelerate linearly but get a massive jolt of deceleration the instant you open the umbrella. So you decelerate at far more than 8G’s at first, killing you and destroying the umbrella long before you can ever reach the “safe” speed of 22 m/s.

Caldazar, I think I’d be happy to be unconscious when I hit the water.

Here’s another interesting scenario. Suppose it was you and your buddy falling from a height. Suppose you arranged it so that your buddy hit first, and immediately below you. Would the cavitation caused by the first strike provide a nice deceleration layer, reducing the force of impact to something survivable? And if so, wouldn’t this result in some interesting moral dilemmas on the way down?

“After you, Alphonse.”
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