Jumping off from a high altitude and the landing.

[Tasem]

So I recently watched "Warm bodies" with some of my friends. There was a scene where a the man jumps from a relatively high building while holding the girl in his arms, and lands into a pool, while keeping the girl on top, so as to "save" her from the impact.

One thing really bugged me ; Can someone actually save a person from a fall's impact by placing something under it ? I was thinking, since the sudden change in momentum usually kills people that fall from high altitudes, wouldn't that thing need to be REALLY soft (like a pile of pillows) so it could expand the time that the object's momentum change would take place, and actually reduce the force exerced on them from the ground ? (or water, but I wouldn't think that those two different states would really act differently when hit by something as fast as somebody falling to it's death)

I was also wondering something that my friend said about the subject when I asked him ; We were talking about a scenario where someone would jump off a 150m bridge with a bed underneath them, and what would happen to that person. I was thinking that since the bed couldn't really make that well of an "Impact time extender", the guy would still feel as if there were no bed under him (or in other words, I told him he'd die). But my friend kept insisting that the bed would actually "break" the surface tension of the water, and that the man would therefore suffer no great injury.

I'd be really gratefull if someone could help me out with this.

 

[mathman]

In your first example (man holding girl) the man would absorb the hard impact and crumple, so the girl's deceleration wouldn't be as rapid.

 

[Tasem]

Right. But I wouldn't say the change will be enough to make the impact non-lethal, since the guy can actually crush up to a certain point, maybe changing the deceleration process from .5 seconds to 1 second, tops. (In which case the force will be halved. A very good step towards surviving the fall, but is it enough ?)

 

[tiny-tim]

Welcome to PF!

Hi Tasem! Welcome to PF!

… But my friend kept insisting that the bed would actually "break" the surface tension of the water, and that the man would therefore suffer no great injury.

I'm not sure, but I think that the main problem with falling into water is the surface tension …

if you can get someone or something else to break the surface for you, the water itself is actually fairly soft.

 

[CWatters]

If you dropped something large and heavy into the water first (eg a big rock or perhaps another human) it will most likely penetrate the surface and drag some water and air down with it. That might well effect the rate of deceleration with which the next person stops.

Some diving pools have an air bubble system that reduces the average density of the water reducing the impact force on divers.

 

[jbriggs444]

I would think that rather than reducing average density, the important effect of bubbles in a diving well would be that of making the resulting mix compressible. So instead of having to accelerate a lot of water all at once in a sudden impulse, the diver can accelerate just the local bit of water.

 

[davenn]

Some diving pools have an air bubble system that reduces the average density of the water reducing the impact force on divers.

... and more specifically ... it breaks the surface tension :)


Dave

 

[technician]

I don't think surface tension plays any significant part in falling into water unless you are an insect.
Surface tension is a property of a water surface...it does not 'break'. It can be made weaker with soap etc.

 

[cjl]

... and more specifically ... it breaks the surface tension :)


Dave

I suspect the density reduction is a far more significant effect than the surface tension reduction.

 

[technician]

100% agreement with cjl. We had a TV program here about training of potential olympic divers. Just as a diver is about to dive a mass of bubbles is released and it was stated clearly that this cushioned the impact because of reduced density of water+bubbles (foam).
Not to be confused with the small streams of bubbles that are used to indicate the position of the water surface.

 

[Nugatory]

I suspect the density reduction is a far more significant effect than the surface tension reduction.

Back of the envelope agrees - 100 kg of person entering the water at a speed of 20 m/sec (appreciably less than terminal velocity) will displace by about a meter 100-odd liters of water, in tens of milliseconds. That's a fairly substantial impulse, easily enough to break bones.

You look at things the other way, consider what happens when a large ocean wave hits a stationary object... Water is only soft when you aren't asking it to move quickly.

 

[Tasem]

First of all, thanks for taking your time and sharing your thought about the subject. But let's take a few steps back. Our psyched friend wanted to jump off a 150m bridge with a bed underneath him. The air-bubbles that the bed will be creating seems to make a somehow enough "impact time increaser" for our little guy on a short jump. But as Nugatory stated, even a guy entering water at 20m/s will have trouble on the deceleration (Our guy jumping off the bridge will be moving a little faster than that). I also agree that surface tension doesn't have any big "impact" on the matter (See what I did there ?) as the deceleration will still happen in a relatively quick amount of time.

P.S. : To those out there wanting to do some calculations, let's say that our guy weights 70 kg, had a patato for breakfast and the bed's mass is 5 kg with it's bottom surface being 3 m^3. The water's volumic mass is 1g/cm^3.

 

[BruceW]

I'd agree, that cushioning your fall is not going to make a massive difference. It could make some difference though, so if I was falling with a mattress, I would still try to fall on top of the mattress, rather than just hit the bare ground. Also, with these kinds of things there is a lot of luck involved. For example, people have survived from ridiculously high falls and been 'relatively OK', but then people have also fallen from practically a few meters, and still done themselves serious long-term damage. So it is not like you can just say 'he will be fine as long as he falls from less than height y' or so. And I suggest that there is the same uncertainty with falling onto a cushion. Even with falling onto water, you could accidentally fall stomach-first (edited: I deleted a pretty graphic description)

 

[Tasem]

Can't argue with that, obviously. But the aim of my thought experiment was to screw a bit around with physics, see what people would think about it, and how certain people might just show up and propose a new idea that I hadn't thought of (Haven't been let down so far; the density thing was pretty mind opening). So really, this is just to see what people would come up with, rather than a "Well let's see who was right *jumps off a cliff*" thing.

 

[BobG]

So I recently watched "Warm bodies" with some of my friends. There was a scene where a the man jumps from a relatively high building while holding the girl in his arms, and lands into a pool, while keeping the girl on top, so as to "save" her from the impact.

One thing really bugged me ; Can someone actually save a person from a fall's impact by placing something under it ? I was thinking, since the sudden change in momentum usually kills people that fall from high altitudes, wouldn't that thing need to be REALLY soft (like a pile of pillows) so it could expand the time that the object's momentum change would take place, and actually reduce the force exerced on them from the ground ? (or water, but I wouldn't think that those two different states would really act differently when hit by something as fast as somebody falling to it's death)

Usually, it's the fact that every single one of a person's ribs have snapped, thereby turning them into lethal weapons, that causes the fatalities. Fatalities are usually caused by the ribs puncturing lungs, heart, and other vital organs.

Cliff divers diving off of 40 meter cliffs hit the water at around 26 - 28 meters/sec, but also decelerate at a slower rate because of their orientation to the water.

Of 169* people jumping off of the Golden Gate Bridge (about 75 meters with a velocity around 33 m/s after taking wind resistance into consideration? {it would be around 38 m/s in a vacuum}) between 1937 and 1966, only two survived, but 45 of the deaths were due to drowning (making this a particularly grisly way to die as they suffered serious, but non-fatal injuries, that they had to endure while drowning). Of the drowning victims, 8 were relatively uninjured aside from drowing, meaning at least 10 could have survived. Only 62 of the jumpers suffered brain injuries (with 30 of those suffering skull fractures).

The only one of those even remotely similar to the man holding the woman scenario was a pregnant woman that jumped off the bridge. There was no injury to the uterus or fetus. That's not really very similar to the man holding the woman, though.

Seeing as how the snapping of the man's ribs and the compression of of his torso will reduce the rate of deceleration of the woman's torso (and I could just see Keanu Reeves explaining that to the woman he was saving), I could see her possibly surviving the fall provided none of his ribs pierces her body instead of being deflected by her ribs. I think she'd survive the deceleration itself if her ribs could survive the deceleration. In fact, just wearing a life jacket would offer some protection.

And keep in mind, the protection doesn't have to absorb all of the impact - just enough that her body can absorb the rest without major injury. Increasing the impact duration by 20% would be enough for one of the highest bridges in the US - even less for lower heights.

In other words, I think mathman could be right for most bridges. Personally, I wouldn't want to test it out (although at least one stunt man tried the Golden Gate Bridge).

Fatal Injuries Resulting from Extreme Water Impact

*The 169 were jumpers where medical data was available via autopsies, hospital records, etc. It doesn't include all jumpers and can't be used to calculate reliable survival rates.

** One other note. Most bridges measure their height from the highest point, which tends to be a tower above the bridge rather than the deck itself. Golden Gate Bridge would be considered a 227 meter bridge even though the deck is only about 75 meters above the water.

 

[mrspeedybob]

It seems to me that the most significant effect would come from the mans extra mass. A 50 kg woman hitting the water will present almost as much surface area as a 50 kg woman + a 80 kg man (if 1 is on top of the other as described). Similar surface area would mean a similar force exerted by the water. Since the combined mass of 130 kg is significantly larger the deceleration imposed by the water should be reduced.

Essentially the man is using his momentum to move some of the water out of the way, creating a column of air into which the woman is falling

 

[BobG]

Provided they control their rotation to ensure he hits first. It's going to be really hard for the average person to estimate how much rotation they need for extremely large heights, let alone actually control their rotation. It's just as likely for the woman to wind up being crushed between the water and the man, thereby cushioning his fall, as it is for the man to cushion the woman's fall.

That would be a rather unchivalrous result, to say the least.

Plus the extra mass with an unchanged cross-sectional area means they'll hit the water at a higher velocity than either would individually.

And there's still that pesky problem that the person on top may well be landing on a bed of spears (broken ribs) instead of on top of a soft body.