Asking a Physicist! Kinematics falling problem (Updated OP)

[RickDaily12]

Hi there. To give some context- I thought this example up to explain a physics problem encountered in a story I'm writing. How realistic is the conclusion?

Please note, the example has been updated (to avoid serious injury), as of this post: (http://www.giantitp.com/forums/showthread.php?549105-Asking-a-Physicist!-Kinematics-falling-problem&p=22780983#post22780983)

A teenage girl who is of average build and height, and weight at 73 kg, falls 80m straight down into a well.

However, at 20/40/60 meter depths, she strikes a shallow pool of water- 1 meter deep, which rests over a large rubber barrier. When she strikes the water filled barrier, it bursts, and she continues falling to the next barrier until the bottom.

At the bottom of the well is a 2 meter depth of water. Suppose that Air resistance is negligible.

Are these stipulations enough such that she could realistically survive the drop under these conditions, without severe injury?

(If needed, I mentioned in story that it took her 7 seconds to complete this fall- I didn't bother mentioning this bit in the event it were quite inaccurate in the calculation- but here it is if I'm mistaken.)

[Grey_Wolf_c]

Hi there. To give some context- I thought this example up to explain a physics problem encountered in a story I'm writing. How realistic is the conclusion?

A teenage girl who is of average build and height, and weight at 73 kg, falls 80m straight down into a well.

However, at 20/40/60 meter depths, she strikes a shallow pool of water- 1 meter deep, which rests over a large rubber barrier. When she strikes the water filled barrier, it bursts, and she continues falling to the next barrier until the bottom.

At the bottom of the well is a 2 meter depth of water. Suppose that Air resistance is negligible.

Are these stipulations enough such that she could realistically survive the drop under these conditions, without severe injury?

(If needed, I mentioned in story that it took her 7 seconds to complete this fall- I didn't bother mentioning this bit in the event it were quite inaccurate in the calculation- but here it is if I'm mistaken.)

Insufficient information: you need to specify the qualities of the rubber barriers. Specifically, how much and how fast they slow down the speed of the girl.

Without that information, I can see the full gamut from "the barriers are irrelevant, and we might as well ignore them and assume the girl fell 80m" to "the barriers slow her down completely, and thus it's just an issue of what kind of damage a 20m drop into somewhat shallow water would do"

GW

[RickDaily12]

Hmm.

The way I imagined her striking the barriers were more like, her downward momentum is always nearly completely absorbed by the barrier, but she can barely rupture the thing. So I guess her initial velocity from there is between 0-1 m/s?

I'm not sure how long that would take. Seven seconds sounds way too fast then, but eh.

Would she be falling too quickly for the water on the barriers to join the cushion at the bottom?

[Grey_Wolf_c]

Hmm.

The way I imagined her striking the barriers were more like, her downward momentum is always nearly completely absorbed by the barrier, but she can barely rupture the thing. So I guess her initial velocity from there is between 0-1 m/s?

A fall from 20 m into water is not anything to sneeze at, but it won't necessarily kill her. My own googling (https://physics.stackexchange.com/questions/9059/jumping-into-water) tells me that solid numbers are mostly behind paywalls, and mostly are statistical analysis on suicide, so not exactly a cheery read.


Stone states that jumping from 150 feet (46 metres) or higher on land, and 250 feet (76 metres) or more on water, is 95% to 98% fatal.

ETA: Note that I'm assuming she is not damaged at all by the 3 slow downs due to the rubber barriers. If she decelerates fast enough, though, she could break something or seriously hurt herself before the final impact - she's effectively falling 20 meters four times in a row, so if those things hurt her, she's statistically more likely to be seriously hurt in 4 attempts than just one at the end.


Would she be falling too quickly for the water on the barriers to join the cushion at the bottom?

Not if she slows down to 0 at each barrier. After that point, the water and the girl experience the exact same acceleration, so they will all fall at the same time. If she still has some speed left over, then she'll stay ahead of the water.

Grey Wolf

[Leewei]

Jill and Jack
A hill did tack
To draw from a well quite shoddy
When on their trip, acrobatically
Both problematically
Became a falling body

[RickDaily12]

Jill and Jack
A hill did tack
To draw from a well quite shoddy
When on their trip, acrobatically
Both problematically
Became a falling body

Lol

Well, thanks for the insight, Grey. I think another part of the issue is, I'm a metric user- which is nice when calculating Physics, but then one is quick to forget just how big a drop 20 meters really is.

Actually, yeah. The well is probably closer to 100 feet down, then. Going by that, if I just use two barriers at 35 and 70 feet, she's far more likely to be okay. Plus, with the decelerations that keep happening, 7 seconds sounds a lot more sensible now to complete that fall.

Thanks again!

[tyckspoon]

Hmm.

The way I imagined her striking the barriers were more like, her downward momentum is always nearly completely absorbed by the barrier, but she can barely rupture the thing. So I guess her initial velocity from there is between 0-1 m/s?

I'm not sure how long that would take. Seven seconds sounds way too fast then, but eh.

Would she be falling too quickly for the water on the barriers to join the cushion at the bottom?

Each segment of fall should take a bit more than 2 seconds; it takes just a bit more than 2 seconds to fall 20 meters at standard Earth gravity, and then there will be a little additional time due to deceleration when hitting the water and bursting through the barriers. If the energy applied is just enough to break the barrier and effectively zero the girl's motion at that point, I would expect those falls to be fairly safe (if unpleasant to experience) depending on how yielding the rubber is. The last one at the end might be harmful but, I think, unlikely to be fatal, depending on how she hits the water - might result in broken bones in the foot or whatever other part makes first contact with the bottom.

[factotum]

The last one at the end might be harmful but, I think, unlikely to be fatal, depending on how she hits the water - might result in broken bones in the foot or whatever other part makes first contact with the bottom.

According to the calculation I just did, if we're ignoring air resistance then the girl hits the water at the bottom of the well travelling at around 44mph. That's pretty darned fast! I'm pretty sure she's coming out of this with at least broken bones regardless of how she hits the water.

To use another thought experiment: have a look at the high dive sometime. This is only from 10m, so half the height and about two-thirds of the impact velocity. If the diver goes into the water smoothly then they easily go down well more than 2m into it--if they hit badly, then they can injure themselves just from the impact with the water. So, she either injures herself from hitting the water, or from hitting the bottom of the well under the water.

Oh, and she'd probably pick up injuries at the rubber mats, too--if those are genuinely just strong enough to stop her nearly in her tracks, that means she has several rapid decelerations from 44mph to 0 to look forward to!

[Chen]

Hi there. To give some context- I thought this example up to explain a physics problem encountered in a story I'm writing. How realistic is the conclusion?

A teenage girl who is of average build and height, and weight at 73 kg, falls 80m straight down into a well.

However, at 20/40/60 meter depths, she strikes a shallow pool of water- 1 meter deep, which rests over a large rubber barrier. When she strikes the water filled barrier, it bursts, and she continues falling to the next barrier until the bottom.

At the bottom of the well is a 2 meter depth of water. Suppose that Air resistance is negligible.

Are these stipulations enough such that she could realistically survive the drop under these conditions, without severe injury?

(If needed, I mentioned in story that it took her 7 seconds to complete this fall- I didn't bother mentioning this bit in the event it were quite inaccurate in the calculation- but here it is if I'm mistaken.)

Without the rubber bits, it should take 4 seconds to fall 80m ignoring air resistance. If she loses all velocity due to the rubber things, she's using up 3 seconds for all 3, so 1 second each. She's falling at a speed of 20 m/s when she hits each rubber thing. She needs a net acceleration of 2g upwards to negate all her velocity in 1 second. That would be what, 3gs of upward force? For her 73 kg that would be 2.19 kN. So would be that 3 times, then the 20 m fall into the water. Seems unpleasant.

[Lvl 2 Expert]

A fall from 20 m into water is not anything to sneeze at, but it won't necessarily kill her. My own googling (https://physics.stackexchange.com/questions/9059/jumping-into-water) tells me that solid numbers are mostly behind paywalls, and mostly are statistical analysis on suicide, so not exactly a cheery read.

10 meters is roughly the highest height from which people do the belly flop into a shallow layer of water stunt. There's little you can really do wrong from this height. 20 meters is where professional cliff divers have an ambulance standing ready, you need to stick the landing. (People still do bridge jumps from this height, it's not that hard after all to jump off a thing feet down, but you need to know you're getting it right.) 60 is almost always fatal. There are people who catch a lucky wind current, and there's at least one kayaker who went down a waterfall this high (aim for the foam where all the bubbles come up, it's slightly softer because it's an air/water mixture), but generally you're dead.

73 kg is a pretty normal grown woman weight. The fall described in the opening post would be easier to survive for a younger teenager with less mass. But honestly I don't like our victim's chances here. From 20 meters you need to be able to land right, you can't so that in a meter of water. I think her best approach is if she cannonballs, taking into account the rubber bottom of each bucket the water might just not injure her badly but still slow her down enough. Then you need the rubber floor to give out a little slow, so that she really loses almost all of her speed between drops, if she doesn't she'll have even more speed than what you would normally have from 20 meters, and that's not nice. While the bucket is giving out she needs to reorient herself so that she can do a nice feet down head up cannonball again, no mean feat under the circumstances, especially if she's not a stunt performer or something who has done something like this before. The two meters depth at the bottom would normally not be enough, but it's better than the 1 meter buckets, so if she can survive those she can survive the landing at the bottom.

Most people don't have a feel for how strong and dangerous water can be, even if they know the numbers, so for the purpose of impressing your readers it can't hurt to exaggerate a little. This seems reasonable in that regard, if a bit of a far fetched situation. But realistically speaking that girl is probably going to be in a bad shape if not dead when she gets down. Although it could probably be done with exactly the right technique. The odds look better with a much lighter person, like a midget (a little person afflicted with proportionate dwarfism). Some of those have a nice sturdy full grown body and still only weigh about 10 kg and are a meter tall. That would help. Young teenagers also stand a reasonable chance physics wise, but you need one with good coordination and such, maybe a gymnastics girl.

TL;DR: Don't pick me. I'll go paddle a nice sort of safe waterfall somewhere.


Lol

Well, thanks for the insight, Grey. I think another part of the issue is, I'm a metric user- which is nice when calculating Physics, but then one is quick to forget just how big a drop 20 meters really is.

Actually, yeah. The well is probably closer to 100 feet down, then. Going by that, if I just use two barriers at 35 and 70 feet, she's far more likely to be okay. Plus, with the decelerations that keep happening, 7 seconds sounds a lot more sensible now to complete that fall.

Thanks again!

Yeah, that's way better. That's the sort of thing a crazy person might do for sport, if you can make the buckets reliable enough. And it still sound pretty impressive for a book.

[RickDaily12]

By the way, for even more context, since you guys keep bringing up the paywall examples of professional diver vs attempted suicider:

This victim is trying to take her own life, but not of her own free will.

And to the outside observer (the main character), he calculated a drop of 240 meters (free fall of 7 seconds), which leads everyone to presume that she died.

She didn't; she survived without injury, but this is me trying to come up with the answer of how. So yeah, 100 feet -> 35/70 ft rubber barriers seems to be the best way forward to explain this. Sure, she might have had an uncomfortable, even painful time falling through this, but the goal is having her believably walk out without injury under these circumstances. As long as I can believably say she didn't so much break or sprain anything, we're all good to go.

(Uh, also, this is major spoilers! :smalltongue: )

[Bucky]

Have you considered a parachute?

[jayem]

Another angle...
Each fall through air is 20 times the distance of each fall through water. If she were to just stop at each sheet, then that would require uniform deceleration from the water of 20g (this of course isn't what will happen). This is quite a significant force (especially up and down the spine).

a) The fluid resistance of water depends on the speed of the object (I think in the case on the square of the speed), the initial force will thus be much higher!
b) We've said she's stopped, she could still have some speed:
However if it's less than say 50% of the original speed then it's still going to be 15g.
And if it's more that say 50% the next fall is at a higher speed (in the 50% case equivalent to 25m) and so (at whatever fall she stops at) requires a greater deceleration (if there were only one trampoline then we're needing 12.5g at the base, with 4 it will be much worse). So we're stuck with one of the impacts being worse than 15g.
c) We have some interplay between the fluid and the rubber, however the rubber will have to been stretched by the static weight of the water already. I don't see it doing much.

[factotum]

Actually, that's a good point about the weight of the water. The rubber mat has to be capable of withstanding a weight of probably well over a ton (a cubic metre of water weighs 1 tonne, and a typical well is well over a metre across--this one especially, since if it's too narrow she's going to injure herself bouncing off the walls), presumably without bulging out like a balloon--is the addition of a 73kg woman travelling at 20m/s really going to be enough to break it?

[Brother Oni]

Is she falling head first or feet first?

There's a number of morbidity and mortality studies on falls at height which I looked up for another thread. Generally speaking feet first, there's slightly under a 50% chance of death (no stats on the injuries sustained), while if she's freefalling head first (as opposed to diving), it's quite likely to be fatal.

Even if the initial falls weren't fatal, if she's injured enough so that she can't stay afloat in that 3m of water, she'd going to drown regardless.

I also agree that 73kg is a bit heavy for a female teenager unless she's near fully grown or highly athletic - it's above the upper quartile range (ie top 25%) on most juvenile female weight/age graphs.

[RickDaily12]

A friendly reminder to the thread that the parameters of the fall have changed slightly. I'll update the OP to reflect this, but just to recap:

The pit is now 100 feet, not 80 meters. The two barriers are now placed at the 35/70 ft level depths, but otherwise the example remains as is. I'm concerned about the likelihood of any serious injury (which in this case would be sprain or breakage) If you estimate a greater than 50% chance of such injuries, please feel free to share- my understanding is that the much shorter distance fallen into water from here is far more likely to make it believable to survive unharmed.

In short, if I say "this girl survived her fall without serious injury" and the first words of your mouth aren't something like "wow, she is miraculously lucky", or "she should be a professional diver" then this example is a success in my opinion.

I'm not too concerned about the 73 kg value. It's close enough to the upper 25% mark, and unless a few kg lighter impacts the above desire to a large degree, it shouldn't affect the outcome too much. Such is googling "what is average weight for teenage girl" and then not cross-checking multiple sources, the most I can say is ¯\_(ツ)_/¯.

Drowning isn't a concern because of story reasons. As long as she survives the fall without any serious injury, this girl is in no actual danger.

Keep in mind, the last relevant context is worth noting is our victim did jump into the well with the intent of committing suicide. That said, I'm not really too concerned with flawless neck snap targeting on this topic. The way I've written the scene, she jumps in feet first. For the sake of considering body damage, merely assume she doesn't strike the rubber barriers in a way that she lands neck first.

[Kato]

My quick search (aka Wikipedia) suggests 20m is standard for high diving, so it should not be a problem from that pov but as people mentioned those rubber sheets with shallow water are not really the same. I feel like it would be hard to get proper numbers on this depending on the exact setup but I feel pessimistic to be honest. Reducing the height as mentioned is probably the easiest way to make this work. Make the well shorter and put in a barrier every 10m or so. Won't be pleasant but should be fine. Or just every 5 to be safe.

[RickDaily12]

Actually, that's a good point about the weight of the water. The rubber mat has to be capable of withstanding a weight of probably well over a ton (a cubic metre of water weighs 1 tonne, and a typical well is well over a metre across--this one especially, since if it's too narrow she's going to injure herself bouncing off the walls), presumably without bulging out like a balloon--is the addition of a 73kg woman travelling at 20m/s really going to be enough to break it?

I don't see why this bit needs to be a stipulation. Don't defy me to name a particular substance which foots this bill, but uh, I see nothing wrong with a bit of a bulge here. :smalltongue:

She hits the barrier, and stretches it further down as the water and rubber absorb most of the kinetic energy. She has too much, however- breaching the barrier and continuing the fall onto the next barrier, and so on.

The material is strong enough to carry this much water, but not to withstand in addition, a woman falling at ~14 m/s. Is this too unreasonable a concept?

[Kato]

The material is strong enough to carry this much water, but not to withstand in addition, a woman falling at ~14 m/s. Is this too unreasonable a concept?

I think the point was carrying that much water requires a rather sturdy sheet material but the addition of the person is quite minimal, even going at that speed. So the margin between "not being able to hold the water" and "not rupturing when jumped into" is quite small.

Short search suggests high divers experience forces around 2 to 3 good, so if we take a 100kg diver for easy numbers that gives us a force of 3000N=3kN. But just to hold 1m^3=1t of water the rubber needs to already hold 10kN. So far this seems reasonable to me but you would likely need more like... 60 to 100t of water for a large enough cushion to be safe. and then having it rip at an added 5 or 3% is much harder to measure. It is not unmanageable but it likely takes some trial and error to get the sheet just right.

[Brother Oni]

I think the point was carrying that much water requires a rather sturdy sheet material but the addition of the person is quite minimal, even going at that speed. So the margin between "not being able to hold the water" and "not rupturing when jumped into" is quite small.

You could potentially argue that as she's going in feet first, she'll make contact with the rubber sheet in a very small area, which may be enough pressure to tear it.

Assuming that she retains her form down through this barrier, it's likely that rupturing the second barrier would remove enough kinetic energy to make the remainder of the fall non-lethal.

The whole scenario reminds me of the ~60ft drop in the clock tower scene from the Jackie Chan movie Project A (link (https://www.youtube.com/watch?v=T0xIZpKC5b8)) - there's an outtake where he bounces off the second canvas awing instead of tearing through it and required immediate medical assistance. He suffered cervical spine damage, but was able to make a full recovery.

If you add water to each of the stages, plus make it impossible for her to miss the catch points, I'm inclined to think she would survive with no major injuries, at worst, a sprain or broken ankle/foot due to rupturing the rubber/canvas layers feet first.

[Lvl 2 Expert]

Just as a side track: if the goal is for her to jump into a well and for someone else to hear a splash so many seconds later that they figure the well was deep enough to kill her: could she use a ledge instead? Or a rope, or the walls of the well, or a plank wedged into small cuts she grinded into the wall beforehand, or anything easier to set up than a series of rubber buckets carrying a ton of water? She jumps in, grabs onto whatever, jumps again 4 seconds later. The well is 20 meters of air and below that 4 meters of water. Since it's a simple jump she sticks the landing no problem, and it's deep enough to give a realistic sound.

It's not as spectacular as the alternative, but way easier to create, and probably sounds more convincing than the triple splash to boot.

[Kato]

You could potentially argue that as she's going in feet first, she'll make contact with the rubber sheet in a very small area, which may be enough pressure to tear it.


Hm... But isn't the necessary assumption that she loses most of her velocity to the water or it would not serve much of a purpose? So it would mostly be spread out or we can skip the water and go for just the sheet. Of course there is a middle ground somewhere but finding it seems not trivial either.

[Chen]

In short, if I say "this girl survived her fall without serious injury" and the first words of your mouth aren't something like "wow, she is miraculously lucky", or "she should be a professional diver" then this example is a success in my opinion.

Honestly if the fall involves hitting intermediary barriers and them breaking to reduce velocity, I'm not believing someone falls through without serious injury. It's too far a fall and reducing velocity in multiple steps, while better than all at once, still requires large force impacts. There will definitely be injuries. I could believe the person survived though.

[factotum]

Honestly if the fall involves hitting intermediary barriers and them breaking to reduce velocity, I'm not believing someone falls through without serious injury.

It does depend how *many* barriers, though. Nicholas Alkemade, a tail gunner in a Lancaster bomber during WW2, survived an 18,000 foot fall without a parachute and with no injury worse than a sprained ankle. How? He fell through the branches of a pine tree and into the snow beneath, with the snapping of the thin branches slowing him down and the snow providing a nice soft surface for him to land on. If you put something similar at the bottom of the well rather than having just one or two slowing events then the woman's survival would be far more believable.

[Brother Oni]

Hm... But isn't the necessary assumption that she loses most of her velocity to the water or it would not serve much of a purpose? So it would mostly be spread out or we can skip the water and go for just the sheet. Of course there is a middle ground somewhere but finding it seems not trivial either.

From the initial leap and considering her aerodynamic profile, the first water layer at 1m deep will do very little and she may as well be jumping straight onto the rubber. The subsequent water layers would be much more effective in slowing her fall.

For an idea of how little water will do to slow her fall for the first stage, an olympic 10 m diving platform has a minimum safety depth of 5 m, and as far as I can tell from the footage, the rate of deceleration isn't linear.


Here's how I see the course of events - she finds a well, goes 'Goodbye cruel world' and jumps.

She goes through the first water layer like a hot knife through butter, and tears through the supporting rubber, possibly spraining or fracturing a foot/ankle.

This is incidental however as since she wasn't expecting to hit something so soon, surprise causes her legs to buckle, resulting in her hitting the second layer in a sitting position. Since she now has a larger aerodynamic profile, this causes a more significant slow down by the water, and she barely tears through the next layer. which is additionally cushioned by her posture.

This leaves a simple drop to the final layer which is now more than deep enough to catch her safely.
Assuming she's not completely stunned by the impacts and survives the disorientation/crushing effects of an additional 2 tonnes of water joining her at the bottom of the well, she floats to the surface and wonders 'WTF?!'.

[Chen]

It does depend how *many* barriers, though. Nicholas Alkemade, a tail gunner in a Lancaster bomber during WW2, survived an 18,000 foot fall without a parachute and with no injury worse than a sprained ankle. How? He fell through the branches of a pine tree and into the snow beneath, with the snapping of the thin branches slowing him down and the snow providing a nice soft surface for him to land on. If you put something similar at the bottom of the well rather than having just one or two slowing events then the woman's survival would be far more believable.

Exceptional circumstances I can certainly believe. Repeat that a number of times and I bet you, more often than not, there is some serious injury. I'm not even arguing that she may survive but to survive without significant injury? Doubtful in most cases.

[jayem]

I don't see why this bit needs to be a stipulation. Don't defy me to name a particular substance which foots this bill, but uh, I see nothing wrong with a bit of a bulge here. :smalltongue:

She hits the barrier, and stretches it further down as the water and rubber absorb most of the kinetic energy. She has too much, however- breaching the barrier and continuing the fall onto the next barrier, and so on.

The material is strong enough to carry this much water, but not to withstand in addition, a woman falling at ~14 m/s. Is this too unreasonable a concept?
I don't think that's too unreasonable that it breaks, it does need planning, however the material won't stretch much further (assumming it's hookian-which of course it won't be because it's about to break).
Then if 10,000N stretches it say 1m, then the static case 1,000kN from her weight will only stretch it an additional 10cm. By Newton's 3rd law any additional force she puts on the material to extend it further has to have an opposite force through her.

Also I think you hear the series of 4 splashes from far too soon to far to late, which might be a bit odd.
Silly thought, can you get the first rubber sheet to fail as she is half way too it? And if so what is the effect on that on the situation.
It seems to me that her relative speed to the water will be much less [or at least if it started higher so she hit it at the same point], and it will have a bit longer too decelerate.

[factotum]

Exceptional circumstances I can certainly believe. Repeat that a number of times and I bet you, more often than not, there is some serious injury. I'm not even arguing that she may survive but to survive without significant injury? Doubtful in most cases.

It doesn't have to be something that will work for every single person who jumps in the well, it just has to be something that is believable enough to work for some people--this is a story, not a science experiment. So, if the entire well is empty apart from ten thin rubber sheets 1 foot apart at the bottom (and some water, presumably), I would believe that has a better chance of slowing the woman's fall to survivable speeds than having just two braking devices at one-third and two-thirds distance down.

[Bucky]

Or you could skip the layering and have her fall into much higher than expected water 20 meters down. Which she sinks into and then deliberately drains somehow to avoid drowning.

If she runs into water at 20 meters, everything below that is redundant.

[RickDaily12]

Or you could skip the layering and have her fall into much higher than expected water 20 meters down. Which she sinks into and then deliberately drains somehow to avoid drowning.

If she runs into water at 20 meters, everything below that is redundant.

The problem here is that she clearly does not sound to have fallen far enough where death is an assumption to concentrated observers. The only reason the main character presumes her to be dead is because he doesn't know she isn't actually free falling- a free fall lasting more than 4 seconds all but guarantees death.

As was discussed earlier, a 20 meter free fall is nothing to sneeze at, surely something that causes bodily harm, but characters would not give up on our victim here if it didn't sound like she fell a long way down.



It doesn't have to be something that will work for every single person who jumps in the well, it just has to be something that is believable enough to work for some people--this is a story, not a science experiment. So, if the entire well is empty apart from ten thin rubber sheets 1 foot apart at the bottom (and some water, presumably), I would believe that has a better chance of slowing the woman's fall to survivable speeds than having just two braking devices at one-third and two-thirds distance down.

This shouldn't be too difficult. The only thing that needs considering here is a well is supposed to look like a well. There are points that are canon in the story where such a deep well appears to actually be filled entirely with water (rather, it's the water on top of a barrier) as people end up using it to fill buckets of water. It's already a written plot device, so I can't really back down from here. I can certainly say she plunged the first 10 or so meters before striking a barrier, but that first barrier needs a decent amount of water and needs to be a decent distance down.

Perhaps from there, she tears through a bunch of thinner barriers to keep her deceleration under control. They'll be too thin to lower her velocity too much, but perhaps thick enough to prevent further acceleration until a second barrier?

If the final impact is of concern, I can just say 3 meters at the bottom and that will surely be enough.


The other thing worth considering- the sound of water-filled barriers making a sound- I might be able to gimmick my way out of this. A bunch of other characters are screaming at the top of their lungs while this is happening (of course) so it might not be too far fetched that they're too busy screaming to notice the multiple sounds, as well as the main character who has to filter out the background noise to notice multiple sounds as well.

[factotum]

The problem here is that she clearly does not sound to have fallen far enough where death is an assumption to concentrated observers.
.
.
.
There are points that are canon in the story where such a deep well appears to actually be filled entirely with water (rather, it's the water on top of a barrier) as people end up using it to fill buckets of water.

You have a contradiction here. If the well is full up to near the top then people should be expecting the splash to come quite soon, but you're saying that the splash has to come really late to prove she fell far enough to be killed. Which is it?

[Knaight]

It's a well. She could easily be slowed by just hitting the sides a lot, clothing getting caught temporarily and causing slowdowns, etc. - there's no particular need for some sort of weird elaborate water layering system.

[RickDaily12]

You have a contradiction here. If the well is full up to near the top then people should be expecting the splash to come quite soon, but you're saying that the splash has to come really late to prove she fell far enough to be killed. Which is it?

From the first chapter to this point in the story, people who have used the well noticed a depth of about 10 meters (the location of the first barrier).

But when she falls into the well, they notice a drop of 7 seconds. Then they peer over the well into a dark black void.

Strange things have already happened in this world (another massive spoiler, it's actually a virtual creation), but what happens the very next chapter after the Main Character times her fall, is he is about to note how on the night prior, he thought he noticed a higher level of water of the well the night before, and he's noticing a high level of water now. But when she takes her life, he sees no visible water.

This well has always been 100 feet deep, but because the rubber barrier has always been there unknowingly, people have thought until now that this well has always been filled relatively high.

(This also has to do with why the girl drowning at the bottom is not a worry.)


It's a well. She could easily be slowed by just hitting the sides a lot, clothing getting caught temporarily and causing slowdowns, etc. - there's no particular need for some sort of weird elaborate water layering system.
This does nothing to help the girl survive a 100 ft drop straight down.

The layering system is there to disguise how deep the well goes down, while allowing a way to masquerade someone's death. Trust me- this example is already a headache, which is why I'm here, but I wouldn't be here if the problem of how to keep her alive through a 100 ft fall under these conditions weren't complicated.

You guys have already helped me greatly refine my big reveal of this "death", for that I'm grateful, but yes, typically dropping into a deep well is fatal without some kind of interruption in the fall. Walls are not enough.

[factotum]

But when she falls into the well, they notice a drop of 7 seconds. Then they peer over the well into a dark black void.


But that doesn't work? The first barrier is covered in water, so they should hear a splash after a couple of seconds. What should be the surprise is that they then hear two more splashes a couple of seconds apart. Also, I'm not sure that you would actually be able to see the water 30 feet down in the well, unless a light source is shining right down into it--it should be a dark void before and after.

[Bucky]

If the timing's the only important observable to the people above, she can get quietly snagged on something after 3 meters, then lose her grip after a few seconds. She falls 10-20 meters into water, but it sounds like she fell a lot farther.

[RickDaily12]

But that doesn't work? The first barrier is covered in water, so they should hear a splash after a couple of seconds. What should be the surprise is that they then hear two more splashes a couple of seconds apart.

The other thing worth considering- the sound of water-filled barriers making a sound- I might be able to gimmick my way out of this. A bunch of other characters are screaming at the top of their lungs while this is happening (of course) so it might not be too far fetched that they're too busy screaming to notice the multiple sounds, as well as the main character who has to filter out the background noise to notice multiple sounds as well.

Meh. Not my best defense, but it's the card I've dealt myself. =/


Also, I'm not sure that you would actually be able to see the water 30 feet down in the well, unless a light source is shining right down into it--it should be a dark void before and after.
This I'm less concerned about.

When characters gaze over the well, it's usually at Night and a full moon hangs overhead. If that's insufficient, then I guess I've earned myself another meh xD




If the timing's the only important observable to the people above, she can get quietly snagged on something after 3 meters, then lose her grip after a few seconds. She falls 10-20 meters into water, but it sounds like she fell a lot farther.
Putting aside for a quick second the logistics of what she could get snagged on after 3 meters of falling, the actual problem is the distance still needing falling afterward.

There's something at the bottom she needs going to.


EDIT x2: Besides which, it's important to remember that the other problem is the intent of the well. The person who set up this well illusional death is going to be banking on the appearance of death. A 20 meter fall into water is not obviously fatal, and you're using the equal of a VERY haphazard catching device. It's not going to be reliable for a stunt, least of all on someone trying to kill themselves.

I find my idea less farfetched than this idea, with all due respect. =/

[Lvl 2 Expert]

I find my idea less farfetched than this idea, with all due respect. =/

I don't, with all due respect.

Your idea is that the villagers know the well is 30 meters deep, except sometimes it's suddenly only one meter deep and they can just scoop the water up. Currently the water level sits at ten meters, as confirmed by mr Henderson who went to get water for his chickens this morning. Then someone jumps in, bystanders hear a lot of water noises, but the final one is after enough seconds for her to have fallen 60 meters, so the bystanders conclude she must have fallen 60 meters into this 30 meter deep well that was actually only 10 meters deep this morning, and she must have died. She didn't die though, despite several tons of water smacking down on her at several points during the fall. All of this happened because the person making the jump spend several months experimenting with forces on rubber sheets sealing off a well watertight despite several tons of water sitting on top of them. She did this so she could fake suicide into a well that just happens to contain something important at the bottom that she didn't actually know about, and which probably will also provide her a way out of the well, a thing she didn't think about during her months of preparation. Is that about right or am I missing something?

The alternative is the people don't really care how deep the well is, it's just deep. The person jumping used the rope and bucket to measure it, and comes out somewhere between 20 and 30 meters, dangerous but survivable if you stick the landing. She grabs on to something, possibly something she put there herself beforehand, for a few seconds to make her fall sound longer in a way that doesn't involve people also hearing several tons of water crashing into the bottom of the well. People conclude from how long that fall was that it was far enough to kill her or if they do know the depth of the well that she must have hit the wall a lot on her way down. They scream after her but get no response. Someone suggests "she could just be unconscious, lower me down!" but they're told that if she landed in the water unconscious she'll drown before the rescuer gets there, making him risk his life for nothing (well, for a water source that doesn't have a rotting corpse in it, but otherwise for nothing).

It's still a bit farfetched, but if you want the book to be more realistic than that honestly I'd scrap the well scene entirely. It's never fun to give up your babies, the things you've been thinking off as the core of the experience you've been crafting for months now, but if the rest of the book is realistic enough that falling distance into water is a concern than the whole rubber sheet construction is probably just going to break people's immersion. (If you'll excuse the pun.)

If you want to keep the scene in some broader water related sense I'm willing to try and help if you want that. You can PM me any details you don't want to spoil publicly and I can try to see what I would think are the most realistic sounding scenario's. I'm not a physicist, but I am a whitewater kayak instructor. In a river for instance you might be able to come up with a current that pulls her under a rock and then deposits here in some natural tunnel, depending on the environment an old magma tunnel or a limestone cave, or some underground tributary river that has barely any water this season (not spring). People waiting for her to come out the other side are going into a panic and conclude she got caught on something and is a lost cause when three minutes of angling don't give any results. That's a perfectly reasonable response when dealing with sieves and siphons. You're not jumping in after a corpse. The tricky bit is making the whole thing intentional, that she knew she was making an escape, not actually committing suicide, but that's where you as a writer might have some great ideas.

Good luck with the project any way you decide to handle it, but do try to think through the concerns other people have. Your rubber bucket construction is very clever, but it has some clear downsides that I think make it less suitable for the situation. Mostly that it's not sneaky at all, it takes a lot of time to prepare, leaves a lot of visible evidence and makes a lot of noise in a situation where you want people to be able to hear a single person falling into water.

EDIT: O, I just now caught on that she is actually trying to kill herself, and someone else saves her without other people knowing about it. That only makes the bucket solution less suitable in my opinion. She'll need to get that right, too much things to hurt your head on in a construction like that to just fall like a ragdoll.

[factotum]

EDIT: O, I just now caught on that she is actually trying to kill herself, and someone else saves her without other people knowing about it. That only makes the bucket solution less suitable in my opinion. She'll need to get that right, too much things to hurt your head on in a construction like that to just fall like a ragdoll.

It's unclear on whether this mysterious third party is actually actively watching for her to throw herself into the well. Seems to me that if they *are* they could have some sort of active mechanism in place to save her--for example, have a net pop out of the well wall and catch her. The only difficult bit then is arranging for the big splash, but I'm sure there are things other than this woman who could be dropped into the water at the bottom. That, to me, would be far more believable than this entirely passive method.

[Brother Oni]

But that doesn't work? The first barrier is covered in water, so they should hear a splash after a couple of seconds. What should be the surprise is that they then hear two more splashes a couple of seconds apart. Also, I'm not sure that you would actually be able to see the water 30 feet down in the well, unless a light source is shining right down into it--it should be a dark void before and after.

I'm not sure you would hear splashes 2 and 3 as the sound would have to travel through the bodies of water above them. You would hear the water bodies from layer 1 and 2 impacting with the bottom however.

[Cespenar]

Make the first barrier just an elastic barrier that deforms and ruptures, without any water. Then she falls down to the actual water.

Then you bypass both the "two sounds" issue and the "the barrier should carry 1 ton of water but rupture at the small addition of the girl" issue. Also, it's much easier to install than the two layered pool idea.

[Lord Torath]

Make the first barrier just an elastic barrier that deforms and ruptures, without any water. Then she falls down to the actual water.

Then you bypass both the "two sounds" issue and the "the barrier should carry 1 ton of water but rupture at the small addition of the girl" issue. Also, it's much easier to install than the two layered pool idea.Except then you can't have people filling their buckets at the well, because there's an elastic barrier between them and the water...

[Cespenar]

Except then you can't have people filling their buckets at the well, because there's an elastic barrier between them and the water...

Yep, turns out that's a thing not included in the OP. Anyway then, toodaloo.

[crayzz]

There are materials that deform when a certain amount of stress is applied slowly, but break when the same amount is applied quickly (actually, most materials are like this, it's a matter of degrees). Polymeric materials tend to be like this, since their molecular structure has more freedom to rearrange, but it takes time. Plus, water is really good at transmitting pressure, so a lot of the impact force will be transmitted directly to the rubber.

A stiff elastic near-ish to it's breaking point failing under a (somewhat) sizeable sudden shock isn't that unscientific. It's a little hand-wavy/contrived, but I'd go along with it without complaint.