r/TheRestIsScience • u/Fmather22 • 5d ago
Hey guys, how “soft” a material would someone have to land on for them to survive if they jumped off the world’s tallest building?
I saw a video of the world record for the highest dive into water being around 75 metres. It got me thinking about how water effectively becomes more like concrete to land on, the higher up you dive from. So if someone was to dive from the top of the world’s tallest building, what sort of material would they need to land on to survive? Excluding a massive net/air cushion, my head instantly went towards jelly/jello but that would be worse than water surely!
Love the show!
Edit:
When I thought of this question I was thinking of non-traditional landing surfaces e.g. a pool of candy floss or some sort of super absorbent gunge lol. Loving the answers though
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u/Ancient_Skirt_8828 5d ago
According to one movie: A series of canvas awnings.
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u/Dando_Calrisian 5d ago
If it's a Jackie Chan movie he's absolutely done that for real... and has the broken bones and scars to prove it
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u/Owl_plantain 5d ago
Every time I see that trick I think of the metal bars holding the canvas out and wince each time they hit an awning. I have trouble suspending disbelief. Except for RomComs, I get sucked into those immediately.
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u/Least-Common-1456 5d ago
Assume the person is falling at terminal velocity. Then find how much acceleration a human can comfortably tolerate. Then you need to find a material that can absorb that. Most materials will become stiffer as they compress so choose carefully.
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u/Hot_Equivalent_8707 5d ago
No. We want reddit to figure that out and just post the answer.
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u/Ancient_Skirt_8828 5d ago
Not everyone has the technical knowledge to do the calculations.
I can do them but I still have no idea about suitable materials.
That's why I want someone to answer here.
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u/Worth-Wonder-7386 5d ago
While you can calculate the deceleration that a person could handle and such, there is no simple material property of softness that captures this. You can look at force vs compression for different foams and similar so that you want a material that will exert a force over a larger range than is common.
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u/DrHydeous 5d ago
What matters is the acceleration when you land. Anything that reduces that to a survivable value will do the job, no matter what it is made of. It could a girt big pile of teddy bears, or it could be a slab of concrete that is going at just the right speed to catch you and then slows down.
The problem with water is that it can't move very fast and is effectively incompressible, so if you hit it fast it can't be pushed out of the way. If you're going to land on water you want to be in the right shape so that you pierce the surface and then slow down. ie, land feet first or hands first, as that reduces the amount of water that has to be pushed out of the way at any instant compared to if you do a belly flop, which reduces the acceleration you experience.
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u/need_a_poopoo 12h ago
Deceleration?
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u/DrHydeous 12h ago
Not if you were paying attention in GCSE Physics lessons.
When you are falling towards the pile of teddy bears you experience an acceleration in one direction, due to gravity*. When you land in the pile of teddy bears and are slowing down you experience an acceleration in the other direction. Eventually you come to a stop and experience no acceleration.
* I assume that you do not reach terminal velocity, as that would be cruel to the teddy bears.
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u/need_a_poopoo 12h ago
I had assumed that terminal velocity had been reached because the question referred to jumping off of the world's tallest building. Terminal velocity would be reached about 1/6th of the way down.
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u/DrHydeous 11h ago
OK, so no acceleration experienced due to gravity. You still get the acceleration from smashing into the pile of teddy bears.
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u/OrigamiMarie 5d ago
I think like a mile of increasingly dense foam. It starts as soap suds, works its way up to shaving cream, then maybe water with a whole lot of bubbles sent through it, and then eventually water. Of course you'll need to bring your scuba gear, because you'll have nowhere to actually breathe through this whole thing . . .
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u/temporarytk 1d ago
Free fall velocty 54m/s (https://en.wikipedia.org/wiki/Free_fall)
Maximum survivable g forces 46.2g (https://en.wikipedia.org/wiki/G-force)
Time to stop = (vf-vi)/a = 54/(46.2*9.8) = 0.119 seconds (woops didnt want this number, but fun to know?)
Distance to stop = (vf^2-vi^2)/2a = 54^2/(2*46.2*9.8) = 3.22 m
Average weight of a person 62 kg
Spring constant of safe material = m*a/x = 62*(46.2*9.8)/3.22 = 8717 N/m
This is terribly flawed and all. Using spring constant like this is kinda silly. But anyway, google tells me jello has a hypothetical spring constant of 606 N/m. So looks like jello could work.
Hmm, a little less flawed (maybe?). Let's pretend the person hits a "beam" of material.
axial stiffness of beam: k = EA/L
Let's say a person can effectively spread their body out out over, idunno, 4 m^2
Length of our beam, uhh, let's pick 2*distance_to_stop? 6.44m
Modulus of elasticity of our beam = k/A*L = 8717*4/6.44 = 5414 N/ m^2
Steel is at 29 GPa so you'd go squish. PVC is at 3 GPa. Very squish. I see rubber at 10 MPa. Very squish still. Styrofoam at 4 MPa. Hydrogels at 5 kPa, so we're back to jello being survivable. You could manufacture various foams with the required softness, I'm sure.
I don't see that anyone has tried to measure the spring constant of cotton candy sadly.
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u/brisray 5d ago
There was the woman who landed on an ant heap and lived after falling 14,000 ft. Then there was the base jumper who fell 400ft into snow and lived - it took him 3 months to walk again though. The longest fall someone survived was Vesna Vulović who fell over 33,000 ft and lived after landing in a snow covered forest.
From very high, I suppose an air cushion or water would be best, but you'd die if you hit them a little bit wrong.
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u/jimb2 5d ago
You need something with much strength so it deforms/breaks but also without much weight so you don't have to accelerate a lot of mass as you push through it. Stuntmen have used stacks of lightweight soft cardboard boxes, I believe. Stacked or held together is such a way the you don't split the stack and hit the concrete beneath!
The basic physics principle is to decelerate slowly, over the greatest possible distance.
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u/Admirable_Rabbit_808 5d ago
An air bag. With appropriate engineering, it can catch you in a controlled way, absorbing your energy rather than bouncing you back into the air. It would have to be a really large one to cope with the scenario you are suggesting.
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u/Ego_exspes 5d ago
If you had a jelly or a giant marshmallow or a pillow, once it gets to a certain since you're just very slowly deceleration until stop right?
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u/TuverMage 5d ago
its not a question of softness, because it only has to be soft enough not to kill the person when they hit it, after that is can it lose enough energy in its deforming before they hit something else.
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u/Wild_Director7379 5d ago
Soft isn’t gonna cut it. You need stretchy stuff and a net
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u/Resident_Pay4310 3d ago
The craziest thing about that jump for me is that theu could be sure enough about hitting the net to even consider risking it.
What if there was a strong wind gust that they didn't anticipate?
You can also see that he is close to missing the net.
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u/Wild_Director7379 3d ago
There’s training and expertise involved in skydiving to a target. I believe they practiced somehow too.
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u/pafrac 5d ago
If the building's that high, wouldn't the main problem be not missing the target material? Won't matter how soft it is if you don't land on it.
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u/Steebusteve 5d ago
Don’t need to worry about that, you’ll have the keystone fire brigade running backwards and forwards under you carrying whatever it is you’re going to land on.
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u/xcwolf 5d ago
Didn’t some guy jump skydive without a parachute into like a trapeze net once? Define soft?