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For a fantasy race, or for bio-augmented humans, how durable would they have to be to walk away from a 1000ft free fall unscathed? And how would this logically translate to other forms of durability?
Would it make gunfire from calibers like 9mm or .45 entirely ineffectual? What about more powerful rounds like .308 or even .50cal?
Would the force from a bullet that powerful be enough to crack the skull of someone able to survive landing on their head if pushed off a skyscraper?
People have survived terminal velocity falls. In 1972, Vesna Vulović fell over 33,330 ft without a parachute after the plane she was in exploded. She didn't exactly walk away from the fall, however. She spent days in a coma, and was hospitalized for months after that. But she did survive.
Vulović is not the only one to survive a fall that should have killed them. They all had varying degrees of injury, so perhaps it's not quite accurate to say that normal human durability is all that's required if you want to walk away afterwards. But the point is that there are a great many factors involved in surviving a fall, and the height and the body's structure are only two of them.
That said, here are some other suggestions of the types of people who would fall well:
From On Being the Right Size by J. B. S. Haldane:
To the mouse and any smaller animal [gravity] presents practically no dangers. You can drop a mouse down a thousand-yard mine shaft; and, on arriving at the bottom, it gets a slight shock and walks away, provided that the ground is fairly soft. A rat is killed, a man is broken, a horse splashes. For the resistance presented to movement by the air is proportional to the surface of the moving object. Divide an animal’s length, breadth, and height each by ten; its weight is reduced to a thousandth, but its surface only to a hundredth. So the resistance to falling in the case of the small animal is relatively ten times greater than the driving force.
Terminal velocity is a function of (among other things) weight. The lighter you are, the lower the velocity at which you hit the ground, and the easier it is for you to survive.
Wingsuit flying is a reasonably popular sport where flyers skydive in a suit with "wings" between the arms and body, and between the legs. These jumps typically end with a parachute, but in 2012, Gary Connery landed a 2,400 ft jump without a parachute, landing on a "runway" of cardboard boxes.
Also, having wings gives you more ability to steer towards softer ground, which is always a plus.
Bullets kill by piercing - by applying a large amount of force in a small area, they are able to penetrate the protective layers of our bodies and apply that force to the vulnerable parts of our bodies. That's why bulletproof materials function by preventing the piercing effect and distributing the force over a larger area.
That sort of protection is not going to help against a fall, where the entire area of your body is experiencing the massive forces involved. Conversely, the cushioning and strengthening of the enhancements needed to protect against a fall are not going to do much to stop the piercing effects of a bullet.
"They all had varying degrees of injury, so perhaps it's not quite accurate to say that normal human durability is all that's required if you want to walk away afterwards." One of them, Nicholas Alkemade, only suffered a sprained leg. That might make walking away unpleasant, but you can do it.
$\endgroup$
Bullet resistance and falling resistance are different from each other. A bullet that impacts some pierce-resistant skin will cause a shockwave to propogate through the body, which will bruise the flesh underneath and potentially cause bloodvessles to break and organs to be damage enough to die off. If you fall from a great height (or are in a carcrash, or have your body accelerated by a car/truck/whatever hitting you), all your organs will suddenly decellerate (or accelerate) and need to be slowed down over as long a distance as possible without ripping the nerves and bloodvessles.
Against bullet, extra fat would help as it would increase the distance the shockwave needs to propagate through before it reaches something vital. Against falling extra fat would mean more velocity for all your organs counteracting the extra distance the organs can now slow down over and increasing the chance the organs will rip out of their place, it's going to kill you faster!
One thing missing in @ArcanistLupus his answer about falling (https://en.wikipedia.org/wiki/Free_fall#Surviving_falls) is that almost all these falls had something break their fall slightly. They didn't really hit the ground with terminal velocity even though they reached terminal velocity at some point in their fall. It's like saying a parachutist survived a terminal velocity fall when he hit the ground.
This leaves 3 answers:
Make the person lighter without making them smaller. How much I can't say, but this will reduce their terminal velocity and increase their chance of survival.
Increase the person's surface area. This does mean he's going to need something along the lines of a parachute of surface area somewhere on his body to survive a fall.
Redesign their internal organs to have more room for the nerves and bloodvessles to follow the organs during the crash. Additionally the organs would need to be suspended with ligaments to allow them to move more and slow down over a larger distance. Possibly the organs could be molded into a bunch of smaller pieces that will each individually be slowed down. This will likely not be enough alone to help you survive.
as a last option: All of the above.
For a few other options check this similar question I made: Creating a scientifically semi-valid super-soldier, part 3: Physical shock resistance
Cats frequently survives high falls. How durable are cats?
The cat example suggest it is not the durability (because I guess cats are quite similar to humans in this respect) but how you handle the fall. I would imagine an experienced parachute jumper that had trained on landing without parachute would have quite good chances of surviving (but seriously injured).
You d have to look like this
This is a simulation of how you would need to be to resist 100mph car crashes. Free fall is essentially the same problem
The simple answer is no - the ability to survive a fall in no way implies the ability to shrug off a bullet. The answer provided by Arcanist Lupus is correct, but I think it can be simplified considerably.
What you have to realize is twofold: bullets travel much faster than human terminal velocity, and they provide a much smaller impact area. The two combine to make an enormous difference.
Human terminal velocity is not a precise number, but for adults at sea level it runs from about 120 mph (with the body horizontal) to about 200 mph (with the body vertical), or something like 180 to 300 fps. In fact, the vertical position is more survivable than the horizontal, since the legs provide a shock absorber effect, as well as what is called in automobiles a "crumple zone".
Furthermore, all of the energy dissipated in the landing is done so over the entire cross-sectional area of the body, which for a vertical position will be on the order of one to two square feet.
Bullets, on the other hand, travel roughly 4 to 10 times faster than terminal velocity (1000 fps to 3000 fps, roughly), and have an impact absorption area on the order of 2000 times smaller.
So a bullet, on impacting a body, will simply punch through flesh, and while it will eventually be absorbed, this will not happen until it has done localized damage far in excess of the amount done by a fall.
okay so terminal velocity and being bullet proof have nothing to do with each other. the denser you are the more you weigh which increases your inertia. higher density could theoretically make you bullet proof but would increase your inertia, therefore a fall would be likely to be more lethal. higher bone density might help but the impact on organs would be worse. the lighter an object is the less gravity attracts it so the two problems would compound each other. maybe make them bullet proof with technology that helps them fly, or put some really good parachutes on them. (side note most readers would simply accept the being able to fall from heights if they were bullet proof so I would just ignore the lack of explanation on the former)
The first thing to know is that fall-proof and bulletproof are largely unrelated. Sure, if you put enough "durability" on anything, you'll get both, but if you want to optimize your modifications, I'd focus on one or the other.
Optimizing for Bullets
Bulletproofing is fairly straightforward, as it largely comes down to three basic ideas:
The first and second are reasonably interchangeable, where more capacity to distribute force largely obviates absorption capacity, and vice versa. If you can distribute the force of a bullet from an area less than a square centimeter in size (bullet cross-sectional area) to around 2,000 square centimeters (approximate torso cross-sectional area), you've reduced the kinetic energy applied per unit area by 2,000. If instead, you put a wall of concrete in front of you, that doesn't do much as far as distributing force, but it does wonders at absorbing kinetic energy through plastic deformation.
Preventing penetration is largely about putting fibrous materials that will "catch" a bullet before they enter the body, and these same types of fibrous materials (Kevlar, for example) tend to do distribution fairly well.
So some fibers for distribution and prevention, and some ceramics or other plastically deformables for absorption is the typical solution. Note that out of the fibers and deformables, the deformables will help somewhat with fall resistance, but the fibers will not. Which brings us to...
Optimizing for Falls
The first question to ask is what it is specifically that typically kills people in high-altitude falls. I'd imagine it's breaking of the spine, and the bones of the ribs being pushed through the organs. What causes both of these problems is the same two issues: things bending like they aren't supposed to, and jerk being applied to the body.
To stop things from bending, you can include a frame of some sort, either directly to the spine or to a frame outside the body. To stop the jerk is trickier, and comes down to either reducing the force (either absorption like with the bullets or decreasing speed of fall) or slowing down the impact (think a giant pillow, gently bringing you to rest upon impact). Here you have a few options:
Here your creativity is the limit. Plenty of real-life creatures can survive the force of an impact at terminal velocity. For example, most insects cannot be killed by falls from any height, because as you scale down objects, they tend to handle impacts far better. A human-sized ant would be only slightly more fall-proof than a human-sized human. So small size is your friend.
You can also add something like a biological parachute or wing, which can slow you down just before landing. Of course, this isn't as "cool" to a reader as an enhanced human who manages superhero landings.
Another interesting option is crumple zones, areas of your enhanced human specifically designed to smash, absorbing the force of the landing, perhaps quickly healing/regrowing after smashing. You can also try to capture the kinetic energy of the landing, with some electromagnetic damping or something similar "capturing" the force and generating electricity, glucose, fuel, or something else that can be used later.
Finally, cushioning. This one is tricky, because generally you need a lot of it. Crash pads for bouldering (low-altitude rock climbing) are typically around 13cm (5in) thick, and that's just for heights below 6m (20ft). From that height, you'd only be able to reach around 33km/h (20mph) at maximum, disregarding air resistance, well below the estimated 195km/h (122mph) terminal velocity of a skydiver. Basic physics dictates that kinetic energy, the real problem with falling, grows with the square of velocity. This means that if a 13cm (5in) mat can handle a fall at 33km/h (20mph), a fall of 195km/h (122mph) will pack not 6 times the punch, but 36 times the punch. If required mat thickness scales linearly with velocity (which it doesn't, it's probably even worse than that, so this is probably a best-case scenario), that means you'd need 78cm (31in) of pad to take an impact at terminal velocity. Again, this is only a best-case scenario, so you'd likely need much more than that, and 78cm (31in) is already a fairly prohibitively large amount of padding to add to your enhanced humans.
Conclusion
If you want to stop bullets, add fibers and plastically deformables. If you want to stop falls, slow down the fall, add plastically deformables, or slow down the impact with biological padding. Let me know what you come up with! As far as creativity goes, the sky's the limit. Then again, that's sort of the point :)
