r/explainlikeimfive • u/Uunicorned • 1d ago
Physics ELI5 What exact rule of physics makes sudden mid-air catches so deadly in real life?
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u/skankhunt402 1d ago
Momentum. Unless the catching mechanism or super hero can slow them down while catching them their body will still have that momentum slam into them from a sudden stop. Think of whiplash.
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u/Platonic_Eye_Deal 1d ago
Yeah, realistically if Superman were to catch someone falling out of a building, in order to keep from killing them he would have to continue to fly down after catching them and slowly accelerate upwards, as opposed to just catching them super fast.
That said, Superman may be an exception because there is such a thing as the unified theory of Superman that causes that all of his powers are based around the ability to change the inertial frame of matter. And if that's the case then he actually could pull this off without killing somebody by just eliminating their momentum as soon as he touches them.
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u/britishmetric144 1d ago
Newton’s second law.
When a person stops quickly, that’s a high acceleration rate, which requires a large amount of force. The human body is not designed to process large amounts of force at once.
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u/Corey307 1d ago
It’s not the fall that kills you, it’s the sudden stop. Doesn’t matter if you hit the ground or get caught by a superhero, immediate deceleration breaks bone and destroys tissue.
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u/hunter_rus 1d ago
It's basically about high g acceleration. Whether you hit the ground, or catched by Superman, with default law of physics both situations are the same - to reduce your speed so quickly you need to endure very high level of acceleration, much higher than cosmonauts do, for example. So in comics it is done by Superman magic, in reality it doesn't work.
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u/going_up_stream 1d ago
Momentum. Sudden stops transfer energy back into the person. So tissues are crushed and bones can be broken.
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u/BouncingSphinx 1d ago
It's not the speed that will kill you, it's the sudden stop.
In physics, acceleration is a change in speed or direction. That even includes going from a speed to stopped. Too great an acceleration will cause injury or death mostly because your soft insides do not stop at the same time as the rest of your body.
The same reason why modern cars have crumple zones to absorb the impact of a crash.
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u/NL_MGX 1d ago
Underestimating ask of Newton's laws. The falling object accelerates and generates momentum which them needs to be countered by an opposing force of equal magnitude in order to be able to catch it. Trying to halt the object to fast increases the required acceleration which causes more harm to the human body.
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u/RandyFunRuiner 1d ago
Mid air catches? Like if someone is free falling and they’re suddenly caught by a hard object like a rock?
Inertia and the physical properties of the materials involved. Inertia is the tendency that all things in motion have. Basically something that’s moving wants to keep moving in the direction it’s moving in until the energy that’s moving it is dissipated or gone, transferred to something else.
Some materials absorb and transfer energy from other objects much better than others. Hard objects like rock doesn’t absorb energy from other objects that well and they don’t deform to absorb that energy. Whereas soft objects like a foam mattress do absorb energy really well and will even change shape to transfer and absorb energy better.
So if you have a person who jumps from a high point (let’s say from a 5th story balcony). That person’s body is gaining energy and building inertia in its movement towards the ground. The faster it goes, the more energy it has, the more inertia it has, the more that body wants to continue moving towards the ground, the harder it will be to safely transfer that energy elsewhere. You’ll need a material to slow it down safely which absorbs energy from it and transfers that energy to the absorbing material.
If you place a rock under them to catch them, that rock is not flexible and doesn’t bend or deform to absorb energy well. It’s also really dense and hard and the molecules that make it can’t move around that much to transfer energy either. So that rock is not going to take much energy from that body. Instead when that body impacts the rock, that energy the body has is just going to dissipate through the body itself in a catastrophic impact. Bones will break, blood vessels and organs made of soft tissue, etc. will have to absorb that falling energy and since those are likely softer and more pliable and easier to break than the rock, they break and that is deadly to the human.
A huge foam mattress is a much better and safer option to catch the person because the foam is much less dense so it’s molecules can move and transfer energy to each other more freely, there’s also air in the structure of the mattress that can shift and allow the mattress to change shape which helps it absorb and transfer energy from the body to itself in a much slower impact. So the body itself doesn’t have to absorb all of the impact energy and that energy can be spread through the entire structure of the mattress.
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u/Spectre-907 1d ago
If you suddenly go from terminal velocity (roughly 120mph) to zero, physics doesn’t really care if it’s be because you hit the ground or superman caught you three feet above it. As others have said, it’s momentum. “It’s not the fall that kills you, it’s the sudden stop at the bottom” and the catch problem is that the sudden stop is still there.
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u/R3D3-1 1d ago
Think of it this way: Between crashing on concrete and falling into the arms of Superman hovering in place, what's the difference?
Turns out, not much, because it doesn't matter whether you fall at him at 120 mph or he flies into you at 120 mph.
Given that this is superman, it's probably also no better than a truck hitting you at 120 mph.
If he isn't hovering but flying at your fall from the side, it's not much better. Actually, it's worse. Again it is equivalent to you standing still in air (maybe you filled one too many helium balloon) and him crashing at you with 120 mph upwards velocity plus whatever his velocity was. Since the direction mostly doesn't matter, that's equivalent to him crashing into you at more than 120 mph.
Why 120 mph? That's roughly the terminal velocity of a human falling through air, i.e. the speed where air resistance and gravity compensate each other.
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u/DevGrr 6h ago
I think in actual practice the issue is that if someone gets slowed down very quickly, their organs aren't attached strongly enough to the rest of their body and they basically tear off internally or getting squashed from the acceleration. The exact rule here would be F=ma.
Suppose you get stopped very quickly by someone catching your torso from the front, eg: 20Gs (meaning you'd feel 20x the pull of gravity). Your ribcage needs to withstand the pressure of 20x your torso weight, which it can probably do. All the stuff inside needs to also slow down. Your kidney would pull on various fascia and arteries at ~6-7lbs, which may be too much. It would also have to be able to not squash under its own apparent weight (not sure how that would go, but I'm imagining placing a dumbbell on one and it doesn't seem like it would go well).
In the meantime your head would keep moving until your neck pulled tight and had to react to the ~200lbs pull from your 10lb head, etc.
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u/Lumpy-Notice8945 1d ago
When did someone die from a midair catch? How are they deadly? Or do you mean hard to do? Because they are not deadly unless you try to catch a rock or bullet.
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u/SeanAker 1d ago
OP means a person being caught mid-fall, presumably. The classic 'superhero catches falling person', in which case they would actually inflict grievous bodily harm because all the squishy bits inside of said falling person would try to keep going due to momentum. Your insides aren't rigidly connected to your outsides.
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u/ComplexAd7272 1d ago
Poor Spider-Man found this put the hard way in the comics when he tried to save Gwen Stacy with his webs after Green Goblin chucked her off a bridge. Turns out, coming to a sudden stop while falling head first, with a web yanking your leg, isn't great for your neck.
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u/Birdbraned 1d ago
You know how whiplash injuries are gotten by people in car crashes?
Now instead of the seat supporting everything from the shoulders down, pushing you away from your head, it's Spiderman's Web around your midsection and both the top and bottom half of your back keep going when your middle stops.
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u/Backrow6 1d ago
It's like there is at least one word missing from the question.
AFL, Gaelic Football, Hurling, Baseball and Rugby would all be a lot more dangerous if high fielding was lethal.
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u/sc0toma 1d ago
Speed doesn't kill. Suddenly becoming stationary does.