I failed all my science courses, I don’t understand much about science but why doesn’t the water just… move like when you jump in normally?
In: Planetary Science
When you hit water, it hits you back. If you dip your hand in a pool you don’t feel resistance, but if you slap your hand in you feel pressure on your hand. If you do a belly flop, it hurts- the faster you hit it the harder it hits back.
When you fall from a high space, you start to go faster the further you fall until you are falling at your maximum speed. Once you’re at that speed, you would hit the water really hard and it’s going to hit you back really hard, to the point where it feels like you’re just hitting a solid.
It does, but your body has to push it aside – it doesn’t just part for you. Water has weight and inertia like all other matter, and currently it’s “somewhat heavy” and “hardly moving at all”.
Think about atmospheric re-entry of the space shuttle… why doesn’t air just move out of the way of the incoming spaceship? It wants to.. it can, but the ship is moving *fast* and there’s still some resistance there. The result is the space shuttle heating up from compressing the air.
While you’re not moving as fast as the space shuttle nor nearly as large, water helps make up for it by being much denser. The problem remains: your body has to move water out of the way fast enough for your body to go into it. The faster and the larger area you hit, the harder the water resists and hence the more painful the impact. The fact that water doesn’t compress doesn’t help matters either.
Compare walking into a pool to a belly flop. Or, jumping in from the side vs. the belly flop off a high diving board. The water moves the same, but you hit it harder the further up you are, and the more it resists just because of how fast you are trying to enter it. So, from the side, a belly flop is ok, but from the diving board, it smacks you and leaves a welt. That smack is harder the higher you are when you hit it.
There are techniques to survive, and there are stunt divers that do things to dive from very high heights, like dropping something first to get the water moving before they hit, or bubble water in it. But that does the same thing, get the water moving so it’s easier to move out of the way of your body when you hit. (Check the records section, including the failed dives and broken bones – https://en.wikipedia.org/wiki/High_diving)
Water weights 10lbs a gallon and has surface tension that wants to hold it together. Your body needs to displace that weight and break the tension spread over the area you impact.
A diver that turns their body into a spear can penetrate the water by concentrating all their weight into the tips of their fingers making them heavy per square inch than the water while breaking the surface tension. Someone who lands flat spreads their weight out effectively making them lighter per square inch than the water, and not able to break the surface tension.
To enter water, you need to push it aside. It has to go somewhere.
The one place it can’t go is straight down, because as far as your body is concerned, water is incompressible.
Or rather, let me amend the above statement: Water can be pushed down if you force the water below it to also move out of the way. And that water also has to go somewhere, so you have to move all the other water, too.
In other words, you’d need to raise the rest of the water level up. Which is possible… Mathematically. The only problem is the force required to do that by decelerating from high speed also liquifies your body first. Your body can, ahem, spread sideways far easier than the water can be moved down.
So if you hit water in a “flat” way, it basically has nowhere to go and “slaps” you back with equal force. Exactly like a solid surface.
BUT! It is far easier to push water to the side, it actually has somewhere to go, or at least you’re trying to move a massively smaller mass of it. So if you arrange yourself into a shape that does that, you can penetrate the surface. A pointy shape. That’s exactly what people are trained to do if jumping from heights.
Imagine a crowd of people standing shoulder to shoulder. If you walk up to them, you can wiggle your way through the crowd. If you run at full speed at them, you’ll crash into them and get hurt.
The crowd in this analogy represents the water. At high speeds, it doesn’t move aside quickly enough and feels like a severe impact.
Water is heavy and doesn’t compress, which means 1) it has to be moved out of the way and 2) it’s *difficult* to move it out of the way. Its resistance to being moved quickly is felt by you as a hard impact.
That said, there’s no circumstance where liquid water is anywhere near as unyielding as solid concrete. “Like hitting concrete” is a euphemism, not literally true.
It cannot get out of the way fast enough. You can try it yourself next time you’re swimming or with a basin of water. Slap the water. Just hit it with an open palm. It stings, like belly flops do. Very fast aircraft and rockets encounter similar problems with air. It’s all about how fast it can get out of the way.
This may be a terrible analogy but it might also make sense so bear with me: if you push a car that is in neutral using your car, the car in neutral will move. If you take that same car you pushed before and drive into the back of it at 50 mph, the car will still move but it’ll mess your car up a lot more than the first time round. It’s a similar principle. The water will move but it takes work to move it. If you do that work over a long period of time then the maximum force is fairly small. But if you try to transfer all of that energy at once then you get one big spike of force that is high enough to kill you. The water doesn’t act the same as concrete, they just say it’s as good as hitting concrete because the force isn’t spread out over a long period of time so both will kill you
I do t get the displaced water ahead of you theory because eventually you will hit undisplaced water. It just might be a few feet further down.
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