why is jumping off a bridge often fatal, but people are rarely injured in high diving?
In: Physics
Training and practice, physics and a water jet placed at the bottom, basically.
You remember the time you mixed cornstarch and water together to make slime? And how when you poked your finger into it you could pierce into the slime but when you slapped it the slime wouldn’t budge? Well, that slime is basically an exaggeration of how actual water operates when things go into it.
Any body of liquid, be it a glass of water or the ocean, is basically a bunch of little balls sitting in a pile. When you jump into water, you need to push away all of those balls so that you are then able to take up space in that body of water, also known as displacement.
There’s also this funny thing called surface tension. For our purposes, imagine the balls are also attracted to each other, not a bunch, but enough that they kinda don’t want to separate if they’re all touching each other and sitting happily together.
When you dive into a pool, the balls of water, or molecules, are all sitting there happily, touching each other and being attracted to each other (hey stop giggling), but when you jump in, you disturb that and all of a sudden they have to move, and they’re gonna put up a fight.
When it comes to the ‘training and practice’ part of the story, the divers are essentially positioning their body in such a way that instead of smacking the water with their stomach, they’re instead piercing the water with their feet or hands, meaning that they kinda act like a knife instead of a hammer, and cut into the water instead of smacking it with their torso.
As for the physics part of it, the easiest way to explain how the mechanics of displacement change between you belly flopping and you diving straight down feet first is to think of your body like a big ol baguette.
Now, if we cut a baguette widthways into quarter inch thick slices, starting from the tip, and then lay them out next to each other, the first slice is gonna have just the tip of the crust, then the next slice will be slightly wider but now show the fluffy interior, then the next slice will be slightly wider again, then the last slice will be the full width of the baguette. If we consider each of those slices to be representative of moments in time as you make contact with the water when you jump in feet first, the first slice is you breaking the surface tension of the water, so the slice is all crust, signifying that all this water has to move out of the way to make way for your body. The second slice, only having the crust on the outside, doesn’t have to move anywhere near as much water, and all the other slices are about the same.
But if we jump in flat, we are instead cutting the baguette lengthways. As you can imagine, the first slice, still being all crust, is way bigger than the other slice we made when we cut it widthways, so in that moment your body has to move way more water cause you belly flopped instead of with your feet, but once you make that move the next slice is nowhere near as bad. Now technically it’ll take less time to fully enter the water, because when we cut the baguette widthways we ended up with 20 slices but when we cut it lengthways we ended up with only 6, but because we’re entering in a way that forces the water to move so much of itself to allow that, we get met with a lot of force by the water in return, and when you add velocity to the situation, you can essentially expect the water to act like concrete.
One more thing though, the water jet. At pretty much all high diving meets, there’s a water jet spraying the water below. What that sprayer is doing is adding air to the water and constantly breaking the surface tension for the divers, think of it as the diving equivalent of putting a foam pad on the floor, because doing that, more specifically adding air to the water, allows the water to compress a little, like a foam pad, instead of acting like concrete or that slime, which you cannot compress, that slight difference, combined with the accrued skill of the divers and some physics, is what makes a failed landing an injury instead of instant death.
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