Smarter Every Day

Watch: This is what happens when you shoot an AK-47 underwater in slow motion

Whoa.

FIONA MACDONALD
30 DEC 2015
 

Thanks to the Internet, we now know that everything looks better in slow motion... even things that you'd never really wanted to see before. And it turns out that holds true for a high-speed bullet too, as Destin proves in this classic episode of Smarter Every Day from 2013, which has now been watched almost 20 million times.

 

So what's so exciting about an AK-47 being fired underwater? In theory, not much (and we have to admit that we're not really the biggest gun fans), but when you take a look at the slow motion footage you see that the action of firing a bullet forms incredible worm hole-like tunnels underwater, which open up like a rip in the fabric of space time, and then close up just as quickly as they appeared.

After watching the process from a few different angles it's pretty apparent that some crazy physics is going on here. Because how else could you explain the bubbles that expand and shrink and then expand again, even after the bullet has moved on? Or why one of the cavities produced by a bullet looks like glass, while the other one looks more like a cloud?

That's where the physics comes in. As Destin explains, the strange behaviour of the bubbles you see above is mostly thanks to something called the Rayleigh-Plesset equation, which describes the motion of bubbles, and helps explain why they can oscillate so wildly.

But there are also a whole range of fluid dynamics in play, like the Bernoulli's principle, which basically states that liquid moves faster as pressure drops.

That all sounds pretty confusing, but trust us, it all makes a lot more sense when you see it happening at 27,450 frames per second. And, of course, it looks better too.

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