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AK-47 Underwater at 27,450 frames per second (Part 2) – Smarter Every Day 97

Hey it’s me Destin welcome back to Smarter Every Day. So I’ve been learning a lot about guns underwater, which is pretty cool. I mean, in the first video I learned all about what’s happening back here in the action. But the problem is because of limitations in my setup I didn’t get to see what happened when the bullet exits and goes into the water. Now I really want to see that, so I had to think about it for a while and I had an ah-ha moment. You see instead of building an aquarium that was keeping the water in with the gun, I built this, to keep the water out. But the problem is, even though I built all this cool stuff and got my hands on one of the best high speed cameras on the market, I didn’t have enough hands to run it all. So I invited some friends to help me. It’s gonna work.
– Yeah?
– Yeah.
– Sickeningly good. (Destin) I was hoping to get the widest view possible, that we can get inside that mirror. – So, we’re gonna have to be as close to the mirror as possible with the wide angle otherwise we’ll see the rig won’t we.
– I think so. (Destin) What kind of lens do you think you’re gonna use?
– Lens… So you’re gonna be in the middle.
– So we should have a lens, shouldn’t we? [laughs]
– We should have a lens. [laugh] – The problem with doing an AK under water is that I can’t trigger the camera, so that’s the Slow Mo Guys who’ve come to help me. So what are you gonna run?
– Run the Phantom v1610 today which goes up to 18,000 frames per second at 720p. – I can’t count that high.
– Definitely more than.. you’ve got fingers. – Can you zip me?
– Unless you’re from Alabama.
– Alright just to show you how cold it is when we’re doing this, that’s about 40 degrees. And it is cooold. So the camera’s gonna look into the top mirror, it’s gonna bounce down and then basically it’s a periscope under water. I’ve got the gun here… You can see it there, with the phantom. [gun cocking] [gunshot] [gunshot]
[bubbles] [gun clearing] [slowed down gunshot] [multiple bangs] Alright, it has been very consistent. We’re getting about 5-6 feet of bullet travel. [gunshot] [slowed down gunshot] [slowed down gunshot] [bangs] [gun clearing]
OK. So, what did we just learn? You can see that there’s gas that comes out right here. It’s a little gas bubble. And the reason that’s happening is because it’s.. the piston is venting just when it gets past… [laugh] I can’t talk I’m freezing. As the bullet goes down the barrel it passes this gas block and pushes gas against this piston. Now you can see it starts to move the bolt but watch what happens. Right there, it opens up this little gas port. That’s why there’s a bubble right there during the shot. So if we take this off, you can see that the piston vents after it goes only about a 1/4 of an inch back. If you think about it that’s cool because this very short pressure impulse is enough to overcome all the springs and the friction in the weapon and cycle it, simply due to inertial forces. [gunshot] [gunshot] [water spray] Oh gee.. it’s cold.
[laughter] Alright, so… So now we’re gonna do over the shoulder? – Yeah, yeah. Over the shoulder shot.
– Alright. (Destin) All clear to load?
– Clear to load. – Clear to fire? – Clear to fire.
– 3.. 2.. 1.. [gunshot] (Destin) Weapon’s clear. – Ohh. – Did you get it?
– I got it. I’ll go check it. [slowed down gunshot] [multiple bangs] [slowed down gunshot] [music] [slowed down gunshot] [bangs] [underwater gunshot] Yeah. I know that was gonna be cool, but not that cool. Let me try to explain what I think’s going on here. So we got this oscillation that you can see after the shot in the bubbles. That’s awesome. I didn’t understand why the bubble would start back up after it collapsed. But here’s what’s going on. There’s an equation called the Rayleigh-Plesset Equation that describes everything a bubble does under water. It’s too hard to solve by hand, you’ve gotta use computers to figure it out but this is basically what’s going on. At the initiation point you have a super high pressure inside of a bubble and it begins to impart momentum to the fluid around it so it begins to grow. At some point it passes the point where the pressure inside the bubble is equal to the pressure outside the bubble and it continues to grow until eventually the water stops it. Now at this point you have low pressure inside and high pressure outside so it begins to collapse again, and because of fluid momentum it goes again beyond that equilibrium point and begins to compress. So it gets a super high pressure on the inside again and boom, another shock wave and the process starts all over again. This oscillation occurs until you dissipate all the kinetic energy in the system. Now at this point of tightest closure, you have the highest pressure. At this point sometimes something can occur called sonoluminescence. Sonoluminescence occurs when you get a flash of light when a cavitation bubble is collapsing. Now as much as I want to believe this is sonoluminescence I’m pretty sure it’s just a really cool reflection from the sunlight above the pool. But it’s still really interesting that it occurs at the point of collapse. I wonder why that’s happening. OK there’s something else we need to talk about. This is my favourite shot. Originally I thought that the first gases out of the barrel were from where the bullet exits and the expanding gases from the cartridge flowed around it, but look again. Do you see that black colour trailing down the length of the bubble? That’s the burnt gunpowder being released from the barrel behind the bullet. So if you follow that powder down the bubble, it should line up with that bullet. Yeah, there it is. So what’s the first white cloud then? If you have a flowing liquid and you speed it up, the pressure of that fluid drops. Now it seems a little bit backwards from how it should be, but this is what happens. It’s called the Bernoulli principle, where flow is high and pressure’s low. So let’s look at this phase diagram for water. The water we were in was about 4 degrees celcius and about 1 atmosphere. If we drop the pressure of the water below a certain point the water turns to vapour. The inside of the barrel was full of water before we shot right? And so the bullet pushed it out at a very high speed. So where flow is high, pressure’s low. Cavitation is happening in the barrel on the front side of the bullet. I’m still trying to wrap my head around this but there it is. You can actually see it in the video. Once the bullet punches out of this cavitation cloud something else is happening. You’ll notice that the bubble on the right looks like a cloud and the bubble on the left looks more like glass. Andrew Davidhazy took some awesome shatter graphs of bullets in flight which show the shock wave on the front of the bullet. The area behind the shock wave is lower in pressure. It turns to vapour. Because this low pressure region has a smoother flow boundary it looks more like glass instead of the fuzzy cloud look caused by the turbulent flow coming out of the barrel. So there you have it. You’re not just shooting a bullet out of this gun, you’re shooting three different things. Now that we understand the physics behind cavitation you can clearly see the effects of each of these three components in the high speed video. Oh yeah, and we also understand bubble bounce now too don’t we. So a huge thank you to Gavin and Dan the man from the Slow Mo Guys. They came all the way to Alabama to help me shoot this video. That’s a pretty big deal. So we re-did a video on their channel, something I did a while back, pistols under water, only we used a v1610. It’s awesome. Go check it out on their channel, it’s totally worth your time. On my channel the intent here was to make an awesome video that you enjoyed and you also learn something, and perhaps earn your subscription. So, if you think I got close, we got a little chemistry going on, check out part 3 of this video, Russian frogmen guns. Yeah, they exist. So I got my hands on some of those. I still have the ninja scope 3000 or whatever we’re gonna call that thing. You kinda see where this is going. Anyway, I’m Destin, you’re getting Smarter Every Day, have a good one. So we actually selected this pool for a reason. Jimmy Neutron lives in the middle of nowhere, so if the bullet got away from us, everybody down range would be safe. Please be smart, don’t try this. So check out how he cuts the glass. [scratching]
Huh? [snap]
(Destin) What? What did he just do?! [laughs]
(Destin) Get away from my computer, get away from my computer! Alright we got the bullets. There was no damage but you can definitely see the grooves from the rifling in the barrel. [ Captions by Andrew Jackson ] Captioning in different languages welcome.
Please contact Destin if you can help.

Reynold King

100 Replies to “AK-47 Underwater at 27,450 frames per second (Part 2) – Smarter Every Day 97”

  1. i love TheSlowMoGuys, it was actually one of their videos that i found out about SmarterEveryDay because it was in the suggested videos section

  2. Destin, I need a video that shows the effect of tumbling bullets. The M16 bullet starts tumbling when it hits an object. Any chance that you can video the process?

  3. The cavitation effect is taken into serious consideration when designing pumps for various purpose. The inlet of the pump lowers the pressure, and so if you're pumping very hot water from a boiler, for instance, you have to make sure you aren't lowering the pressure below the vapor point for the water, otherwise you get steam pockets (Cavitation) in your pump and… Bad things happen.

  4. What you are seeing is the collapse of a supercavity, which when do so, sends a shockwave outwards, triggering cavitating in smaller pockets in the far field. Impressive camera! We use the Phantoms too, not this resolution though ;(, but better sensor efficiency than some of the other stuff our lab has lying around.

  5. I just tought that the bullet create vacum chamber after the powder blast,, thats why its expand and then back in together… but i was wrong … lol..

  6. Now, what would happen with a full automatic? What happens if you fire through the cavitation? Would the cavitation envelop a stationary object or reflect off it?

  7. When the bullet leaves the barrel and there is a small cut between the shock wave that the bullet creates underwater, I'm guessing that is when it goes from supersonic to subsonic? Am I right?

  8. so whdo love to know if you do have lemanes you shod try this again at night alls ow the shok way look kind of like a lighting storm and thunder and i got ask what did you fell when you fidr it how did the shok way or any of that feell

  9. i would add to your good insight another phenomenon here and propose an explanation. Perhaps both my observation and explanation are incorrect. However, it seems that the glass bubble collapses before the fuzzy bubble (perhaps in spite that the latter had more time for expansion and collapse). The glass bubble should be mostly vacuum – caused by the bullet edges breaking through the surface tension and pushing the liquid water out and forming a void. By thermodynamics, indeed there should be vapor inside, but I posit that there is insufficient time to come to vapor-liquid equilibrium. The fuzzy bubble, on the other hand, was created by water vapor from in front of the bullet, not behind it, and it is more in tune with vapor liquid equilibria. Therefore, the glass bubble has less vapor and thus less internal pressure than the fuzzy bubble. The external pressure (liquid) is roughly the same on both bubbles, and assuming that the both bubbles are roughly similar in area, the overall collapsing force is higher on the glass bubble and it will collapse before the fuzzy bubble, which it does.

  10. Has anybody seen the episode of Mythbusters when they a shot the 50 caliber in the pool and
    the bullet didn't go no more than 5 feet in the water

  11. Really cool video with great explanations thank you! Great job! Would love to see this same test done in the dark or very low light conditions. Might help solve the question of flash?

  12. If you were to tape off the end of the barrel and chamber a round first before submerging the AK then their would not be any water in the barrel for the built to push out so the bullet would me traveling faster and would go a bit father and make a bigger blast cloud at the end of the barrel.  A fun video Thanks

  13. How far does the bullet travel? Looks like after just some 50cm, the bullet would be so slow it can't puncture paper.

  14. I'm sorry but I couldn't comprehend all this with my inferior brain I was just here coz of the slo mo guys.

  15. At the point of implosion when there is a flash of light (and Destin considers sonar luminescence), notice that the image of the muzzle (and the rest of the frame for that matter) jostles back and forth. I would guess that about the same time it takes for the first bubble implosion is when the most energetic wave reaches the mirror box. It would be no coincidence that the bubble oscillation would then be in phase with the waves reaching the mirror. So Destin is likely correct that it is not sonar luminescence and likely a "reflection of the sunlight", but more specifically the change in apparent brightness is due to variation in angle of combined refraction/reflection through the mirror box. If the camera were moved or the box manually vibrated, one might also see a change in the apparent brightness of the overall image. The effect is just more pronounced when it occurs in sync with the bubbles.

  16. Also, remember that pressure is very much directional. It is simply an average force over an area… and the force in a liquid or gas can be directed so that more of the average motion of the particles are in a certain direction and hence the average force exerted is also higher in that direction. In static equilibrium, pressure is essentially equal in all directions over a small surface area at the same depth. But while flowing the pressure is generally only lower at the boundaries orthogonal to the flow. The pressure is actually higher along and in the direction of the flow than against the boundaries of the flow (i.e. tube walls). If the pressure were also lower in the direction of the flow, it would be lower than the water outside the barrel and so it would not escape the barrel, rather it would be pushed back further inside the barrel by the higher-pressure water outside! But that's not what happens. I mention this because like so many other subtle–but often repeated–incomplete ideas in science can spread misconception later on.

  17. between 2:02 and 2:16 Wooahhh did you see the shockwave rippling toward the camera; and how the rifle wobbles up and down like a piece of rubber?
    Wowww it's so cool how around 4:45–4:50 the air which was initially pushed radially from the the bullet then is pulled into the channel created by the bullet through the water.
    Thanks for explaining the Rayliegh-Plesset equation.

  18. Hey Destin, I have a quick question for you. Well maybe two, the first is, is there a way to message you on YouTube without messaging in a random video? And the second, and real question, is do the sounds that come from your throat sound different than the sounds that come out of your mouth? I was listening to an audiobook, and it mentioned a sticker that goes inside of your mouth that acts as a communication device. It's a fictional book and it's like a future technology that's been made up and I was curious if the sound that came from your throat into your mouth would sound the same as the sounds that come out of your mouth when you talk. I didn't know if the sounds were generated by your vocal cords and your lips and jaw inside of your mouth the same as they sound when they come out.

  19. Fantastic video as always. The bullet exiting the muzzle resembles a bullet impact into ballistic gelatin. My question is, what is in the temporary cavity? I understand that the water is pushed apart as in the ballistic gel, but where does the (what looks like) air come from? Soon after the temporary cavity is created, it collapses/implodes in on itself, but no air rises to the surface. What is in the temporary cavity? Thanks

  20. does about the same thing happen if you shoot 2 feet above water into water? does the water slow it down so that it only travels about 4 feet in the water?

  21. I'd quite like to see those beautiful collapsing bubbles happen through a column of dyes or inks in the water.. Destin. get the wet suit!

  22. it would be interesting to see this repeated in a pitch black environment to determine whether it is truly sonoluminescence, or just reflected light like you hypothesized.
    that has been an ongoing debate on so many ballistics gel videos I've watched.

  23. In your footage and the slowmo guys, I noticed something. The snub nose revolver projectile seem to travel straighter and didn't tumble as much. Is this possibly due to the shorter barrel not housing as much water volume, in turn lowering the hydraulic affect? Which would allow the bullet to gather more spin and less pressure to have to push thru?

  24. Would you consider firing more than one shot in as automatic fire in this context?
    I'm thinking about the effect of the compound shock waves, but also if the second round is traveling in the cavity left by the first, the round may travel further and more elegantly.

  25. Following the recent shootings in the US don’t you feel you should stop glorifying guns of any kind as a nation you can’t cope wither the responsibility

  26. This is why I love YouTube. Great video man. You won me over with this video and now just made me a subscriber.

  27. Is it possible to fire a gun fast enough to get bullets to travel through the cavitation bubble of the previous bullet and would this then make the bullet travel further?

    I know this is 6 years old but I think you should do it again but mount the gun and go full auto

  28. still my favorite myth busters show where they fired various guns into water. Amazing how NOT far the bullets went. Unlike all the old hollywood movies that showed bullets streaking down past the heroes. Even the 50 caliber disintegrated

  29. so much force on the front of the bullet. Not hard to now see why the bullet quickly turns off center instead of pointing straight ahead. Then once the bullet gets not pointed straight in the direction of the velocity vector, side forces are huge and the bullet can't go much further or disintegrates into pieces

  30. can you shoot a gun in an enclosed water filled container? would it just burst? and what would happen in the inside?
    Since Water cant be compressed, how would an explosion look like?

  31. 6:30 There's no pressure in a cavitation with no gases or air in it. Some of the "bubbles" from a gunshot are cartridge exhaust bubbles, some are cavitations with no gas inside, especially surrounding the projectile. There was a video somewhere about a fluid in a glass vacuum tube, and the fluid would slam against the glass almost like a solid because there was no resistance or gas in the way.

  32. I don't think the gas from the piston cycles the weapon – it is just enough to unlock the bolt. The pressure inside the barrel is what does the main job. The same pressure that pushes the bullet forwards is what pushes now the empty shell from the cartridge and the bolt backwards. If the bolt is not locked the bolt would go back way to soon and a lot of the gasses and the energy will simply be lost. The reason we have those gas operated rifles is so we give maximum inertia to the bullet possible but not wait to much because if the bolt is unlocked too late then the gases will escape the barrel after the bullet has come out and there won't be any pressure left to cycle the bolt. That's why the position of the small hole on the barrel that takes small part of the gas pressure to unlock the bolt is so important – too close to the end of the barrel – you risk experiencing a jam, too close to the entrance – the bolt will unlock and the pressure will escape way too soon before the bullet gets is maximum speed.

  33. I am curious as to what the effect would be if you were to plug the end of the barrel so it would not fill with water, perhaps you’d get different results?

  34. I’m not trying to be controversial when I say this, but with all your knowledge, how do you convince yourself that that there is an almighty creator? Again I’m not trying to start a beef, I as an atheist am honestly curious. It’s just that all research I’ve done proves evolution to be true! I would love to believe in an all knowing, all loving, perfect God, but I can’t based on what I know.

  35. I understand this is science, but I LOVE my ak. That's really cool what you did, and I think people should at least try firearms out for them seleves SAFLEY.

  36. Hey, why the bullet is tumbling while it's being shot. I observed the same in Slo Mo Guys video, but the pistol did tumbling, not revolver.

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