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For no other reason other that we thought it would be quite nice
September 24, 2012 2:31 AM   Subscribe

Dr Roy Lowry of Plymouth University demonstrates what happens when you allow liquid nitrogen to vapourise in a sealed container, with the assistance of 1,500 ping-pong balls (SLYT)

The delightfully barmy Dr Lowry has also, in the interests of science, roasted jelly babies and broken the world record for the largest number of fireworks simultaneously launched.

All science teaching should be like this!

via
posted by hardcode (83 comments total) 24 users marked this as a favorite

 
And Elijah said, “Fill four pitchers with water and pour it on the burnt offering and on the wood.” And he said, “Do it a second time,” and they did it a second time. And he said, “Do it a third time,” and they did it a third time. Then Elijah the prophet came near and said, “O LORD, the God of Abraham, Isaac and Israel, today let it be known that You are God in Israel and that I am Your servant and I have done all these things at Your word." Then the fire of the LORD fell and consumed the burnt offering and the wood and the stones and the dust, and licked up the water that was in the trench. When all the people saw it, they fell on their faces; and they said, “The LORD, He is God."

But the Priest of Baal then came unto that place and he said, "That's nothing. Check out what I can do with this canister of liquid nitrogen and 1,500 ping pong balls." And the people said, "Wait, we just saw the LORD pull off some fairly wacky shit. This better be impressive." And the Priest of Baal said, "No, seriously, this will fucking blow your mind". And the people said, "OK, cool." And the Priest of Baal called upon the forces of science and Lo, he did indeed blow their minds, and the People said, "Ha ha ha! Wow, that is much cooler than the shit with the burnt offering. You win, Science." But Elijah said unto them, "Sheesh, you people are fickle."
posted by the quidnunc kid at 3:19 AM on September 24, 2012 [30 favorites]


Considering the casual way he was pouring it out of the dewar, I thought I was going to see a demonstration of what happens when you spill liquid nitrogen on your hands. Also it seems he is wearing ordinary eyeglasses and not shatterproof, protective lenses. And hands over the ears instead of proper industrial hearing protection?

So this is as much a demonstration of a woeful ignorance of ordinary commercial health and safety protocols as it is a cool way to explode ping-pong balls into the air.
posted by three blind mice at 3:26 AM on September 24, 2012 [12 favorites]


the world record for the largest number of fireworks simultaneously launched.

I think San Diego might have beat him on that. Not on purpose of course.
posted by three blind mice at 3:30 AM on September 24, 2012 [2 favorites]


Stand back, we're doing science!
posted by Omon Ra at 3:34 AM on September 24, 2012


Mister Moose could not be reached for comment.
posted by ob1quixote at 3:44 AM on September 24, 2012 [3 favorites]


I'm getting a "This video has been removed by the user." for the main link?
posted by HuronBob at 3:47 AM on September 24, 2012


I watched it 10 min ago. Guess they woke up about the safety issue.
posted by seanmpuckett at 3:50 AM on September 24, 2012 [1 favorite]


Yep, I suspect that Dr. Roy gave some thought to his professional standing after three blind mice's safety admonition.
posted by HuronBob at 3:54 AM on September 24, 2012 [4 favorites]


Here's a remarkably similar video on the Ellen show. I note in passing that they put protective headgear and earphones on Ellen , but didn't bother with the earphones themselves or the audience ("they're fine"). I don't think there's that much of a safety issue personally: until the bottle goes in the hot water it isn't going to explode for some time & even then it'll probably just split. Once it's in the water, most of the force of the blast goes into the water itself (look at the way the bucket jumps upwards as the shockwave reflecting off the bottom lifts it upwards by creating a vacuum between the water and the bucket.)
posted by pharm at 3:57 AM on September 24, 2012


Video can still be seen at wimp.com.
posted by ouke at 3:59 AM on September 24, 2012 [2 favorites]


As an undergraduate, I worked at a pharmaceutical site where the dry ice machine made pellets that exactly fit into eppendorfs (sort of small plastic test tubes, about the size of the last joint of your little finger. They have a snapdown top and biologists use them for some nefarious purpose). All of the undergrads would normally hang around after office hours to write up reports for university or use the internet or the coffee machine or whatever.

We would go and visit each other, taking these small dry ice bombs with us. Outside a friend's office, you would snap the top down and then it was only a matter of time before they went off. The challenge was to hide them un-noticed somewhere in the office (wastepaper bins, under their chair, behind their computer etc). My heart is racing as I remember the excitement of trying to act normally and have an innocuous conversation while trying to place the eppendorf somewhere - before the warmth of your own hand cause it to go off prematurely.

The incident I remember most vividly was Andy - we made quite a lot of bombs, went to borrow his stapler and dropped them all in the bin. From outside the 5 cm thick fire door, we could only hear them detonating as a muted "pop-pop-pop" sound, but we could still hear his stream of shouted swearwords and furniture overturning. When we ran back in to do victory dances we found him hiding under the table, convinced that he was being shot at by snipers.

Good times. There was also a bug in MS Outlook that allowed you to launch DoS attacks against your friend's email if they sent out votable questions.

I'm sure I did some API development during that year of my life, but mostly I remember the pranks.
posted by Isn't in each artist (7) at 4:05 AM on September 24, 2012 [7 favorites]


Maybe not a safety issue in the video, but I doubt he or the university want to basically teach kids how to asplode themselves with $5 of supplies from a party store.
posted by DU at 4:06 AM on September 24, 2012


Right after the explosion, my first thought was: Captain Kangaroo!
posted by DU at 4:10 AM on September 24, 2012 [2 favorites]


[I edited the post to use the working wimp.com link; thanks ouke!]
posted by taz at 4:18 AM on September 24, 2012


[Thanks taz and ouke]
posted by hardcode at 4:22 AM on September 24, 2012


What's interesting is that the garbage can flys up as well which I am guessing is because of the ping pong balls providing thrust which forces the can against the floor which causes an elastic-inelastic collision that makes the can bounce in the air ?
posted by Podkayne of Pasadena at 4:24 AM on September 24, 2012 [2 favorites]


The trash can flying flying up was the best part.
posted by sammyo at 4:28 AM on September 24, 2012


Have you all heard of 52 pickup? I've got an even better game!
posted by double block and bleed at 4:31 AM on September 24, 2012 [1 favorite]


"What's interesting is that the garbage can flys up as well which I am guessing is because of the ping pong balls providing thrust which forces the can against the floor which causes an elastic-inelastic collision that makes the can bounce in the air ?"

You've got the basic idea, but the ping-pong balls are largely immaterial, the bottle explosion is directly providing the downward thrust, which appears to cause the can to bounce off of an elastic-inelastic collision with the ground.
posted by Blasdelb at 4:32 AM on September 24, 2012


how to asplode themselves with $5 of supplies from a party store.

Liquid nitrogen?
posted by Horace Rumpole at 4:34 AM on September 24, 2012


That's my university!
Man was I in the wrong department (and also left too early).
That August fireworks championship was always awesome though. It was spread over two nights with the six biggest fireworks companies competing with each other to produce the best show.
posted by Just this guy, y'know at 4:51 AM on September 24, 2012 [1 favorite]


Liquid nitrogen?

Dry ice does the same thing.

Not that you can't find videos and instructions of it on the web, but institutions are so wary of lawsuits that I doubt they'd want to give the appearance of sanctioning a blase, "here's an explosion, just cover your ears" approach.
posted by DU at 4:54 AM on September 24, 2012


This demo also reminds me slightly of a professor at university that taught a course on colloids or something like that. The lectures were really, incredibly boring, even by the standards of a chemistry degree, yet the theatre was always packed.

That's because he was also the head of Health and Safety, and was able to write risk assessments and sign off on his own practical demonstrations.

During the course we saw:

a biscuit burnt in liquid oxygen (complete combustion, so a very bright flame, no biscuit left and no smoke to set off the smoke alarms)

An oxyacetylene torch in a latex glove - this involved producing acetylene in the glove by filling one finger with calcium carbide, another finger with water and then tying the wrist and giving it a shake to mix the ingredients. The glove inflated with acetylene and a hole pricked in the top, plus a match, caused a fairly spectacular blue-white flame about 30 cm long, which kept going until the glove eventually deflated.

Nitrogen triiodide - he had prepared three beakers of this and whacked them all with a stick. The first two did nothing, except spread nitrogen triiodide crystals everywhere. The third went off with a loud bang and a cloud of purple smoke.
Unfortunately, the crystals which had been spread everywhere went off at random intervals throughout the rest of the lecture, each time with a loud bang and a puff of purple. I think he gave up and let us all leave at the point when you couldn't hear him over people laughing. That was when he was gesticulating at an equation on the board using the same stick that he'd used in the demonstration, and the end of it exploded particularly violently.

Detonation of large, novelty balloons full of hydrazine (old school rocket fuel). My friend was phobic to balloons and left in hysterics before the demonstration - probably for the best, as the resulting pressure wave brought down several light fittings in a two storey high lecture theatre.

However, my favourite part with each of these demonstrations was that he would do them on a metal trolley, which he would wheel to a position right in front of the first row of students. He'd then retreat, put on goggles and a labcoat, before starting whatever reaction with a long pole. Watching the ultrakeen scholars at the front squirming rapidly to either end of the row was in some ways better than watching the demo itself.
posted by Isn't in each artist (7) at 4:57 AM on September 24, 2012 [25 favorites]


A prop guy I worked with a few years back told me a story of a dry ice bomb he made out of a handful of dry ice, a two liter soda bottle, and some hot water. Once the bottle started to expand he and the other stagehands ran out of the room and shut the door. The resulting explosion could be heard throughout the 6 floor building and blew down every tile in the drop ceiling in the room.

Man, I love chemistry.
posted by nevercalm at 5:03 AM on September 24, 2012 [1 favorite]


I do hope the video wasn't taken down because of the health and safety nazis, that would be so depressing. There should be more people like this in education.
posted by epo at 5:04 AM on September 24, 2012 [2 favorites]


Lottery drawings should be done like this.
posted by I'm Doing the Dishes at 5:09 AM on September 24, 2012 [8 favorites]


I'm pretty surprised that he could pick up the bottle without wearing gloves
posted by DanCall at 5:11 AM on September 24, 2012 [1 favorite]


So this is as much a demonstration of a woeful ignorance of ordinary commercial health and safety protocols as it is a cool way to explode ping-pong balls into the air.

Oh, please.

He didn't wear gloves because, in general, the fastest way to get a nasty injury from LN2 is to have it either poured inside, or absorbed by, something you are wearing. The best protection is the Leidenfrost effect., and most "saftey" gear takes that away from you. The other way it gets you is it boils off, displaces the air (and most importantly, the O2 in that air) and you pass out from hypoxia, but for that, you need a much smaller room and much more LN2. I'm not even afraid of that much LOX*.

And not only was there water to absorb the energy, there were ~1500 ping pong balls, all of which acquired energy being accelerated to the ceiling, never mind throwing the trash can up a couple of meters -- a trash can with a couple of kilos of warm water, mind you. Never mind the distortion and rebound of the material of the ping pong balls, which are made of nitrocellulose. Nitrocellulose is better known as the key ingredient in nitrate films, which are incredibly flammable, and gun cotton.

Yes, arguably, the most dangerous compound in the entire experiment was the ping pong balls!

The only mistake in procedure I saw was he was screwing the cap on outside of the container. I hold it inside the container out of the water, with my face well away. That way, if it were to go boom instantly -- unlikely, but not theoretically possible -- then the bin helps keep the force away from the crowd, and from most of you, as long as you're not leaning over it. You might hurt your hands. There are arguments about gloves at this stage. Thick enough gloves would certainly protect your hands, but if they reduce your dexterity, they make an injury more likely by slowing down the time it takes to screw down and release the bottle. Safety gear, used improperly, is not safe.

Is this perfectly safe? No. Is living? Hell no. The biggest risk those kids took that day was getting into a vehicle to get to the campus. I could almost fault him for not wearing goggles, but PET bottles generally don't shatter, even at these temperatures, they split. Fragments aren't the problem, it's the shockwave that does the damage, and for that case, the goggles, they do nothing, or at least, very little. But while I get annoyed at the useless to downright dangerous nonsense that "safety" officials want to foist on us, I can never argue with something encouraging people to wear goggles. Eyes are just too damn fragile.

But in terms of safety. I'd do this demo in a heart beat. Well, I've done it. Not with ping pong balls, though, that's "quite nice" indeed.

Finally. I have worked with a few English folks much like Dr. Lowry. What I've learned is when they say "right" in that tone of voice, something interesting will happen soon. Pay attention.



* Did you notice the frost and vapor on the bottom of the bottle as he was filling it? That wasn't spilled LN2. Liquid Nitrogen boils at ~77K (-179 °C, -321 °F).

There are other gases in air. The most important one to us is Oxygen. LOX boils at ~90K (-183 °C, -297 °F), some 20 degrees kelvin higher than LN2. So, what's happening is the outside surface of the bottle, full of liquid nitrogen, drops below the temperature that oxygen liquifies, and it condenses out on the surface, just like water vapor does on a cold drink.

The vapor is the now LOX boiling off, and cooling the atmosphere enough for water vapor condensation to be visible. The white frost, of course, is water ice, condensed and frozen. The effect works much better with a metal can. Fill it with LN2 and LOX condenses on the outside. This can actually be a bit of a hazard if you leave your container open, because LOX will condense on the inside, and the LN2 evaporates faster, so the concentration of LOX increases, and if there's something reactive with oxygen in there, oops.

LOX, btw, is a very, very pretty blue color.
posted by eriko at 5:12 AM on September 24, 2012 [44 favorites]


"What's interesting is that the garbage can flys up as well which I am guessing is because of the ping pong balls providing thrust which forces the can against the floor which causes an elastic-inelastic collision that makes the can bounce in the air ?"

The reason the can flies upwards, in four easy steps:

1) Explosion of the soda bottle creates a shockwave which expands spherically around where the explosion was.
2) Shockwaves being what they are (and I can go into significant detail if you want, but the gist is that information doesn't pass the wave, so what's ahead of the wave doesn't know what happened behind the wave), it pushes the water ahead of it, leaving a low pressure area behind.
3) Shockwave reflects off the bottom of the trash can and pushes all the water up and out. This happens so quickly that it creates a vacuum at the bottom of the trash can, since the water is pushed away but there's nothing there to replace its volume.
4) The vacuum literally sucks the garbage can off the ground.

There's no collision because the can and the ground are already in contact with each other.
posted by backseatpilot at 5:18 AM on September 24, 2012 [5 favorites]


Still on YT here.
posted by unSane at 5:20 AM on September 24, 2012


a biscuit burnt in liquid oxygen (complete combustion, so a very bright flame, no biscuit left and no smoke to set off the smoke alarms)

Sounds like Professor John Salthouse, who used to give a talk called 'Son et Lumiere', which involved making lots of loud bangs and explosions.

I saw him perform at an SF con in Glasgow about 20 years ago, and he took all his props up from Manchester on the train, including a thermos flask filled with liquid oxygen. He told us that the British Oxygen Corporation wouldn't provide him with any LOX because of his reputation as a pyromaniac. However, he *was* able to get hold of liquid nitrogen, and because the boiling point of nitrogen was less than that of oxygen, he was able to use it to generate his own LOX.

There's a video of fairly shaky quality here.
posted by daveje at 5:27 AM on September 24, 2012


@eriko, you are my new hero. I tried to compose a reply to this effect but didn't have the science or eloquence to do so. The over-protection in modern life due to bogus health and safety concerns based on going through the motions (I'm sorry, that should be "ordinary commercial health and safety protocols") rather than honest risk assessment should be a worry to intelligent people everywhere.

BTW I did notice that at one point he had a glove on and held the other but had removed them by the time it came to pouring out.
posted by epo at 5:29 AM on September 24, 2012


BackseatPilot, "4) The vacuum literally sucks the garbage can off the ground." is the actual answer. There's a diagram I saw over the summer that explains this in much detail.
posted by Old'n'Busted at 5:41 AM on September 24, 2012


Call me stupid, but I can't for my life work out where the energy for the explosion comes from. I understand that the gas expanding causes the bottle to explode, but *where does the energy come from*?

/bad geek
posted by zoo at 5:42 AM on September 24, 2012


That blowed up real good!
posted by Obscure Reference at 5:43 AM on September 24, 2012


zoo -- I *think* it's because kinetic energy is expended as a material goes from liquid to gas phase (that's why the gas molecules are so active). Heat/energy is consumed(?) as the matter changes phase state.

(My chemistry knowledge is weirdly specific and narrow, though, so this is a guess supported by some quick & light research.)
posted by kalimac at 5:59 AM on September 24, 2012


zoo: "Call me stupid, but I can't for my life work out where the energy for the explosion comes from. I understand that the gas expanding causes the bottle to explode, but *where does the energy come from*?

/bad geek
"

You're supposed to fill your car tires to 30-40 psi. Imagine if you could fill them to 1000 psi very quickly. The nitrogen gas takes up a LOT more space than liquid nitrogen.
posted by double block and bleed at 6:09 AM on September 24, 2012


Call me stupid, but I can't for my life work out where the energy for the explosion comes from. I understand that the gas expanding causes the bottle to explode, but *where does the energy come from*?

It's the phase change. To go from liquid to gas, the nitrogen must absorb a ton of energy from the surrounding environment. Some of that energy is expressed as potential energy by pressurizing the container. When the container fails, that potential energy turns into kinetic energy.

Interesting side experiment/ explanation. If you have a giant tank of CO2, it is stored as a liquid because the tank CAN contain the vapor pressure of CO2. However, if you are able to flash off the pressure in the tank faster than the liquid CO2 can absorb energy from the surrounding environment, the CO2 will turn solid. When you cap the container again, it will slowly absorb energy, build pressure and melt back to a liquid.
posted by gjc at 6:09 AM on September 24, 2012 [2 favorites]


Just think of the liquid nitrogen as a highly compressed spring (at room temperature and pressure).
posted by unSane at 6:10 AM on September 24, 2012


The explosion happens because the liquid turned into a gas.
The liquid turned into a gas because it warmed up.
The warmth came from the bucket of hot water.

Therefore, the energy for the explosion came from the water, which cooled down slightly.

Water can hold a *lot* of energy in the form of heat.
posted by seanmpuckett at 6:15 AM on September 24, 2012 [1 favorite]


No, that's not right. To turn nitrogen gas into liquid nitrogen it had to be cooled and/or compressed. Both of those things require a lot of energy.

The warmth of the water increased the pressure inside the bottle (cooling slightly in the process), which shattered the container. Then the energy that was used to condense the nitrogen gas is then released.

Imagine if you'd simply broken the glass with a hammer, instead of using the warm water. The explosion would have been just the same.
posted by unSane at 6:18 AM on September 24, 2012 [1 favorite]


See Latent Heat, in particular of nitrogen changing from liquid to gaseous phases, about 200 kJ/kg, which is a lot of energy.
posted by unSane at 6:21 AM on September 24, 2012


I understand that the gas expanding causes the bottle to explode, but *where does the energy come from*?

The LN2, being warmed by the very warm (to it) water, boils off. LN2 gas expansion ratio, at 20 °C, is 1:694. So, for every cubic centimeter of LN2, you have, there will be 694 cubic centimeters of nitrogen gas, at standard pressure.

However, the bottle is sealed. Unlike liquids, gases are very compressible, so that 694 cm3 can be packed into a space. Since the bottle is sealed, that space is well defined and basically unchanging -- though I know the bottle will bulge a bit, I'm going to handwave that away. So, we're now dealing with a gas in a sealed container, where the amount of gas is increasingly rapidly, but the volume does not. Good old Boyle's Law is pV=k, and since k (the amount of gas) is increasing, and the volume is not, pressure must rise. It does so, and with that 1:694 expansion ration, this rapidly increases the pressure in the bottle.

Your typical PET bottle can handle about 120 PSI before it bursts. We know we reach that pressure because, well, it burst. What's the bottle's surface area given, as an educated guess a 3" diameter, 9" tall cylinder?(Yes, it's taller, but tapers. This is also a handwave.) Surface area = πr2 * 2 + 2πrh, or about 7in2 * 2 + 85 in2, or 98 square inches. Call it 100. (Told you, handwave.)

So, you have 100 square inches, the pressure is 120 pounds per square inch, so the total pressure on the outside of the bottle -- that is, the total force being exerted on the container when it fails, is 12,000 pounds force (lbf), or about 52KN. This is a lot of force, and it's put into the water in a very brief amount of time. That's what does the work, work being defined, of course, as force acting over a distance. When you look at the amount of mass being moved (a fair bit, with the water) and the time the impulse takes (very short) you can figure the acceleration -- F=mA, remember? Since A, acceleration, has a time component, since it's defined as ΔV/ΔT. Since the change in time is very short, the change of velocity is very high. I don't have exact number, because I have no idea what the bin masses what ~1500 ping pong balls mass, or worst, how much water was put into the bin, but it's obvious that the velocity was pretty decent, right?

Thus, the balls, they fly.
posted by eriko at 6:35 AM on September 24, 2012 [5 favorites]


You FOOLS. The energy comes from the balls changing from a state of "ping" to a state of "pong". Pong is the ground state of the ball-on particle and ping is the higher energy state, and they all go from ping to pong quickly and radiate energy in the form of smaller particles, namely marbles. That why when you see this you go, "Holy shit, that guy must've lost his marbles!".

I have a Sports Science degree, so I'm your man if you wanna know "the complete balls" on such topics.
posted by the quidnunc kid at 7:03 AM on September 24, 2012 [10 favorites]


I found it interesting that a science class in Plymouth University (in southwest England) was 100% comprised of Asian students. Or might this demo have been for a tour group of some sort?

(On a trip to China earlier this year, I separately met two recent college grads - mid-20s - who each had gone to the University of Kent. Intrigued about the recruitment strategy today in English universities.)
posted by mark7570 at 7:45 AM on September 24, 2012


...err, unless, of course, this was Dr Roy Lowry of Plymouth University giving a demo, but not at Plymouth University.
posted by mark7570 at 7:47 AM on September 24, 2012


Chicken/egg: was he just repeating a segment he saw on Ellen or was it vice-versa?

And now that I'm thinking about it, I'd like to see the experiment done with eggs. Or chickens.
posted by item at 8:00 AM on September 24, 2012 [1 favorite]


But surely the system needs energy for the state change? If I add salt to ice, it causes a drop in temperature as the water melts.

The energy can't be in the liquid nitrogen surely? Maybe it does get the explosive force from the water. The water cools down, the gas heats up and a stuff left over goes into making the bang.

If this is the case, then either water holds a shit tonne of energy, or these kind of explosions don't actually contain that much energy.
posted by zoo at 8:00 AM on September 24, 2012


The energy can't be in the liquid nitrogen surely?

That's exactly what latent heat is.
posted by unSane at 8:07 AM on September 24, 2012


According to the latent heat link...

...the change is endothermic, meaning that the system absorbs energy on going from solid to liquid to gas.

So the energy isn't coming from the phase change. In fact, the phase change requires even more energy from its surroundings to work.

I realise I'm doing that thing where I ask for people to explain something, and then I disagree with their explanations. Apologies for that, but apart from "it comes from the water", I still can't see where the energy for the explosion comes from.
posted by zoo at 8:27 AM on September 24, 2012 [1 favorite]


Mind you, if you're just some kid seeing this on the internet, you do risk jail time. Coworker's kid was doing dry-ice bombs off the deck with a bunch of buddies, and the cops do respond to those kinda hijinks.
posted by k5.user at 8:32 AM on September 24, 2012


If you're willing to wait a bit you can go something like this with 16 ounces of guava juice. I came back from vacation to find a perfectly spherical plastic bottle of guava juice in the refrigerator. I put on goggles and carefully picked it up with a tennis racquet, and dropped it out the window. It detonated, and left a clean straight line of fermented guava up and over all six stories of the building across the street.
posted by StickyCarpet at 8:38 AM on September 24, 2012 [4 favorites]


The liquid N2 is at -321 °F. Let's say the warm water is 110°F. That means the differential is 431 °F.

Let's say the room you are in is at 70 °F. Imagine the amount of energy in a half liter of 501°F material -- that's the equivalent temperature differential (~450 °F is the point at which wood burns).

To use the spring analogy above, springs can store energy by being compressed, but they can also store energy by being stretched. In the same way, very hot things can store energy, and so can very cold things.
posted by fings at 8:40 AM on September 24, 2012 [2 favorites]


3) Shockwave reflects off the bottom of the trash can and pushes all the water up and out. This happens so quickly that it creates a vacuum at the bottom of the trash can, since the water is pushed away but there's nothing there to replace its volume.
4) The vacuum literally sucks the garbage can off the ground.


In order for that to be the case, since vacuum does not by itself move anything at all, air pressure at the bottom of the trashcan would have to be providing the force to lift the can off the ground. But that can't be the case because air pressure acts primarily downward unless it has enough space and the air pressure in the space between bottom of the can and the ground contact points is likely insignificant.

An alternative explanation to my action-reaction guess might be that the explosion flexes the bottom of the plastic garbage can outwards thrusting the can into the air possibly.
posted by Podkayne of Pasadena at 8:44 AM on September 24, 2012 [1 favorite]


The science has all been explained nicely here, but what is a "drinks bottle"?
posted by orme at 8:47 AM on September 24, 2012


Zoo, if I'm understanding your question correctly, one way of looking at it is that the energy originally came from the process that liquefied the nitrogen. It takes a lot of energy to suck that much heat out of something; you get the energy back (modulo the 2nd law of thermodynamics) when the heat equalizes.

Hm, I've explained that badly. OK, think of a river. Now think of a waterfall. The river in this case is the heat of the system, it's just kind of lying around -- maybe we should be thinking of a lake rather than a river, something with no motion in it. But if you dig a giant hole at one end of the lake, you will get a waterfall, and that falling water will have a lot of energy associated with it, until it fills up the hole and the system is at equilibrium again. In order to create the disequlibrium that causes the potential energy to convert to kinetic energy, you need to dig that hole. In heat terms, the LN2 is the hole.

(I am, obviously, not a scientist.)
posted by KathrynT at 9:41 AM on September 24, 2012


unSane: The warmth of the water increased the pressure inside the bottle (cooling slightly in the process), which shattered the container. Then the energy that was used to condense the nitrogen gas is then released.

This is almost exactly backwards.

Heat is energy. Energy is heat. They're the same thing. There's no such thing as negative energy. You can move it around, but you can't make negative amounts of it. (at least, as far as we know at present, barring something really bizarre and probably well beyond our current technology levels). You can't destroy energy, or create it, you can only move it around, and store it in various forms. (matter being one of the largest known reservoirs.)

When they cool the nitrogen down, they are removing heat, and thus energy, from it. It takes a lot of expended energy to remove the energy from the nitrogen. The 'hardest part' of this process is typically at the phase change in most materials; if you cool water to 32F, it will require that you remove a great deal more heat from it before it turns to ice, and likewise, ice will stay at 32F for a long time while it melts, gradually absorbing heat from the environment. That's why it works to chill your drink... it floats around for a long time, precisely at 32 degrees, but losing mass steadily as ice converts to water.

Now, I'm mixing up my terms a little bit here. I'm comparing nitrogen's transition to liquid with liquid water's transition to solid, which is mixing my terms a bit. It would be more accurate to compare it to condensing steam back into water. But we don't have the experience with boiling water and steam that we do with ice, so I'm substituting the slightly weaker liquid->solid analogy instead.

Ok, so, they spent a lot of energy pulling the energy out of that nitrogen. You can think of it like squeezing water out of a sponge; they've been squeezing energy out of the nitrogen, so much that it turns to liquid. If they cooled it even further, it would freeze solid, just like ice. It's the same vapor -> liquid -> solid transition that water goes through. By the sponge analogy, liquid nitrogen is like an unbelievably dry and thirsty sponge, willing to soak up large amounts of heat from the environment. Just like you can use liquid water to cool something hot, possibly boiling the water if the object is hot enough, and turning it to steam, you can use liquid nitrogen to cool things to much lower temperatures, almost certainly boiling it in the process, because it boils at -321F, instead of +212F.

So, when he fills that bottle with LN2, and drops it in warm water, he's putting an extremely low-energy substance into a substance with quite a bit of heat energy, which it immediately starts transferring to the nitrogen. As the nitrogen melts, it expands a great deal. The vapor of the nitrogen gets compressed into the tiny bottle, and increases in pressure very rapidly, likely increasing its heat very rapidly as well, which in turn will boil off more liquid nitrogen. This runaway process eventually results in an explosion.

But almost all the power to cause the explosion comes from the water. Water can just hold a LOT of energy. A little bit comes from the bucket, and from the air, but the great most of the energy in the explosion was already in the water. If the bottle had been placed somewhere that it couldn't gain energy, like more liquid nitrogen, or something even colder, it would not have exploded.

Oh, and as far as the actual explosion and why the bucket comes off the ground; I think you can basically divide the explosion into two rough hemispheres, upward and downward. The upper hemisphere blows water and ping-pong balls upward; that's what causes the big showy part of the process. The other half of the explosion goes downward, hits the bottom of the barrel, hits the floor, and then because the floor doesn't shatter, is reflected back upward again. So you've got a strong push from the floor into the bucket, combined with a partial vacuum of evacuated water right in the middle, so the bucket leaps off the ground, shoved by both the reflection of the explosion and the vacuum in the water. At the same time, the top of the water is sucked down, but since so much of the upward energy from the explosion went into the pingpong balls, the remaining slap-back on top doesn't have much stopping power, and probably retards the bucket's launch only a little.
posted by Malor at 9:47 AM on September 24, 2012


I think there's some confusion between the entropy in the system and the energy in the system. I'm pretty happy with the (dismissed) answer that the energy is in the water. But I'm still aware that I either don't understand what I'm asking or I'm asking a stupid question in a particularly stupid way. Or my understanding of thermodynamics is utterly messed up. That's an option.

on preview: malor's answer makes the most sense to me.
posted by zoo at 9:51 AM on September 24, 2012


Oh, you know, maybe I should have stuck with the water->steam analogy instead.

Think of a steam locomotive. That huge tank on the front is filled with water, right? It's liquid water, just like liquid nitrogen, only warmer. So if the engineer adds heat underneath the water, it starts to steam, expanding, and creating pressure in the tank. That pressure is released in a controlled way to drive the wheels, but if the fires were just left burning, the tank would eventually explode, just like that little plastic bottle did.

The energy isn't in the water. The energy is in the fuel heating it. Without the heat input, that tank would just sit there until it rusted through. It would never explode. But add enough heat to it, and it will rupture catastrophically, because of the enormous pressure from the water -> steam phase change.

Likewise, that LNO would just sit there, happily, as long as the surrounding temperature was kept low enough. You're thinking of it as 'having energy', because if it absorbs enough energy from the environment, it can explode just like a water->steam transition can. It is, however, using input energy to do this work, just like the water is. Keep the water below its boiling point, and nothing happens. Keep the LNO below its boiling point, and nothing happens. But add energy to either, and you can get ping-pong ball showers, or giant steam-boiler explosions.
posted by Malor at 9:54 AM on September 24, 2012 [1 favorite]


In order for that to be the case, since vacuum does not by itself move anything at all, air pressure at the bottom of the trashcan would have to be providing the force to lift the can off the ground. But that can't be the case because air pressure acts primarily downward unless it has enough space and the air pressure in the space between bottom of the can and the ground contact points is likely insignificant.

This is incorrect, sorry. Pressure acts equally in all directions, but that's really a red herring here.

Your system comprises two parts - the trash can and the contents inside, which for the purpose of examination can simply be water and nothing else (we can ignore the soda bottle and the nitrogen gas). In an instant, the mass of water inside the trash can is pushed upwards just slightly from the shockwave of the explosion. That movement creates a gap between the bottom of the slug of water and the bottom of the trash can. There is nothing to fill this gap, because the water creates a tight seal around the trash can's sides, so in this gap is a vacuum. The vacuum draws the water and the trash can back together again, but because the can is much less massive than the water it moves farther and jumps into the air.

Air pressure outside the trash can has no bearing on this. It's the same reason a plunger works - there's a tight seal between the plunger and the toilet bowl/sink/whatever, and when you draw up on the plunger it creates a volume of space where there wasn't any. The plunging process is a lot slower than this near-instantaneous thing that happens in the video, so rather than stuff flying into the air it (hopefully) pulls the liquid up the pipes to fill the void.
posted by backseatpilot at 9:55 AM on September 24, 2012


the great most of the energy

Sigh.
posted by Malor at 9:58 AM on September 24, 2012


Sorry, slightly incorrect what I just said. Ambient pressure has a bearing on this, but the real thing at work here is the vacuum in the can.

Nature hates a vacuum.
posted by backseatpilot at 9:59 AM on September 24, 2012


Yet another example: if he'd had a big can filled with, say, molten lead, and he'd dropped a can of water into it (one, obviously, with a higher melting point than lead's), he'd have gotten more or less the same result, although I suspect the container probably wouldn't move very far. The water container would be much stronger than a plastic bottle, so the explosion would be much more intense, but lead is very heavy. And the splashes of molten lead would probably not be very healthy for the audience.

But it would do basically the same thing, and the energy would be coming from the lead, not from the water. Likewise, in this experiment, the energy is coming from the water, not the liquid nitrogen.
posted by Malor at 10:06 AM on September 24, 2012


Ambient pressure has a bearing on this, but the real thing at work here is the vacuum in the can.

No, the real thing here at work is the water, moving upwards, along with the bin. Remember, work is force over distance, force is mass time acceleration. Vacuum doesn't have mass, therefore, it doesn't do work.

The vacuum created by the shockwave pulls the bin towards the moving water, dragging it along, but the KE to lift, and keep lifting, the bin up is pretty much all in the water that was pushed out and up by the shockwave created by a very short period 50KN release of force. Some of that water was then pushed up by the reflected shockwave, which is why there was little down component to the force.

Of course, we should mention that there was a source of force pushing the bin down. Well, two, the first is our good friend gravity. The second? The ping ball balls flying upwards. What do we call an open ended container throwing mass in one direction? Well, a rocket, because every action has an equal and opposite reaction. So, the force that pushed the ping pong balls up pushed the bin and water down. However, it's obvious that the KE of the water moving up was much higher, and that's a simple mass function. (1 ball= 2.7g, 1500~= 4kg!) -- but the mass of the bin+water would be much higher. Same force, different mass, thus, different velocities. (Ke=1/2mv2, of course.)

Arguably, the energy was put into the entire system when the nitrogen was condensed. Yes, the energy *flow* is from the water to the LN2, but that flow would not have happened unless the energy to condense that nitrogen had already been spent.

But if you take the liquid nitrogen as a given, then yes, the energy comes mostly from the water (a little from the air, and a little from the bin. Mostly the water, though.)

If you start with N2, you put in the energy to get the LN2, but the source of energy was condensing the nitrogen, which allowed it to pull the energy out of the water, heating the liquid nitrogen and cooling the water. The water has to get that energy from somewhere, and from the waste heat of condensing the nitrogen is as good a place as any, closing the cycle.
posted by eriko at 10:46 AM on September 24, 2012


The vacuum created by the shockwave pulls the bin towards the moving water

Right, my point was that if you performed this demonstration in a vacuum the void created in the trash can wouldn't have the same effect - you wouldn't have the pressure differential. You'd probably still get a small jump from frictional forces, but the can's not going to fly in to the air like it did.

Remember, work is force over distance, force is mass time acceleration. Vacuum doesn't have mass, therefore, it doesn't do work.

It's not strictly the vacuum, per se, but the pressure delta.
posted by backseatpilot at 10:55 AM on September 24, 2012


It seems quite obvious to this mechanical engineer that the can achieves its height by the swelling and deformation of the bottom surface, which propels the can upwards. This requires the bottom surface to be compliant. It does not require a collision or for the can and floor to be initially separated.

backseatpilot: your argument that a lower-than-atmospheric internal pressure draws the can upwards doesn't make sense to me. It seems to say that if the can were positioned sideways with its opening pointing left (and nothing holding it in place), that the balls, the can, and the nitrogen would all go left. How would momentum be conserved in this case?
posted by Mapes at 11:20 AM on September 24, 2012


If I were an educator and I did something cool I would never post a video to the internet. Like, why would I want to deal with people who have no experience or training in my field tell me I'm doing it wrong?

And suggest that there be consequences for my career?
posted by danny the boy at 11:38 AM on September 24, 2012 [1 favorite]


How would momentum be conserved in this case?

The problem tipping it sideways is that the water won't stay in the can. You need the water for this to work.

If you change the problem slightly, you can see how it works. Take a cylinder - maybe an oversized tin can - and put a slug in it that fits perfectly inside the can. The slug sits at the bottom of the can (see what this looks like?). Tip the whole thing sideways so the can rests on its side.

Now, the slug still rests against the "bottom" of the can while the can is on its side. But now instantaneously slide the slug out towards the opening of the can. In that instant the slug moves but the can stays still, which creates a vacuum in the can - since the slug was flat against the bottom of the can originally, there was zero air between the slug and the bottom of the can. Now there's a large volume of vacuum between the slug and bottom of the can. Here is where conservation of momentum kicks in - both the slug and the can start to move towards each other. However, the can is a lot less massive than the slug, so it travels faster (and is moving in the direction of the can opening).

Of course, the can is opaque, so you don't see the slug moving. All you see as an outside observer is the can rapidly moving towards its opening. Same thing happens with the garbage can filled with water.
posted by backseatpilot at 11:44 AM on September 24, 2012


I have the strangest craving for a box of chocolate nonpareils...
posted by Splunge at 12:04 PM on September 24, 2012


...because really, think about how fun it'd be to watch if those had been eggs instead of ping pong balls. The answer is lots of fun, I can assure you. Sure, you'd need bigger containers and more of the magic evil potion, but lots.

Or, once again, live chickens instead of ping pong balls. Think of the fun. think of it. G'head. Do it.

Lots.
posted by item at 12:08 PM on September 24, 2012 [1 favorite]


Sorry, backseatpilot, I don't buy it. The center of mass of the original system can move upwards only through interaction with the ground. The contraction of a volume internal to a system cannot levitate its center of mass, period.

Your scenario might apply for a can that jumps an inch or two. For a can that jumps more than its own height, not a chance.

I'm wondering why you think sudden bulging of the bottom of the can during the explosion is not sufficient to launch the can?
posted by Mapes at 12:58 PM on September 24, 2012


The center of mass of the original system can move upwards only through interaction with the ground.

Not true; the center of mass of the system is moved by the shockwave pushing the slug of water into the air. The can will rush up to meet it (because of the aforementioned void created between the water and the bottom of the can) but the water is still moving so the can is just going to keep following it upward.

You could tie that whole system to a rope and hang it from a tree and it would do exactly the same thing.
posted by backseatpilot at 1:02 PM on September 24, 2012


Good lord, I knew I saw something about this recently: xkcd.
posted by backseatpilot at 1:21 PM on September 24, 2012 [1 favorite]


1 ball= 2.7g, 1500~= 4kg!
That's the number for tournament-quality balls, which retail for over $100/gross.
If I were buying for this experiment, I wouldn't be using 3-star balls. I'd be getting a bunch of beer-pong balls from eBay, which weigh less. They also may have used 38 mm balls, which are even cheaper if you can still find them. That would be around 3.5 kg.
posted by MtDewd at 1:49 PM on September 24, 2012


So this is as much a demonstration of a woeful ignorance of ordinary commercial health and safety protocols as it is a cool way to explode ping-pong balls into the air.

Pfft! Can't kids just have fun anymore.

Science is kool.
posted by BlueHorse at 1:54 PM on September 24, 2012


People are obviously having a bit of trouble with the question of "where the energy comes from" in this setup.

It might help to think in terms of work and potential difference instead. Remember that in a heat engine (and this is a heat engine) work can only be done as a consequence of a *flow* of heat energy from a hotter body to a colder one. The total work done is a function of the heat differential between the two which defines the maximum possible thermodynamic efficiency of the engine and the total energy flow. (Real world heat engines always have a lower efficiency than the thermodynamic limit of course.)

Now, in order for a heat engine to work, you need to have a heat differential between your hot thing and your cold thing. In the heat engines we use every day (like a car engine) we do this by heating some gases (by burning petrol or diesel) and dumping the heat into the atmosphere which acts as a heat sink. In the case of the liquid Nitrogen and the water, we've created the heat differential by cooling the liquid Nitrogen.

You could think of it by analogy with a ball rolling down a slope: for there to be a slope, either we have to push the ball up a hill and let it go, or alternatively we have to dig a hole and let the ball roll into the hole. In this case, we've dug a hole (made the Nitrogen very cold) and the work (throwing water and ping pong balls everywhere) is done as the nitrogen and the water come into thermodynamic equilibrium by heat transfer from the water to the Nitrogen.

Does that make sense?

(Oh, and I'm pretty sure that Malor is correct that the reason that the vessel launches into the air is because the shockwave reflects off the bottom and creates a vacuum underneath, allowing air pressure to push the vessel up as the water flies upwards. No deformation required: this would work even if the vessel was completely rigid. (I have a suspicion that if it could be done by deformation then the impulse would rip the bottom off the bin, but I haven't run the numbers to prove that.)
posted by pharm at 1:59 PM on September 24, 2012


)
posted by pharm at 1:59 PM on September 24, 2012 [1 favorite]


StickyCarpet: If you're willing to wait a bit you can go something like this with 16 ounces of guava juice

1. Eponysterical
2. Orange juice bottles in a hot car work well for this, too. But smell more, I am pretty sure.
posted by SLC Mom at 2:55 PM on September 24, 2012


They had a pretty strong can, there.

When I was an undergrad we may or may not have broken one of the trash cans in our study area doing approximately the same thing..
posted by nat at 3:46 PM on September 24, 2012



I realise I'm doing that thing where I ask for people to explain something, and then I disagree with their explanations. Apologies for that, but apart from "it comes from the water", I still can't see where the energy for the explosion comes from.
posted by zoo


This is quite a good question.

Unlike an ordinary combustion driven explosion, in which chemical energy is converted into heat, causing a net heat energy increase in the explosive taken together with the surroundings, in this explosion heat energy is converted into a kind of chemical energy, namely the latent heat of vaporization of the nitrogen, resulting in a net decrease in heat content of the explosive and the surroundings all taken together-- nor does the conversion of heat to chemical energy drive this explosion.

Explosions and implosions result from huge imbalances of forces, not necessarily large releases of energy.

I think Podkayne is right that the can jumps into the air when the bottom flexes against the ground.
posted by jamjam at 4:45 PM on September 24, 2012


namely the latent heat of vaporization of the nitrogen, resulting in a net decrease in heat content of the explosive and the surroundings all taken together-- nor does the conversion of heat to chemical energy drive this explosion.

The total amount of energy never changes, it just moves around. Assuming no other changes, if there was X amount of energy in that auditorium when he dropped the nitrogen into the can, there was X amount of energy after the explosion settled, just in different places.

In this case, the conversion of heat, mostly from the water, to phase change in the nitrogen, to pressure against the container, to eventual rupture, is what drives the explosion. The warm water is like fire under a steam boiler.
posted by Malor at 5:54 AM on September 25, 2012



The total amount of energy never changes...

That's why I said "heat content" rather than energy, Malor.

However, from a classical point of view, even that constancy of energy is disputable.

As I understand it, heat energy that goes into chemical bonds, such as in this case, the heat energy that goes into breaking the bonds that hold the nitrogen in a liquid state (considered separately from the energy that raises its temperature) actually shows up as increased mass of the nitrogen in the gaseous state.
posted by jamjam at 2:16 PM on September 25, 2012


That blowed up real good!
posted by Obscure Reference at 1:43 PM on September 24 [+] [!]


Eponysterical?
posted by klausness at 4:05 PM on September 25, 2012


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