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Flying circus of physics

Chap 2 (fluids) archived stories part E

Friday, February 06, 2009


For Chapter 2, this is part D of the new stories and also the updates to the items in the book, including many video links and journal citations. If you want all the video links (hundreds) and journal citations (thousands) for this chapter, go to

First, a list
--------- New items (not in the book):
2.176  Pub trick --- egg tricks
2.177  Pub trick --- raisin in champagne trick
2.178  Pub trick --- oil blobs in water
2.179  Pub trick --- piercing a plastic bag of water with a pencil
2.180  Pub trick --- bottle cap blown into bottle
2.181  Fake video --- free standing water
2.182  Controlled breach of the Condit Dam
2.183  Pub trick --- separating two pub glasses
2.184  Pervious concrete
2.185  Fly in gun blast
2.186  Attack of the tumbleweeds
2.187  Pub trick --- pouring a rainbow
2.187  Pub trick --- pouring a black and tan
2.188  Poured art
2.189  Escape from a sinking car
2.190  Pub trick --- beer-spots in bottle caps
2.191  Water is heavy
2.192  Pub trick --- adding pins to a full glass of water
2.193  Pub trick --- a puzzle in evaporating wine
2.194  Pub trick --- fast pouring beer or soda
2.195  Boyle's perpetual motion device
2.196  Pub trick --- opening a wine bottle with a shoe
2.197  Pouring water in an inverted plane
2.198  Royle's sefl-pouring teapot
2.199  Sloshing
2.200  Spiral arms of water
2.201  Spin painting

2.202  Disaster: the Boston Molasses Flood

Now the stories:

2.176  Pub trick --- egg tricks
Jearl Walker
July 2011  Here are three pub challenges involving hard-boiled eggs.

Separating the egg from the shell. The common way is to knock the egg against the table and then peel the cracked shell from the egg. However, this procedure is frustrating because it is slow and usually requires picking off small bits of the shell. Is there any better way, a way in which you can separate the egg and shell in a few seconds?

Egg into a bottle. Once the shell is off a hard-boiled egg, position it with its sharply-curved end down on the opening of a fairly wide bottle (wider than a beer or soda bottle, but narrower than the egg). Can you make the egg go into the bottle without cutting the egg up into small pieces?

Egg out of a bottle. Once the egg is in the bottle, can you get it out without breaking the bottle?


Tap each end of the egg on the table so that an end piece can be removed, each about the size of a small coin. Roll the egg over the table so to crack it and to separate the egg from the interior surface of the shell. On the end with the smaller opening, encircle the opening with your thumb and index finger, press your mouth snuggly against the thumb and finger, and then blow hard. The egg should pop out the other opening. No hard physics here ---- on your end of the egg you increase the air pressure above the atmospheric air pressure on the other end. The pressure difference shoves the egg through the larger opening. Because the egg is an elastic solid, it can be distorted enough to slide through the opening without being ripped apart.

To get the shelled egg to slide into the bottle, drop a bit of burning paper into the bottle and then position the egg over the mouth, pressing down on it slightly to get a tight seal. While you are moving the egg into place, the fire heats the air inside the bottle, causing it to expand. Some of that air is forced out of the bottle. As the fire burns out (due to the elimination of the paper or the oxygen in the bottle), the air remaining in the bottle begins to cool, which causes the air pressure to decrease. The air pressure on the lower end of the egg is then less than the atmospheric air pressure on the top end of the egg, and the pressure difference shoves the egg into the bottle. Again, the egg can squeeze through a narrow opening because it is a fairly elastic solid.

To get the egg back out of the bottle, tilt the bottle up so that the egg slides down near the opening. Then blow hard into the bottle. As you blow, the air lifts the egg away somewhat, but as soon as you stop, the egg slides back into place near the opening, sealing off the escape route of the air. The air pressure on the top end of the egg is then greater than the atmospheric air pressure on the bottom end, and the pressure difference shoves the egg through the opening.

more links


2.177 Pub trick --- raisin in champagne trick
Jearl Walker
August 2011   If you drop a raisin into water, it simply sinks to the bottom and is rather boring. But if you drop it into soda, clear beer, or champagne (for those special times when the celebration requires a raisin or two), the raisin will repeatedly move up and down. Here is a video taken when apparently the excitement required a raisin:

The champagne (or soda or beer) has lots of dissolved carbon dioxide. Some of those molecules can come together to form bubbles (the characteristic bubbles of champagne) but they need some nucleating site for the bubble formation. The bubbles cannot easily form in the liquid itself because such a bubble is initially tiny with a large curvature of its surface. That large curvature means large surface tension, which collapses the bubble. So, although the bubble tends to expand by collecting more and more carbon dioxide molecules from the liquid, it is almost immediately collapsed by the surface tension.

Instead the bubbles form in tiny crevices on the glass surface or on contaminates lying on the surface, or, in the case of our party girl and her glass of champagne, on the surface of a raisin. In such cases, the initial is not spherical but is down inside a crevice, and the crucial feature to its survival is that its surface with the liquid is not highly curved but only slightly curved. So, the surface tension is not large enough to immediately collapse the bubble. Instead, it remains in place as more and more carbon dioxide molecules pass (diffuse) into it. Eventually it is large enough that it is no longer in danger of collapse, even if the curvature of its surface is large.

As bubbles form and then grow on the raisin, they produce an upward buoyant force that is large enough to bring the raisin to the champagne’s upper surface. As soon as the raisin reaches the air, many or most of the bubbles pop open and release their carbon dioxide. Thus, the raisin loses much of it buoyancy and sinks to the bottom of the champagne. This cycle repeats until the champagne loses most of its dissolved carbon dioxide (goes flat) and bubbles stop forming.

If you want to experiment, try various seeds and other small objects in a carbonated drink. Or, using a carbon dioxide dispenser as is now commercially available, carbonate a glass of water.

Dots · through ··· indicate level of difficulty
Journal reference style: author, title, journal, volume, pages (date)
· Stavrinidis, V., “Dancing lemon seed,” The Physics Teacher, 40, 286 (May 2002)

2.178  Pub trick---oil blobs in water
Jearl Walker
January 2012   Here is another trick from the delightful book by Eric Muller of the Exploratorium in San Francisco. Pour enough water into a clear drinking glass to fill it to about 75% capacity. Then pour in salad oil to make a layer about 1 centimeter deep. (Muller suggests olive oil or sesame oil; I used peanut oil.) The oil floats on the water because it is less dense than the water. The challenge is to make blobs of the oil descend into the water and then rise back up to the oil layer, without your touching the glass or stirring the contents.

Here is the answer: Sprinkle salt into the glass. Salt grains are denser than both oil and water and thus will sink through the oil and then begin to sink in the water. As they move through the oil layer, some of the oil clings to the salt grains and is pulled downward with them. The descending oil is in blobs surrounding salt grains.

As the grains descend, then also begin to dissolve, and then salt molecules (sodium chloride) diffuse (spread) through the oil. As the molecules begin to diffuse out of a blob and into the surrounding water, the density of the oil blob decreases until it is less than that of water. Then the blob floats back up to the oil layer, merging with it.

When I heavily sprinkle salt on the oil layer, a dimple forms on the top surface, under the weight of the salt. I can then see salt grains drawn into the dimple and down through the oil, where another dimple forms on the water-oil interface.

The blobs that break free of the oil layer and descend through the water to the bottom of the container are noticeably opaque because of the salt grains they hold. But as a blob sits on the container bottom, its top portion clears because the salt descends to the bottom of the blob. When a blob breaks free of the container bottom and begins to ascend, it is noticeably clearer.

If I adjust the illumination correctly, I can see a downward flow of the water along the center even when there is no blob activity. Salt molecules are entering the water from the oil in that region, and the heavy salty water then descends. When a blob breaks free of the container bottom and bobs back up to the oil layer, it can leave a trail of descending salty water along its path.

If I add enough salt to the glass container, all this action noticeably slows down or even dies out. The salt concentration in the water is then so large that the salt molecules in a blob will no longer diffuse out of the blob and thus the density of blob does not change. The show is then over, and if you are doing all this in a pub or restaurant, maybe it is time to leave before the owner sees the mess you have made.

Muller, E., While You’re Waiting for the Food to Come: Experiments and tricks that can be done at a restaurant, the dining room table, or wherever food is served, Orchard Books (1999)

2.179  Pub trick --- piercing a plastic bag of water with a pencil
Jearl Walker
March 2012 Partially fill a plastic food bag (such as a Ziploc bag) with water and then seal it (close the interlocking plastic ridges and grooves at the top). Can you tear the bag below the water line without losing any water?

A similar question was asked at the onset of World War II by everyone flying aircraft over enemy territory (on both the Allied and German sides). If ground fire hit a fuel tank, the hole would allow fuel to leak out, which would limit the flying distance. Even more important, the leakage could cause a fire or explosion. The leakage was especially a problem where a round exited a tank, because its penetration into the tank probably set it tumbling and thus it would rip out a larger hole at the exit point than at the entrance point. One obvious solution was to attach heavy plates to a tank, to stop the rounds, but the large, extra weight was just impractical on any long-range flight.

The acceptable solution for the Allied aircraft was to line the tanks with a strong fabric layer and then several layers of a treated (vulcanized) rubber and untreated rubber. When a round pierced the multiple layers and allowed the fuel to wet the untreated rubber, that layer would swell so much as to cover the hole and thus eliminate (or at worst, greatly diminish) the fuel leakage. Of course, the system was not perfect, but it did give the pilots a better chance at completing a mission and returning home safely.

Something similar happens with the pub trick with a Ziploc bag except there is no swelling. Take a sharp pencil and ram the pointed end into and through the lower part (water-filled part) of the bag. The pencil can rip a hole through both sides of the bag without any water spilling out.

After the pencil point penetrates the front of the bag and as progressively thicker portions of the shaved, cone-shaped region pass through the opening, the pencil grabs the perimeter of the hole and pulls it into the water. This action stretches the plastic around the perimeter. When the pencil stops, this stretched plastic hugs the pencil shaft so tightly that water cannot leak out around the shaft. The hole is sealed down onto the shaft.

When I pull the pencil backward so that point leaves the far hole, water (of course) pours from that hole. When I push the pencil back through the hole, I can see how the upper end of the shaved portion again grabs the plastic around the hole and stretches it, resealing the hole.

The water serves two purposes: It demonstrates how well the hole is sealed and it adds bulk (mass) to the bag so that a sharp blow by the pencil allows the pencil to penetrate the side. I find that pint and quart freezer bags, with their thicker plastic walls, work better than the sandwich bags, with their thinner walls. (My family was amused with my frequent failures with the sandwich bags.)

Of course, there is a limit to how far below the water surface the pencil can be used. If you have a fairly tall column of water, the water pressure near the bottom will be enough to push water outward between the plastic and the shaft. If you investigate this pub trick with a tall column of water, let me know what you find.

2.180  Pub trick---bottle cap blown into a bottle
Jearl Walker
April 2012 Take the cap from a beer bottle (or the drink of your choice), bend it over onto itself (you might need pliers), lay the bottle down on a table, and then place the bent cap in the neck of the bottle. The challenge is to blow into the bottle so as to push the cap into the rest of the bottle.

If you simply blow into the bottle as instructed in the challenge, the cap flies out of the bottle, not farther into it. Here is a video demonstrating both the unsuccessful method and the successful method of meeting this pub challenge.

As the video performer explains, simply blowing into the bottle sends lots of air past the cap and into the rest of the bottle. This incoming air effectively turns around and comes back of the bottle (because of the increased internal air pressure). That outflow of air pushes the cap out of the bottle.

I can add to the explanation. When you bend the cap onto itself, it is bent around a diameter. When you then put the cap in the bottle neck, it can sit only with that diameter along the length of the neck, not across the width of the neck. This orientation means that the cap presents only a small cross sectional area to the air that is blown into the neck. Most of that air stream passes over the top of the cap. Thus, the air stream that comes back out of the bottle must be along the lower part of the neck and thus right up against the cap, pushing it out of the bottle.

Try replacing the cap with something with a larger cross-sectional area in the incoming air stream. Or put tape around the cap several times to increase its cross-sectional area. Do you find that the cap still flies out of the bottle? Or, when the cross-sectional area is large enough, do you find that the cap moves into the rest of the bottle?

2.181  Fake video --- free standing water
Jearl Walker
April 2012

At least 6 million people have viewed this video in which a young man seemingly suspends water in midair until he pricks it with his finger (well, he actually never touches the water). He first fills a glass with water, places a card over the open mouth, inverts the glass while holding the card in place, places the inverted card and glass on a table, and slides the card out from beneath the card. A bit of the water spills from the glass during that last part. Then he yanks the glass upward while giving it a twist. The water stays exactly in place.

A lot of curious things can happen in the physical world, especially in quantum physics. So, some people are quite willing to throw away any common sense when watching a video like this. They something like, “Well, anything is possible. Maybe the twisting somehow turns off gravity or makes the water rigid and no longer fluid.”

Yes, well, there is a lot of stuff in quantum physics that defies my everyday experience, but water is not weird. I’ve using it for decades. I bathe in, swim in it, stir it for tea, and clean my clothes in it. Never once has swirling the water solidified it. Other than freezing it, there is nothing I can do to transform it to a solid.

There is also nothing I can do to turn off gravity. Every time I stumble and fall, as I fall I always scream out, “Off! Off! Gravity off!” Alas, the command has never been metI have always landed hard. The only consequence of my screams is that everyone around me is quite uninterested in helping me up. In fact, they always move away from me as quickly as possible, usually with wary glance back over one shoulder. Perhaps they are worried that if I could somehow turn off gravity, we all would float into space. Well, probably it is just the screaming.

One of the strongest arguments for everyone studying science in school (physics included) is that everyone needs to build up a credibility checker. Educationalists call it “critical thinking.” However, I think we could just call it “avoiding someone making a fool out of you.” Then if someone shows you a video in which water seemingly defies gravity, you immediately know it is faked.


2.182  Controlled breach of the Condit Dam
Jearl Walker
August 2012 In 2011, in south central Washington, a tunnel was drilled through the Condit Dam on the White Salmon River, which feeds into the Columbia River. For environmental reasons, the dam was no longer economically viable and the power company running the hydroelectric system there decided to remove the 1913 dam and re-establish the White Salmon River. When the tunnel breached the dam near the base, the water pressure on the lake side of the dam propelled water through the tunnel, moving not only the water but about 1.6 million cubic meters of lake sediment through the tunnel in about six hours. Here is the dramatic video by Andy Maser, who is an acknowledged cameraman, producer, and film edito. (Press the advance button to start the video.)

The explosive flow was due to the hydraulic head on the lake side of the tunnel: The water there was about 38 meters deep, which created a pressure near the bottom of about 0.37 million pascals (about 3.7 times atmospheric pressure).

In the video, note how the first rush of water is clear and thereafter it is deeply blackened by the sediment, with blackened water rushing under the clear water and then rising in front of it. Here is a screen capture from Maser’s video:

Also, note the hydraulic jump (or bore) where the flow has a stationary high point just downstream of the flow. Another screen capture: information about Andy Maser Condit Dam National Geographic Society scroll down to Nov 6

More video


2.183  Pub trick --- separating two pub glasses
Jearl Walker
August 2012 A pint pub glass is held with the hand just below the wider section of the glass.

Here is this month’s challenge. Insert one empty pub glass into another and lay both on a table. Can you separate them without touching either one of them, either with your hand or some other object? Maybe you can roll them by blowing on them. Well, no, they just roll together. Maybe you can tilt the table. Well, no, they will slide off onto the floor and then you’ll have to drink your Guinness from a coffee cup, which is just sad.

Here is the video answer:

In case you cannot reach Youtube, here is the answer. When you lay the glasses down on their side, leave a small gap between them. Then blow hard into that gap through a straw. The sudden increase in air pressure inside the exterior glass will push out the interior glass.

2.184  Pervious concrete
Jearl Walker
September 2012 One of the disadvantages of concrete is that water can pool on it or be channeled by it. Pedestrians and cars then must wade through the water. Worse, if the cars are moving quickly or through a turn, they tend to slide out of control.

Pervious concrete eliminates the hassle and the danger. It is as strong and enduring as normal concrete but allows water to quickly flow down through pores in the concrete. Under the concrete layer, layers of small rocks have been laid. The lowest contains larger rocks, with about the same size. Another uniform layer, with smaller rocks, lies above it. A third uniform layer, with even smaller rocks, lies highest, just beneath the concrete.

With this arrangement, water simply flows down to the lowest layer of rock, where it can then seep into the surrounding soil. Here are two videos demonstrating just how rapidly water is removed from the top of the pervious concrete: UNH Stormwater Center 1500 gallons. Filtercrete pervious concrete

Gravity, of course, tends to make the water move downward. However, if the pores in the concrete and between the rocks are tiny, the surface tension of the water (due to the mutual attraction of the water molecules and their adhesion to any solid surface) would stop the downward flow. So, the concrete pores are kept, on average, large enough to allow the water to flow, rather than just cling.

Here is a detailed video that shows the construction of the several rock layers and the pouring of the concrete. Installing a permeable paver driveway

2.185  Fly in gun blast
Jearl Walker
November 2012 This video is mostly for fun. It shows a common fly coping with the blast from a P-22 handgun as the bullet crashes into a donut. (Never mind the complete disregard for safety by someone firing into a target at very close range.) You can tell the direction of the air blast because the fly is rolled over counterclockwise by the airstream just below it. So, the stream is coming down to the right, reflecting from the target below the donut, and then swirling up to the right and over the top of the fly.

Here is a video that shows the blast from a gun:


2.186  Attack of the tumbleweeds
Jearl Walker
January 2013 Tumbleweeds are bushes that have withered and become detached during the fall, typically in arid regions. Once free, the dried branches can catch the wind and function like a sail, which drives the bush over the land. The advantage of the motion is that the bush can then drop seeds along its path and thus disperse the seeds. However, the really curious feature is that because a tumbleweed is roughly spherical, it can end up rolling and hopping along the path.

Even if you have seen a rolling tumbleweed, you may not have noticed it but here are two videos where the tumbleweeds come in like science-function monsters. tumbleweed invasion attack of the giant tumbleweeds

Of course in a very strong wind, they tumbleweeds fly instead of tumbling. Here is one of the strangest sights I have ever seen. Lots of tumbleweeds have been captured in a vortex. So instead of a whirlwind or dust devil, we see a tumbleweed devil.

Tumbleweeds are a common sight in western films, usually symbolizing bleakness. Here is a montage of clips:

Martian tumbleweed rover

The general concept of tumbleweed locomotion has inspired several designs for Martian rovers that would be propelled by the winds there. Such a rover would have a spherical arrangement of sails that would catch the wind, causing the rover to roll or bounce over the surface, even a very rocky surface. It could be partially deflated so that it could be stationary in order to transmit observations from its internal instruments or to deploy stand-alone, solar-powered instruments. The rover could then be re-inflated and rolled away. The advantages of a tumbleweed rover are that it could quickly cover much larger distances than the current Martian rovers and it could travel into regions where landing a normal rover would impractical or impossible. Here is an animation showing tumbleweed rovers:

Here is a senior project at Case Western Reserve University showing a unique design for a tumbleweed rover. Note how it shrinks so that it would then catch less wind and thus remain in place.

More links explanation of tumbleweeds caught by news car

Dots · through ··· indicate level of difficulty
Journal reference style: author, title, journal, volume, pages (date)
Baker, D. V., J. R. Withrow, C. S. Brown, and K. G. Beck, “Tumbling: Use of diffuse knapweed (Centaurea diffusa) to examine an understudied dispersal mechanism,” Invasive Plant Science and Management, 3, 301-309 (2010)

2.187  Pub trick --- pouring a rainbow
Jearl Walker
January 2013 A bartender lines up several shot glasses and then begins to pour from a shaker container into them, one after another. Amazingly, the colors of the liquids in the glasses are different. The challenge this month: How can we get separate colors from the same container?

The secret is that the bartender has chosen alcohols and various bar liquids that have different densities and that do not readily mix with each other. Then the bartender layers the liquids in the shaker container, with about one shot-glass measure of each. The densest goes in first. Then the second densest goes in second. And so on.

However, as you can see in the next video, a denser liquid might go in after a lighter liquid if they do not readily mix. The denser one merely drops below the lighter one and then spreads out.

Bartenders who build layers use an alcohol density chart, such as the one at this web site:

The specific gravity listed in the chart is the ratio of a liquid’s density to that of water. For example, the common bar syrup grenadine has a specific density of 1.18, which means that its density is 1.18 times that of water. Although such charts are helpful, experience is also needed because the density of a liquid depends on temperature. Recall in the second video here that the bartender poured ice into the shaker container. The ice cools the liquids and increases the density. (The ice is blocked by the strainer when the bartender pours out the shots.)

Here is a nonalcoholic version of layering (not pouring). multiple layers of liquid of different colors

More videos:


2.187 Pub trick --- pouring a black and tan
Jearl Walker
November 2015  A black and tan is a glass of two types of beer, one denser than the other. If the denser beer is poured into a glass first, then the light beer can be poured on top of it such that there is little or no mixing of the two beers. The visual effect is best if the two beers have different colors. For example, bartenders typically use the light-colored Bass Ale or Harp Lager for the denser beer and the dark Guinness draft for the lighter beer.

The bartender first fills a glass halfway with the denser beer and places an inverted normal spoon (convex side upward) just above the denser beer. Then the bartender very slowly and carefully pours the lighter beer onto the spoon such that it gradually drains from the side of the spoon down onto the denser beer. There it will float provided that the drainage is gradual. If, instead, the lighter beer hits the denser beer with too much momentum, it will penetrate into the denser beer, mixing the two types of beer.

Here is a tool for making a black and tan that functions better than the back of spoon.

After I pour in the denser beer (Bass Ale), I place the tool across the top of the glass. Then I gradually pour in the lighter beer (Guinness Draught). The small holes around a ring in the tool allow the lighter beer to drain gradually down onto the denser beer. I found that I still need to pour slowly or the lighter beer would penetrate the denser beer and begin to mix by turbulence. I also found that I needed to half fill the glass (at least) with the denser beer, or the long fall of the light beer would lead to penetration and turbulence.

This tool can be purchased at many places online. Here is an Amazon link:

Here is a video showing the tool in use, in a failed attempted and then in a successful one:



2.188 Poured art
Jearl Walker
February 2013

One of the stories in the Flying Circus of Physics newsletter of a few years ago was about the poured art of Holton Rower. Yes, I mean that he pours the paint, cup by cup, onto a flat surface. pillar vertical posts

The beautiful pattern of alternating colors is due to the way that a stream of paint hits a horizontal surface. The paint is thick (viscous) and spreads sluggishly away from the impact point, but the interesting feature is that while it hits the surface, it forms a shallow crater. As new paint pours into this crater, it pushes the previous paint away from the impact point. Holton Rower hit upon the brilliant scheme of pouring different colored paints one after another, in a slow, controlled way so that each new paint pushes the previous paint radially away from the impact point.

If he poured a new paint without making a crater, the new paint would just spread out over the previous paint. You can see that happen in at least one of the pours in these next videos. Notice that in the second video, some of the paints are not viscous enough to push a previous paint out of the way but instead flows into it:


2.189  Escape from a Sinking Car
Jearl Walker
April 2013 Every year lots of people die while trapped in a car that has gone off a road and into water. How can you use physics to escape from such a car?

I’ve gathered several videos (see below) that show what happens if someone is in a sinking car. The commentators make several points:

1. Immediately unbuckle your seat belt and unlock the car door.

2. Unless you open the door immediately, the water pressure on the door exterior will soon prevent you from opening it. However, as water seeps into the car through various openings or if you open the window, the compartment will soon fill with enough water that the interior water pressure matches the exterior pressure. Then the door can be opened (provided it is unlocked). Of course, by then the air in the compartment has almost disappeared. So, you have competing effects here --- you need to open the door but you also need to breathe.

3. Because you might have to escape through the window, leave the key in the ignition so that you can lower the window. The electrical system should still work for a few minutes.

4. If you cannot open the window or door, you can try to break the window. However, many makes of cars (such as Volvo) have exceptionally strong windows, so you probably won’t be able to break the glass with the side of your fist. As one video suggests, you might pull a headrest off its tracks and then use one of its metal rods as a crowbar wedged between the glass and the door frame. By leveraging the rod, you might be able to crack the window. camera inside car, 3rd attempt fails goes wrong part 1 Top Gear part 2 Top Gear animation. Electric system will still work for a while

In the 1960s one of the most popular cars in the world was the now-classic VW beetle. As you can see in this advertisement, the car was almost airtight and thus also watertight. If a beetle were driven into water as we see in the advertisement, it would float for a relatively long time, long enough for the driver to roll down the window and climb out. The classic VW beetle will float

More links: using headrest as crowbar to break window tip on seat belt can break window Discovery Channel Mythbusters part 1 Mythbusters part 2 mentions child in back seat


2.190  Pub trick --- "beer-spots" in bottle caps
Jearl Walker
December 2013  This is not so much a pub trick as a pub observation. Although I have opened countless beer bottles in my (long) lifetime, I noticed this effect only in the last month. After I pried open the cap on a bottle of dark beer, I happened to look at the underside of the cap. A small spot of the beer sat at the middle. Strange, I thought. I do not see such spotting in all beer-bottle openings, and any lighter colored beer can leave a spot that is easily overlooked. However, with dark beers, I find most leave “beer-spots”.

Here is what I think is happening but invite counter explanations. Initially, the beer and the gas pocket between it and the closed cap are under high pressure (around three times atmospheric pressure). That pocket contains primarily carbon dioxide gas but also some of the vapor from the beer.

When I pry one side of the cap upward, the internal pressure is suddenly reduced as some of the gas flows under the lifted part of the cap and out from the bottle. The pressure reduction is so rapid that the gas cannot exchange thermal energy with its surroundings. Yet, the gas needs energy to do work in expanding against the external atmosphere. That energy comes from the thermal energy of the gas itself; that is, in order to expand, the gas rapidly cools in what is called an adiabatic expansion. This sudden cooling causes condensation of moisture in the escaping gas. Some of that condensation coats the underside of the bottle cap.

In addition, the decrease in pressure inside the beer causes bubbles to suddenly form, rush to the upper surface, and then pop open. Some of the resulting spray also reaches the underside of the cap.

The underside of the cap is coated with a material to prevent the metal of the cap from chemically interacting with the beer. That material does not wet well. If you place a small amount of water in an inverted cap, the surface tension of the water tends to pull the water either into a bead or to collect it along a circular ridge on the cap. Thus, the water does not spread out uniformly over the material.

So it goes with the condensation and spray: the liquid tends to form a bead. Often the bead lies in the center of the cap because of the crease created across the cap when one side is pried upward. The crease outlines the lifted region of the cap.
The greatest flow of the escaping gas (through the widest opening between the cap and bottle) passes under the midpoint of the crease. So, liquid is deposited there and then surface tension pulls it into a bead. When the bead dries, a beer-spot is left.

I plan on repeating these beer-spot observations to sharpen my arguments. Well, to be honest, I would be opening more beer bottles even if beer-spots did not form. 

2.191  Water is heavy
Jearl Walker
December 2013  When an air tanker (or water bomber) drops water on the flames of a forest fire, the water is released over several seconds as a spray so as to spread it over as much burning area as possible. That technique also decreases the damage done to the trees and other vegetation. Here is an example in which the spray is used to dampen a truck fire after the truck and a roader grader had collided. Notice that neither the truck nor the grader are noticeably damaged by the released water.

One of these air tankers can release several thousand liters of water, which weighs several thousand tons (or tens of kilonewtons). Had the water simply been poured out of the aircraft, the impact would have been far more severe. Indeed, here is a dramatic example:

Water is heavy!

2.192 Pub trick --- adding pins to a full glass of water
Jearl Walker
January 2014  Fill a drinking glass with water up to the rim. Then drop in a metal pin. No water spills out. Put in another dozen pins. Still no water spills out. But why? After all, each pin displaces a volume of water equal to its own volume, and the water surface started out at the rim.

The added pins does indeed raise the water surface, but surface tension (the mutual attraction of water molecules along the surface) prevents the water from flowing over the rim. If you look carefully, you will see the water surface is actually slightly above the rim height and curves down to the rim, seemingly glued there.

How many pins can you put into the glass without water flowing out? The answer of well over 1000 is demonstrated in this video:


2.193 Pub trick --- a puzzle in evaporating wine
Jearl Walker
February 2014
This month the item is more of a puzzle than a trick, a puzzle that I discovered in a partially filled glass of red wine that a guest left in my house and which was neglected for several days. Throughout the history of The Flying Circus of Physics, I have argued that physics puzzles lie around us in the everyday world, if we would only take the time to notice them. In that neglected glass of wine I saw a physics puzzle that took me several days to solve. Well, I think I solved it --- you can let me know.

Here is a photograph of the pattern I find in the wine glass after the glass sat undisturbed for several days.

Perhaps surprisingly, I believe one important clue to the cause of the pattern was the extremely cold weather we were experiencing here in Cleveland, Ohio, due to the polar vortex dipping south into North America. Let’s start with the main features of the pattern.

1. At the top, a series of horizontal bands formed below the initial level of the wine.

2. Lower, there is a region that is fairly uniformly red but with dark red spots surrounded by clear rings, resembling a moat surrounding a medieval castle. However, when I magnify that region of my photograph I can tell that it too consists of bands that are too close to be distinguished with my unaided sight.

3. Farther down there is a fairly clear region with red spots.

4. At the bottom of the glass there is a pool of sludge that eventually dried out.

The higher horizontal bands. I believe these are due to the evaporation of the alcohol from the wine. The behavior of evaporating wine (or any strong alcohol) has been studied for about 150 years. However, let’s start the story with pure water. The water molecules along the surface strongly attract one another. Those next to the glass are also strongly attracted to the glass surface. Both attractions are described as being surface tension. The result is that a short, curved “curtain” of water is pulled up along the glass wall, with the height limited by the weight of the curtain. This arrangement is static; that is, there is no flow in the water.

If we add alcohol to the water, a curtain still climbs up the wall but the liquid mixture is not static. Instead, there is a slow but continuous flow of liquid up into the curtain from the nearby bulk liquid, driven by surface tension. Here is the reasoning: when alcohol is added to the water, the water molecules are no longer entirely surrounded by other water molecules, to which they would be strongly attracted. Now there can be alcohol molecules in the way. Thus, the presence of alcohol decreases the surface tension.

In the curtain, however, the alcohol is fairly rapidly evaporating. So, with the loss of the alcohol molecules, the surface tension in the curtain is greater than that in the nearby bulk liquid. The stronger surface tension in the curtain pulls liquid from the nearby bulk liquid, setting up a circulation system.

As liquid is pulled into the curtain, red sediment (molecules and larger particles of grape) are deposited on the wall. This collection of red sediment forms a red band that can be seen in my photograph.

But why is there a pattern of red and clear bands in the photograph instead of just a red region left as the general level of wine decreased? The question stumped me for a while.

The glass of wine was left in a room that was heated by a gas fireplace during several especially cold days, with the outdoor temperatures well below freezing. I think the periodicity of the bands reflected the periodicity of the fireplace activity as the thermostat automatically turned the fireplace on and off. When the fireplace warmed the room, the evaporation of alcohol from the curtain increased, and thus so did also the flow into the curtain and the deposit of red material on the glass wall. In the meantime, the general level of the wine in the glass decreased. Then when the fireplace was next turned off, the evaporation, the flow was less, and the deposit of material was less. Thus, a red band was created when the fireplace was on and a relatively clear band was created when it was off.

The lower, uniformly red region. Well, to my eye it looks uniform but when I magnify the photograph I can see thin, horizontal bands here too. I believe that the bands are now much thinner because, after several days, the alcohol content of the wine had decreased so much that the flow of liquid into the curtain had waned. Now there was only a moderate difference in surface tension in the curtain and in the nearby bulk liquid.

The red spots with clear surrounds. The spots were larger bits of grape that were pulled onto the glass wall by surface tension, in a process now dubbed the Cheerios effect (click here to see my earlier discussion of the effect). Wherever a bit stuck to the wall, a curved liquid film clung by surface tension to the bit.

The flow from the bulk liquid up into the curtain was diverted by this film. As the level of the curtain gradually dropped, a relatively clear area was left around each bit of grape on the wall.

The clear region. by the time the liquid level reached this region, all the alcohol had evaporated, leaving water with bits of grape. There was no longer any flow of liquid into the clinging curtain

Sludge. Most of the sediment ends up in a watery sludge that eventually dries out.

I have repeated the experiment using the same type of glass and the same type of wine. The only difference is that the outdoor temperature is about the freezing point, which means that the fireplace turns on far less frequently. I see the hint of only one band, certainly not the multiple bands in my photograph. I also repeated the experiment with the glass of wine left in the basement where the temperature is nearly constant. No bands appear.

References on surface-tension driven flows in strong alcohol, including “tears of strong of stong wine”

For a large number of citations, go here in my archives

and go down to item 2.88 Tears of wine and other liquid surface play.

2.194  Pub trick --- fast pouring beer or soda
Jearl Walker
March 2014  Can you pour a carbonated beverage into a drinking cup quickly without generating lots of foam, so that the cup can be filled quickly? If the liquid stream hits the liquid pool already in the cup, the resulting splashing will generate lots of foam. You then must wait until the bubbles pop open and the foam collapses before you can pour in more liquid.

Bubbles in a carbonated beverage cannot form in the bulk liquid. Any chance formation of a small bubble there is almost immediately squeezed out of existence by the surface tension in the wall of the bubble --- small bubbles have a wall with a small radius of curvature, which means that the inward force due to the surface tension on the wall is strong. However, if a poured stream hits the pool, the turbulence that occurs in the splashing produces bubbles, which form the foam. To pour in more requires waiting for the foam to subside.

A bar server (or worse, an airplane server) does not want to spend that much time pouring a carbonated beverage. So, the server tilts the cup in order that the stream hits the side of the cup and then slides down the side into the liquid pool. In that way, very little splashing occurs.

This tilted cup technique is used by many servers. However, airplane servers must move quickly and have developed a pouring technique that is even faster than the tilted-cup technique. The problem of bubble formation is more serious in an airplane than a pub because the air pressure inside an airplane at flight altitude is lower than atmospheric pressure. That lower pressure allows the dissolved carbon dioxide in a carbonated beverage to more easily form bubbles when poured.

The airplane-server technique of pouring is this: A server opens a beverage can, slips an inverted cup over the top, inverts the can and cup, and then gradually pulls the can upward. The beverage then flows only a short distance (a few millimeters) from the can to the cup and there is little chance for splashing. Also, the bubble formation is diminished because the can’s top surface is kept inside the pool or just above it. Here is a link to a blog about the airplane-server technique. pouring Coke in an airplane the fast way. Activate the video.

2.195  Boyle’s perpetual motion device
Jearl Walker
March 2014  Robert Boyle (1627-1691) designed a perpetual motion device in which water continuously drains from a container into a tube and then flows through the tube and back above the container where it falls back into the container.

An immediate argument against the scheme is that the water should not rise any higher in the tube than in the container. Although the weight of the water in the container is much greater than the weight of the water in the tube, the water pressures are equal at any given height. Pressure is force divided by area. At the bottom of the container, the large weight of the water bears down on the large area of the bottom. At the same level in the tube, the small weight of the water above that level bears down on a small cross-sectional area of the tube. Although the weights and areas are different, the pressures (the ratio of force to area) are the same. Here is photographic evidence of the equilibrium: Pascal vases

A counter argument is that if the tube narrows considerably, a capillary force will pull the water up the tube. This force is really due to the attraction of the water molecules for themselves and for the tube molecules. You can see this electrical attraction in an ordinary glass of water. Fill the glass almost full and then examine the surface near the glass wall. That surface is curved upward along the wall. But also notice that the water climbs only a short distance up the wall. If the water is in a narrow tube, it can climb higher.

If the tube in Boyle’s arrangement is very narrow, the water can climb significantly higher than the water level in the container. However, that high climb means that there is a significant force holding the water to the tube wall. If the tube is bent over and down as in Boyle’s arrangement, that force is too strong to allow the water to leave the tube. So, the water might climb higher than the container but certainly cannot flow.

Other liquids behave just like water. However, here is a video that supposedly demonstrates how carbonated beverages might actually flow as envisioned by Boyle.

The argument supporting this video here is in two parts.

(1) The presence of foam in the tube means that the liquid density there is smaller than the liquid in the container. Thus, at any given level the height of the foamy liquid would have to be greater than the height of the non-foamy liquid in order to have the same pressure.

(2) Because the bubbles in the foam are pushed upward by buoyancy, they drive the liquid up the tube.

Even if what we see in the video were real, this still would not be a perpetual motion device because the bubbles would eventually break, equilibrium would be established, and the liquid would be stationary.

However, I and many others believe that the video has been faked. The flow is just too vigorous to be driven by buoyant bubbles. When I pour a beer into a glass too quickly, the foam can overflow the glass rim but not with the vigor I see in the video. Most likely a small motor is hidden in the tube in the region where it is supported by the wood arm.

Here is another video where no such motor is used:

When I wrote “The Amateur Scientist” department for Scientific American magazine, I received numerous designs for perpetual motion devices along with earnest letters explaining how they would work if built. Although disproving such designs can be interesting, convincing someone that a design cannot work can be extremely time consuming. Some people simply want to believe in perpetual motion regardless of the fact that not a single working perpetual motion device has ever been built. (I leave out continuous motions that are possible with superfluids and superconductors.) Here is a video about such beliefs:

2.196  Pub trick --- opening a wine bottle with a shoe
Jearl Walker
May 2014  Yes, a shoe! I cannot recommend this way of opening a wine bottle because you might get hurt. However, several videos, including news reports, show how you can loosen the cork in a wine bottle by placing the base of the bottle inside a shoe and then repeatedly hitting the bottom of the shoe against a wall or some other hard surface. Here is the first video I came across.

Several more links farther down here show other (far more sober) people loosening a cork with the same technique but only one link gives an explanation:

However, I really don’t understand the part about the bubbles somehow causing the cork to move outward. To me, the cork is simply being pounded outward by the forceful sloshing of the wine.

Suppose the bottle is being accelerated leftward toward a wall, as in the first video. The gas pocket (normally in the neck in an upright bottle) moves to the top left as the neck of the bottle is forced against the wine. When the bottle hits, the bottle quickly stops but the wine is still moving leftward until it slams until the base of the bottle, producing a high pressure pulse against the base. This high pressure pulse then rebounds from the base, passes through the length of the bottle and then slams into the cork.

If the impact on the cork is strong enough, the friction between the cork and neck is briefly overwhelmed and the cork moves outward along the neck for a short distance. If the procedure is repeated, the cork eventually moves outward enough that it can be removed by hand (or, as we see in the video, by teeth). However, as you see in one of the following videos, if the bottle strikes the wall only weakly, the pounding on the cork by the pressure pulses is too weak to overwhelm the friction and so the cork does not move.

Why a shoe?

Why doesn’t someone simply slam the bottle itself against a wall? Well, that probably will just break the bottle because the impact on the wall will be against a small section of the base of the bottle. That will crack the glass. The role of the shoe is to spread the impact over the full base, decreasing the chance that any portion will be hit hard enough to crack the glass. Here are my sketches of the impact.

As the person slams the shoe and bottle against the wall, the leading portion of the bottle’s base hits first, compressing the shoe and slowing the bottle.

The bottle then rotates around the point of first contact until it is nearly perpendicular to the wall.
By then all of the base is compressing the shoe and the bottle slows to a stop. The shoe decreases the force in the collision because it compresses and thus increases the time the bottle takes to stop but, more important, it spreads the collision out over a wider area so the glass is less likely to crack. using a screw, screwdriver, hammer

2.197  Pouring water in an airplane during a barrel roll
Jearl Walker
March 2015   First I will tease you with this image that has been widely circulated. A pilot in a military jet pours water from a bottle into a cup while the jet is upside down --- the water stream is downward in the image, away from the ground that appears at the top of the image, but that means the water flows against gravity.

Has gravity been somehow reversed? That is impossible, of course. One key to explaining the image is that the airplane is not continuing to fly upside down but instead is in a barrel roll (the airplane is rotated 360 degrees around its long axis while continuing to fly forward). That rotation might prompt you to say something like this, “The rotation is slinging the water away from the ground.” Or this, “A centrifugal force is pushing (or pulling) the water away from the ground.” I don’t think so, but I will first examine another airplane stunt that seems to be related but which is actually different. Then I will come back to this perplexing image.

Cup of tea upside down.

Here is a video of an airplane doing a barrel roll while a cup of tea is placed upright on a platform (just over the control console --- disquieting!). The tea stays nicely in the cup even when the airplane is upside down.

What keeps the tea in the cup? Or rather, why doesn’t the tea pour out when the cup goes through the upside down orientation? Those are actually two different questions, depending on whether we take the perspective from inside the airplane or from the ground.

Let’s begin to answer the second question with a common classroom demonstration. A bucket of water is rotated rapidly in a vertical plane and yet the water does not pour out of the bucket when the bucket passes through the upside down orientation. The important word in that description is “rapidly”. If the speed is too slow (or the angular speed of the rotation is too slow), the water will pour out.

Let my dot in this next drawing represent the water at the top of the vertical circle and for moment let the cup magically disappear.

As the water goes through this position at the top of the circle, it is about to be launched horizontally into projectile motion, much like you can throw a ball horizontally. The shape of its path depends on the launch speed. If the speed is slow, the water will take the more curved path. A faster speed gives a flatter path.

Now put the cup back in place. That cup is following the circular path drawn with the wide line.

Notice that the slower launch allows the water to fall away from the cup as the cup rotates along its circular path from the top. However, with the faster launch, the water runs into the cup and is force to move along the circular path. So, if the young man in the video rotates the bucket of water fast enough, the water is trapped in the bucket, but if he moves it too slowly, he will be drenched.

Some people prefer to describe the physics as seen in the rotating system, as if we could ride along with the bucket. Because the system is continuously being accelerated (to keep it in circular motion), we say that the system is non-inertial, which is a fancy way of saying that Newton’s laws of motion may not work. In this case, they do not work because an upward force seems to counter the downward gravitational force.
That upper (centrifugal) force is not real but only a convenient way of explaining why gravity does not pull the water downward and out of the bucket.

In the airplane video, we are effectively in the rotating airplane with the camera and thus are in a non-inertial system. So, a non-real force seems to keep the tea in the cup even when the cup is inverted. However, if we could see this stunt from the ground view, as we did in the water bucket video, we have no need of the non-real force and could simply say that the water is just not allowed to go into projectile motion.

Pouring water

Now let’s go back to the first image where water is being poured from a bottle to a collection cup. The physics here is very different because the water does go into projectile motion as it moves from the bottle to the cup. Another key point is that the portions of the water stream we see in the image did not leave the bottle at the same instant. Instead, the portion that is about to enter the cup left the bottle earliest and when the bottle was lower in its circular motion in the barrel roll.

Let’s consider the common launching of a ball and the resulting projectile motion. In my drawing here, three balls are thrown with the same launch speed but at different launch angles. The one with the largest launch angle travels the highest and along a path with the most curvature. The ball with the smallest launch angle does not travel as high and its path is flatter.

To apply those general results to the poured water stream in the jet plane image, let’s first divide the stream into five portions. Here is how they would look from a ground perspective. The bottle mouth is moving counterclockwise along the heavy circular path. Portion 1, which is about to enter the collection cup, was released when the bottle mouth was low on the side of the circular path. Portion 2, somewhat farther from the cup, was released when the bottle mouth was somewhat higher on the circular path. Portion 5 is near the bottle mouth, which is highest on the circular path.

Each portion was launched with the same speed but with different launch angles. The launch angle of portion 1 was greatest, so portion 1 will move highest (just as with the launched balls) to reach its position in the image. A short time later portion 2 was launched but the bottle had move along its circular path by then. The launch speed was the same but the launch angle was less. So portion 2 moves along a flatter path to reach its position in the image. And so. Portion 5 was launched with a small angle and has move only a short distance to reach its position in the image.

Because the image is a photograph and not a video, we don’t see any of this motion. Rather we see only the locations of the portions at the instant the photograph was taken. Visually connecting the portions at that instant we see a curved water stream. From the perspective of the camera in the jet, here is the arrangement.


The water resembles what we would see with a stream from a garden hose, and we are misled into thinking that there must be some mysterious force directing the stream away from the ground.

More videos very irritating music Walter Lewin at MIT, go to last few minutes of this hour-long lecturex

2.198  Royle's self-pouring teapot
Jearl Walker
May 2015 

This teapot differs from all others because you do not need to pick it up and tilt it in order to pour out a cup of tea. same video but with music

This type of teapot, now an expensive collector’s item, was patented in 1886 by John James Royle of Manchester, England. Tea leaves are put on a grill at the bottom of the teapot and then covered with hot water and allowed to steep. The lid (a cylinder) is then raised. When it is forced back down, a finger covers a small hole in the center, thus not allowing air to escape from the cylinder. As the cylinder with its trapped air descend, it pushes exactly one cup of tea out through the spout, which is bent over so that the tea pours neatly into the waiting cup.


2.199 Sloshing
Jearl Walker
July 2015  Sloshing may be fun in a bathtub but it is irritating in a cup of hot coffee as you walk away from the coffee counter. Such sloshing can be a problem even if the cup has a standard plastic lid attached to the top. Beer in a mug, however, is less likely to slosh, especially if the beer has a thick head of foam. Here are a few notes about sloshing.

The first trouble with walking with a cup of coffee is that the natural rhythm of your walking (the up/down, forward/back, left/right motions) sends waves across the top surface of the coffee. The second trouble is that the rhythm can almost coincide with the rhythm at which the wave action in a cup can build up. That match is said to be resonance. A similar example occurs if you periodically push on a playground swing when, say, a child is in the swing. Although each push may be small, if it is timed so that you push each time the swing comes back to you, the extent of swinging increases. The pushing is said to be in resonance with the swinging. So it goes with the oscillations you give a cup of coffee while walking --- those oscillations (almost) match the wave motion across the top surface of the coffee, and if the wave increases in amplitude, the coffee can spill over the rim of the cup.

Of course, there can be a big difference depending on the type of walking. Brisk walking, with sudden starts and stops, can set out spilling waves even before resonance is set up. The coffee can also swirl. So, unless you are careful, you are likely spill a bit of the coffee in a full cup.

However, most coffee venders also give you a plastic lid that is pressed down on the (paper or Styrofoam) cup. Still, if the cup is full, coffee spills through the small opening through which you are to drink the coffee with the lid in place. I wondered if the location of the opening affects the spillage. Let me describe the location in terms of an analog clock. If the opening is at the 12:00 or 6:00 positions,


spillage is more likely than at the 3:00 or 9:00 positions


When I remove the lid and watch the sloshing, the extent of sloshing is indeed more in the forward and rearward directions than off to either side. That must be due to the accelerations of the cup when I begin to move forward or when I stop.

One way to decrease the sloshing is to float an object in the coffee. A hollow sphere would do the trick. Another way has long been used by restaurant servers in some countries. Place the coffee container on a horizontal platform that is then held by string. As you walk with the device, it and the coffee container swing like a pendulum.


The stability of the coffee (the lack of sloshing) can be explained from either our viewpoint or from the viewpoint of someone riding along on the platform. From our viewpoint, the circular motion of the platform during the swinging causes the platform to exert a force (a centripetal force) toward the center of the circular motion. That force eliminates the wave motion. From the platform viewpoint, the circular motion produces a radially outward force (a centrifugal force) that acts as an artificial gravity. Again that force eliminates the wave motion. carrying device

Sloshing of coffee is common in other situations where there are abrupt changes in motion, such as in a car.

But in a smoothly moving train or airplane, a cup of coffee might have very little sloshing.

Sloshing can be less of problem with a glass of beer if the beer has a head of foam. Although walking with the glass tends to set up oscillations on the beer surface, the rubbing of the foam on the glass wall damps out (weakens) the oscillations. Even a head with only three or four millimeters thickness can make a difference. Here is one example

Many beers, all with foam

Here is a quick video that compares the sloshing in Guinness, Heineken, and coffee. Guinness, with its characteristic thick head of foam, quickly settles down after being shaken but the coffee sloshes out of the glass.

The calculations and technical analysis of the experiment are shown in this next video:

Fishing boats
Sloshing can be a serious problem in small fishing boats which take on lots of water when their catch is hauled onboard. In some cases, if the water does not drain off the boat quickly and if the sea is rough, the sloshing can actually tip the boat over. Marine engineers attempt to design boats to minimize that danger.

Cruise ships
Here are some videos of sloshing in swimming pools on cruise ships. Although there is no danger of such a large ship being destabilized by the sloshing, it can be dangerous to a careless passenger who can be knocked down by the sloshing or who can slip on the spilt water.

Here is one more example of sloshing but one in which the oscillations help out. A girl cleverly refloats a sunken canoe by sloshing the water out of it to the left and right. (same video but cropped, more difficult to see)

More videos: beer video sloshing in space

Dots · through ··· indicate level of difficulty
Journal reference style: author, title, journal, volume, pages (date)
··· Mayer, H. C., and R. Krechetnikov, “Walking with coffee: Why does it spill?” Physical Review E, 85, article #046117 (7 pages) (2012)
· Holmes, B., “Getting sloshed: It’s the way you walk,” New Scientist, , 2896 (31 December 2012)
· Lenton, A., “A walnut would do,” (letter) New Scientist, , 2900 (16 January 2013)
··· Turner, M. R., and T. J. Bridges, “Nonlinear energy transfer between fluid sloshing and vessel motion,” Journal of Fluid Mechanics, 719, 606-636 (2013)
··· Cappello, J., A. Sauret, F. Boulogne, E. Dressaire, and H. A. Stone, “Damping of liquid sloshing by foams: from everyday observations to liquid transport,” Journal of Visualization, 18, 2, ? (2015), available online arXiv:1411.2123v1 (8 November 2014)
· Aron, J., “Bubble physics explains beer’s own anti-spill mechanism,” New Scientist, online (27 November 2014)
··· Sauret, A., F. Boulogne, J. Cappello, E. Dressaire, and H. A. Stone, “Damping of liquid sloshing by foams,” Physics of Fluids, 27, article #022103 (15 pages) (2015) 

2.200 Spiral arms of water from a spinning sponge ball
Jearl Walker
November 2015  Here is a novel video in which a water-soaked sponge ball is released with a significant rotation. The water seems to be thrown off the ball in spirals around the equator, in a pattern that resembles the arms of stars, gas, and dust around spiral galaxies.

Can water really be shot out into spirals?

When the ball is stationary, the water stays within the ball because of the mutual attraction between water molecules and also between the water molecules and the sponge molecules. If the ball is rotated slowly, those forces can still hold the bits of water inside the ball. The forces are said to be centripetal forces because they are radially inward and provide the centripetal acceleration for the bits of water to move in circles around the rotation axis.

The greatest inward force is needed along the ball’s equator because the radial distance of the surface from the rotation axis is greatest there and so the water bits travel in a large circle. Little or no force is needed to hold on to the water bits at the north and south poles because they travel in small circles.

When the ball is rapidly rotated, the forces along the equator are insufficient to hold on to the water bits, and so the bits begin to leave the surface, each traveling in an approximately straight line from the release point. Their composite gives the illusion of a spiraling path but that is only because the bits leave the equator one after another while the ball rotates between the releases. The first bits out are the ones on or near the surface, but then water bits from deeper in the ball come to the surface and leave.

I am surprised that we see somewhat discrete spiral arms instead of a continuous, horizontal spray of water. I think the reason is that the water bits are more easily released at several points on the equator than at other points. Perhaps those points are more directly connected to the maze of openings and connections in the ball’s interior.

2.201  Spin painting
Jearl Walker
January 2016 Here is a curious way to paint --- spin the canvas so that globs of paint slide radially outward.

You can see the actual in this video.

The wet paint is a viscous fluid. When the canvas begins to spin, the bottom of a glob is held in place because the bottom layer adheres to the underlying surface, but the top of the glob is held to the bottom only by surface tension. So, when the bottom is forced into rotation, the top can only be partially dragged along. It moves, but in not a circular path with the bottom. The result is that the glob seems to be thrown radially outward, much as you feel if your car takes a fast turn.

Here the artist produces a more varied painting:

Videos by other artists


2.202 Disaster: The Boston Molasses Flood
Jearl Walker
March 2016 Shortly after noon on Wednesday, January 15, 1919, a tank containing about 2.3 million gallons of molasses both burst and exploded in the North End of Boston, sending a giant wave of the liquid down the streets. The wave, perhaps 8 meters, high knocked over and crushed people, domestic horses, buildings, and even the support pillars for the elevated train.

Below is a video documentary describing the molasses flood but it does not fully answer the question: “Why?” One answer was soon obvious in the investigations and lawsuits: The tank was hastily built in order to be ready for the shipment of molasses coming by ship into the Boston harbor. Moreover, it was built with steel sheets that were too thin for the job and which were held in place by rivets. The tank was about 20 meters tall but the only time it was tested was when it was filled with water only 3 centimeters deep.

The molasses was pumped into the tank over several days. Although Boston had experienced cold weather for weeks, the temperatures rose significantly at the start of the week of the disaster. So, when the last of the molasses entered the tank on Monday, it was relatively warm compared to the molasses already in the tank. The temperature difference caused mixing inside the tank, which produced noticeable rumbling of the tank walls. The warmer temperatures probably also increased the fermentation of the molasses, which released gas. So, with the tank almost full, the gas pressure and the rumbling of the wall were set for a disaster.

The bottom of the tank ruptured (probably due to the fluid pressure at the bottom) and then the top exploded (probably due to the gas pressure), sending the lethal wave of molasses through the streets. Here is the story and photos:

More videos:

Dots · through ··· indicate level of difficulty
Journal reference style: author, journal, volume, pages (date)
· Potter, S., “January 15, 1919: Boston Molasses Flood,” Weatherwise, 64, No. 1, 10-11 (January/February 2011)

2.203 Levitation by fire hose
Jearl Walker
March 2016 If you hold onto a garden hose while watering the lawn, you do not experience any noticeable backward push from the water emission. But if you hold onto a fire hose under pressure, the water emission can knock you backward and off your feet.

The effect is similar to the propulsion of a rocket when fuel is burned and the exhaust then shoots out toward the rear. However, with the fire hose, why should the hose be propelled backward or even sideways? There are two reasons. One is that the nozzle on the end of the hose is narrower than the hose, and so the water speed must increase as the water passes through it. Thus, there is an increase in the momentum of the water in the forward direction. In reaction, there is a backward push on the nozzle and thus also the hose.

The second reason is that the hose is nearly always bent near the nozzle, so the water must change its direction of travel as it nears the nozzle. That too produces a push on the hose, tending to straighten out the hose and making the hose hard to handle.

Can this backward push on a fire hose be strong enough to lift a person if the person directs the fire hose downward? No. The push from one hose would be too unstable, but several hoses in a rigid mount can do the trick: same video

About a car? Can multiple fire hoses levitate a car? Well, levitating a full size sedan would take a great hoses, but levitating a small car that has had its engine block removed is possible. Here is one example, with firemen having a blast: same video

And here is part of a segment on The Myth Busters showing the same trick. Myth Busters

Ah, I should have realized that powerful water jets can levitate people because this novel device came on the market a few years ago.


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