ChatGPT and Buoyant Force

I posted previously about ChatGPT's response to a projectile motion question. It got the answer correct, but with a wrong starting point, which made the whole response rather puzzling, inconsistent, and self-contradictory.

This time, I decided to ask a question that I had asked my students in a poll during a lesson on buoyancy and Archimedes principle.

A block of Styrofoam floats on water while a lead block of the same size block lies submerged in the water. Which one has the larger buoyant force?

The buoyant force on an object is equal to the weight of the fluid displaced by the object. The Styrofoam block is less dense than water and floats because it displaces an amount of water equal to its own weight, which is equal to the buoyant force acting on it. On the other hand, the lead block is more dense than water and sinks because it displaces an amount of water that is less than its own weight.

Therefore, the Styrofoam block has a larger buoyant force acting on it than the lead block of the same size because it displaces more water.

This time, ChatGPT got the answer wrong. Unlike the first time, it started out with the correct idea, which is that the buoyant force is equal to the weight of the displaced fluid.

However, in this case, the Styrofoam is displacing a very small amount of fluid because it is floating above the surface, while the lead block is completely submerged in the water, displacing its entire volume. This means that the lead block is displacing a larger volume of the fluid, and thus, having a larger buoyant force, despite it sinking in the water.

So in this case, ChatGPT once again exhibits its inconsistent and self-contradictory behavior.

I followed up trying to correct this error.

This is incorrect. The lead block displaces more water than the Styrofoam.

You are correct, I apologize for the mistake in my previous answer. The buoyant force acting on an object is equal to the weight of the fluid displaced by the object, and the amount of fluid displaced depends on the volume of the object. Since the lead block and the Styrofoam block have the same volume, the lead block will displace more water than the Styrofoam block because it is denser.

Therefore, the lead block has a larger buoyant force acting on it than the Styrofoam block of the same size, even though it is submerged in the water while the Styrofoam block floats on the surface.

It gets it now, but it is arguing that the lead block displaces more water than the Stryrofoam because it is "denser". While that is somewhat correct, it is not the reason why it displaces more water. It is because it is completely submerged in the water and displaces a volume of water equal to its volume.

If I reshape the same mass of lead into the shape of a bowl, the object itself will have the same "density", but this time, it can float on the water. It now displaces enough of the water such that the weight of the displaced water is equal to its weight, and thus, it will float. Granted that the whole "object" now includes the air that it encompasses so that its "total" density is less than the density of water, but the density of the lead itself hasn't changed. It is still denser than water but this time it floats!

Once again, if you don't know the material, it is hard to know whether you are being given not just the correct answer, but whether the whole response is consistent and not self-contradictory.

More to come.....

Zz.

Electrons Behave Like A Fluid - Exhibit Vortices

This is a rather cool experiment.

They have a direct observation, for the first time, of electrons behaving like an ordinary fluid and exhibiting vortices  when going thorough a channel.[1]

In contrast, electrons flowing through tungsten ditelluride flowed through the channel and swirled into each side chamber, much as water would do when emptying into a bowl.

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“That is a very striking thing, and it is the same physics as that in ordinary fluids, but happening with electrons on the nanoscale. That’s a clear signature of electrons being in a fluid-like regime.”

So far, "ordinary" electron flow behaves like a "Fermi liquid", which is not like ordinary fluid flow. To get electrons to behave this way, they had to make sure that the electrons do not bump into the crystal lattice (the bulk material), so this is not easy since normal-state electrons usually have such interaction (non-zero resistivity).

Just to be clear, this is not the first observation of electrons exhibiting vortex flow. This is a common observation when they are in a superconducting state, where vortices form around magnetic flux lines that penetrates Type II superconductors. But in that case, these electrons are in a superfluid, and what is flowing is the paired electrons (Cooper pairs).

In this experiment, these are individual electrons not in a superconducting state, so this truly is a river of electrons.

Z.

[1] A Aharon-Steinberg et al., Nature 607, 74 (2022).

My Favorite Web Applications - Part 5

Previous posts:

My favorite web applications - Part 1

My favorite web applications - Part 2

My favorite web applications - Part 3

My favorite web applications - Part 4

This time, it is an experiment that mimics the fabled Archimedes experiment where he supposedly determined for the "king" whether the crown was made of pure gold or not. This web application basically allows a student to perform a similar virtual experiment to determine the density of the object being investigated.

There are two reasons why I like this app. The first reason is that if you change the default settings for the mass and the volume, you will given rather random values. This means that each student will have different values for the mass and volume, resulting in each student having a unique set of data and calculation.

The second reason why I like this "experiment" is that it actually is the same experiment that we would do in a f2f lab. We use PASCO's Capstone system, and one of the experiments that we do is practically identical to what is shown in this virtual experiment, where a student has connected a weight sensor to a hanging mass, and then he/she slowly lowers it into a beaker of liquid. The sensor sends a reading of the hanging weight value to a data collection system that plots the value of the weight in real time. As the weight is lowered into the liquid, the data being plotted looks almost exactly as what is shown in the virtual experiment in this app. The weight changes due to the buoyant force of the liquid.

The analysis of the experiment and the extraction of the value of the object's density are similar for both the f2f lab and this virtual lab. So in that sense, the student is not being deprived of much of the physics. There are, of course, more errors involved in the real experiment because often the object isn't hanging still, and the movement adds more noise to the data. The app doesn't allow the data to be extracted directly, so no curve fitting or calculation of average value can be made for a range of the data points, something the students in the f2f lab are asked to do to be able to determined the weight before and after immersion.

Still, it is an adequate virtual experiment, especially since each student will have to do his/her own analysis on a unique set of measurement. I actually have used this as part of an assessment where this app was part of an exam for a f2f class (before the pandemic). The student had already done the actual experiment, so they should be familiar with how to find the density of the object using this app since things should look rather familiar.

Zz.

 

Why You Need To Wash Your Hands For 20 Seconds

Finally, the reason why we should wash our hands for a minimum of 20 seconds has a physics explanation.

The research work was published in the Physics of Fluids journal, which so far has been quite an active journal in publishing papers related to the physics of the coronovirus transmission, making them available widely to the public.

Zz.

Tripple-Layer Mask Blocks Secondary Atomization of Cough Droplets

I had already posted several physics papers on the efficacy of masks, even single-layer ones, in reducing the airborne aerosol. Now comes another paper that deals with how cough droplets actually can break up into smaller-sized droplets that may pass through single and double-layered masks, especially when moving at such high speed from a cough.

It turns out that having a mask with 3 layers or more might be the most effective here (I wonder at how many layers will we will of suffocation? :)) The new research is to be published in Science, and you can get the paper at the link to read to your heart's content.

A review of this paper can be found here. It is fascinating to read that expertise in the study of jet engines are being leveraged in studying the dynamics of this problem. But do you think people who don't believe in wearing masks to reduce the virus transmission will buy any of this? They believe in smartphones and jet engines, don't they?

Z.

Mask Physics

There is no controversy about the need to wear a mask to reduce the possibility of COVID-19 transmission. Every scientific research that I've read supports that.

Here are some more from the physics side of it. This is approaching it from purely the point of view of physics of fluids.

Visualizing droplet dispersal for face shields and masks with exhalation valves

Visualizing the effectiveness of face masks in obstructing respiratory jets

On respiratory droplets and face masks

Zz.

Non-Newtonian Fluids On America's Test Kitchen Show

I've seen this episode of America's Test Kitchen before. It is Ep. 1 of Season 19. However, during a recent rerun of the show, I did a double take when I read the description of the show being displayed on my TV menu guide:

There as an entry that said "non-Newtonian fluids".

Like I said, I've seen this show before, at least twice, and I don't quite remember them mentioning this type of phenomenon.

When I saw the show again, I realized what it was. They had a "Science" segment on "fluids" such as ketchup and liquid thickened by corn starch. These two are common examples of..... you guessed it ... non-Newtonian fluids.

But interestingly enough, no where in the show or during this segment, did any mention of the phrase "non-Newtonian fluids" ever appeared. It was odd that they would discuss the phenomenon, but not mention the name given to it. Yet, it appears on the description for this episode. At the very least, giving the phenomenon a name not only allows someone who wants to know more about it something to Google on, but also relates known physics to a common observation.

Or maybe they don't want to mention it so as not to scare away their audience?

Zz.

Space Coffee

It's amazing how much physics and engineering go into just getting the ISS occupants to have their cup of Joe while on board the space station.



They should just open a Starbucks franchise up there. It would have been easier!

Zz.

"Proof" of Pilot Waves from Fluid Dynamics?

This Wired article seems to indicate that the evidence for the validation of Bohmian's "Pilot Wave" idea for quantum mechanics can be found in some fluid dynamics experiment.

In a groundbreaking experiment, the Paris researchers used the droplet setup to demonstrate single- and double-slit interference. They discovered that when a droplet bounces toward a pair of openings in a damlike barrier, it passes through only one slit or the other, while the pilot wave passes through both. Repeated trials show that the overlapping wavefronts of the pilot wave steer the droplets to certain places and never to locations in between — an apparent replication of the interference pattern in the quantum double-slit experiment that Feynman described as “impossible … to explain in any classical way.” And just as measuring the trajectories of particles seems to “collapse” their simultaneous realities, disturbing the pilot wave in the bouncing-droplet experiment destroys the interference pattern.

Droplets can also seem to “tunnel” through barriers, orbit each other in stable “bound states,” and exhibit properties analogous to quantum spin and electromagnetic attraction. When confined to circular areas called corrals, they form concentric rings analogous to the standing waves generated by electrons in quantum corrals. They even annihilate with subsurface bubbles, an effect reminiscent of the mutual destruction of matter and antimatter particles.

OK, let's be VERY clear on this, shall we? In this experiment, there is a very important feature here that needs to be  pointed out. We can DETECT these "pilot waves" that are steering these droplets. This is a very, VERY, important point here. In QM, these pilot waves have NEVER, EVER, been detected. That is a very significant difference, and the main factor why the pilot-wave model hasn't caught on! Trust me, if there's physical evidence for it, physicists WILL adapt it! As of now, there are no deviations between the predictions of the conventional QM versus the pilot-wave picture. So how can one tell which one to accept beyond just a matter of taste and personal preferences?!

The other thing that needs to be pointed out is that this is, at best, an ANALOGOUS situation to the pilot-wave picture. It is NOT an identical situation. The "discovery" of magnetic monopole in the spin-ice system did not turn elementary particle and EM upside down, because while these "monopoles" sure have the same characteristics of the bare monopoles, they are, at best, only analogous to them. These are still NOT what we are looking for! It is not the same thing.

Until there is direct evidence of such pilot wave, or until there is evidence that support the prediction of pilot-wave but not the regular, conventional QM picture, then we have no strong evidence to support or falsify one or the other. Period. It is irrelevant how many droplets and wave experiments one performs.

Zz.

Explosive Beer Trick Explained

If you were ever half-drunk at a bar and started to wonder the physics of that explosive beer trick, now your curiosity can be set to rest.

But of course, no funding agency will pay for someone to study the neat tricks one can do with beer. So there is a more "useful" consequence to this.

Explaining this phenomenon may make you the life of your next party, but Rodriguez-Rodriguez and his colleagues studied beer in order to understand bigger-picture gaseous eruptions. One example is the Lake Nyos disaster in Cameroon. Volcanic activity under this lake dissolves carbon dioxide in the water. In 1986, the lake rapidly degassed a large amount of carbon dioxide all at once, suffocating 1,700 people and thousands more livestock. This rapid degassing event, possibly caused by a landslide, could share similar physics with an erupting beer bottle.

Like I've already said many time, a lot of things are inter-related.

Zz.

The Physics Of Urine Splashback

Oh, I kid you not, dear readers!

First of all, I'm sure you can guess that this isn't just something trivial with no applications elsewhere. But still, it is rather amusing that there is an active research on the problem of urine splashback.

If you don't know what it is, or maybe this is not a problem you normally have (especially if you have a different anatomy than a man), let me explain. It is the splashing that occurs when a thin stream of water hits a water surface. OK, sounds familiar? Good!

So what's the problem, you ask?

"In response to harsh and repeated criticisms from our mothers and several failed relationships with women, we present the splash dynamics of a simulated human male urine stream," reads their conference abstract.
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One might think the physics of aiming urination had already been summarised by the formula: "get it all in the bowl". But micturation is still a messier business than it needs to be, according to the research.

Taking measurements live "in the field" did not appeal to the scientists, so the duo built a urination simulator. The "Water Angle Navigation Guide" is a five-gallon bucket with hoses connected to two types of synthetic urethra.

OK, that's just way to hysterical. And oh, there's a video in the link above to show you the dynamics.

So, is anyone here going to attend the APS meeting where this will be presented? I would appreciate someone reporting back.

Zz.

The Physics of Whistling Tea Kettle

I kid you not.

I would think that, considering that this is quite common already, people would have known the intricate physics of the whistling kettle. Turns out, I was wrong!

Cambridge University researchers recently published a paper in the journal The Physics of Fluids, describing what’s considered the first accurate model for kettle whistling dynamics.

Think this is trivial research? It actually has more far reaching implications. According to the press release, these dynamics could be used to stop pipes in household plumbing from squealing or car exhausts from sounding, well, exhausted.

There you have it!

Zz.

Zero Gravity Coffee Cup

This video is more about fluid mechanics in microgravity, but hey, coffee is important too, even in space! :)



Zz.

Sonoluminescence

Here's Minute Physics description of sonoluminescence.



Zz.

The Physics Of Spilled Coffee

As someone mentioned at the end of this article, this work is destined for the Ig Nobel prize.

If you wish to know more about how and why coffee often gets spilled when being transported in a mug, this might be of interest to you. It was even published in PRE.

A fluid's back-and-forth movement has a certain natural frequency, and this is determined by the size of its container. In their paper published last week in Physical Review E, Krechetnikov and Mayer show that everyday mug sizes produce natural frequencies that just happen to match those of a person's leg movements during walking. This means that walking alone, without any other interference, is tuned to drive coffee to oscillate in a mug. But the researchers also found that even small irregularities in a person's walking are important: These amplify the wilder oscillations, or sloshing, which bumps up the chance of a spillage. 

 So go for either very small mugs, or very big ones (I vote for big). :)

Zz.

The Physics of Floating Pyramids

Or as the title of this article goes, it is the UNUSUAL Physics of Floating Pyramids.

It turns out that, based on the research being reported in the article, a top-heavy pyramid appears to be better at straightening itself out when it is floating on a stream of air from below.

The researchers placed hollow paper pyramids inside the cylinder. The objects were about 1 to 5 centimeters high and were made of tissue paper or letter paper on carbon fiber supports, like tiny homemade kites. Physicist Bin Liu led the experiments, attaching a beadlike weight to a post running down the center of the pyramid and changing the height of the bead to give the object a different center of mass. Common sense says that the pyramid should be most stable when the bead is at the bottom of the post, like ballast in the hold of a ship. But when the team released the pyramids over the subwoofer, the opposite was true: The bottom-heavy pyramids were likely to flip over and fall, whereas the top-heavy ones remained upright and continued to hover (see first video), the group reports in an upcoming issue of Physical Review Letters.

The video is available in that link. I'll keep an eye out on PRL and update this entry when I have
the exact citation for the paper.

Zz.

Edit: We have a synopsis of this work AND the exact reference to it.

The Explicit Siphon

I'm not sure what "explicit" means in the title, but that's what we have in this latest paper by Binder and Richert published in Physics Education. This is a follow up to an earlier article that I mentioned a while back that tries to correct a correction on the physics of a siphon. It certainly turns out that a common phenomenon such a siphon can stir up (no pun intended) such lengthy discussion, which isn't that unusual in physics. In fact, some of the most fascinating discussions that I've had were on such "mundane" physics.

Zz.

Surface Tension

This is a nice video of a very good and simple demo of surface tension. Kids, you can try this at home!




Hum... iPhone grip? This was taped using an iPhone?

:)

Zz.

The Physics of Coffee Rings

Here's another example of the "mundane" curiosity that I love very much.

This time, it is on the coffee rings that we typically see after a drop of coffee dries. And the physics is important enough, and has important applications, that something that appears as mundane as this gets published in Nature, no less.

The edges of a water drop sitting on a table or a piece of paper, for example, are often "pinned" to the surface. This means that when the water evaporates, the drop can't shrink in circumference but instead flattens out. That flattening motion pushes water and anything suspended in it, such as coffee particles, to its edges. By the time the drop fully evaporates, most of the particles have reached the edge and are deposited on the surface, making a dark ring.

There's also a video that accompanies this:

Zz.

The Physics Of Ultrasonic Cleaning

As someone who uses ultrasonic cleaning quite often, it is fascinating to me that we are still learning on what actually goes on when we put something in an ultrasonic cleaner. This study, published in PRL no less, examines the physics of how things actually are cleaned on the surface using an ultrasonic cleaning process.

In some experiments, the researchers coated one side of every bead with gold, so they could see the beads' rotation. The beads rolled back and forth during a single cycle of the bubble's expansion and contraction, some at a rate corresponding to 150,000 rotations per second. This rapid rotation implies a large twisting force (torque) on the beads. The torque arises because the beads rest partially in a thin, stationary layer of fluid at the glass surface and partially in the fluid above, which moves rapidly in response to the bubble. "The rotation is a very clear indication that this boundary layer is important," says Ohl. Similarly, in ultrasonic cleaning, a dirt particle stuck to the surface will extend partly out of the boundary layer and feel a large torque that can break its connection to the surface.

The link also has a video of the action.

Zz.