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July 17

Beer composition

Beer is an alcoholic drink made by fermenting malt and hops. But what exactly is the finished drink composed of? As far as I have understood, it's a solution of alcohol in water, with microscopic bits of malt and hops floating in it. Am I correct here? Can someone supply a more technical description? JIP | Talk 03:09, 17 July 2021 (UTC)

[1], [2], [3], [4], [5] In short, your intuitive analysis is pretty much accurate: Approximately 90% water, 5% ethyl alchohol (on average), and the rest yeast, hops, malt, and trace amounts of sugars, spices, preservatives and other additives. Snow ^{let's rap} 05:44, 17 July 2021 (UTC)

Note that the "microscopic bits of malt and hops" are in large part proteins, oils and other chemical compounds that have been extracted from malt and hops, or are the products of reactions between them, the "liquor" (brewing-speak for water), the products of the yeast's metabolism and cellular substances, and other added trace constituents. If the beer is "real ale" and therefore unfiltered, there should be an appreciable quantity of still-living and recently expired yeast cells. There will of course also be a good deal of dissolved CO₂, either as a metabolic product of the yeast, or (if the beer is not "real" but pasteurised) added artificially under pressure. (Amounts vary, but typically at least 1 unit of dissolved CO₂ at STP per unit of liquid beer.) {The poster formerly known as 878.81.230.195} 2.122.177.31 (talk) 14:33, 17 July 2021 (UTC)

Now this is a good answer, thank you. So it's a solution of alcohol, proteins, oils and other compounds in water. JIP | Talk 15:53, 17 July 2021 (UTC)

Some of the constituents will be in solution, some in suspension.. Also, there will be (hopefully small) traces of any substance(s) used for fining the beer. {The poster formerly known as 87.81.230.195} 2.122.177.31 (talk) 21:36, 17 July 2021 (UTC)

Heat pumps

"But heat pumps are different, as they don’t generate heat. Instead, they move existing heat energy from outside into your home. This makes them more efficient. Since they deliver more heat energy than the electrical energy they consume." [6]

"It costs about 4.6p per kilowatt hour (kWh) to heat a home with gas. And it’s about 9-16p per kWh using standard electric heaters. A typical air source heat pump might cost about 4.7p per kWh to run. But if you use cheap-rate Economy 7 electricity or an GoElec tariff this could be as low as 2.3p! [7]

If heat pumps "deliver more heat energy than the electrical energy they consume", why would they cost money to run? They should make money.

"Air source heat pumps (ASHPs) absorb heat from the outside air to heat your home and hot water. They can still extract heat when air temperatures are as low as -15°C." [8]

How can it be possible to transfer heat from a lower temperature (-15°C) to a higher temperature (inside your home) without net using energy? Doesn't this violate a fundamental thermodynamic principle?

It DOES use energy in the form of electrical energy. It is identical to using electrical energy to power a refrigerator. Refrigators operate by pumping energy (in the form of heat) out of the cold space and jettisoning it into the surroundings. Neither a heat pump nor a refrigerator violates the first law of thermodynamics or the second law of thermodynamics. Dolphin (t) 22:42, 17 July 2021 (UTC)

Well, clearly not. However, if heat pumps "deliver more heat energy than the electrical energy they consume" then why can a portion of the delivered heat energy not be converted (somehow, in principle anyway) to the electrical energy required, or the energy required to turn the compressor, leaving "free" heat energy left over?

I just don't get why there is any benefit in using electricity to drive a heat pump instead of using the electricity to heat the water directly (let's assume for the purposes that we are heating water), having, if necessary (if the outside temperature is greater than the starting temperature of the water) first let the water naturally reach ambient outside temperature. Again, it seems like "something for nothing". Where is the extra "something" coming from? 2A00:23C8:7B08:6A00:E4E6:ABBB:EB80:8B84 (talk) 08:48, 18 July 2021 (UTC)

A heat pump simply transfers heat energy from elsewhere. I suggest you read our articles on heat pump and latent heat.--Shantavira|^{feed me} 09:57, 18 July 2021 (UTC)

Thanks, but this answer does not really help me answer my specific questions. Also, if one could willy-nilly transfer (heat) energy from one place to another, then we could all boil our kettles at no cost from ambient heat, which is clearly impossible. If anyone is able to shed any light on all of this, it would be very helpful to me. 2A00:23C8:7B08:6A00:145A:9AC7:ECA2:BB33 (talk) 10:38, 18 July 2021 (UTC)

Why can't we use some of the heat from the surroundings to power the engine that drives the heat pump? Nobody seriously proposes to do that because the proposal violates the second law of thermodynamics which I admit is one of the more challenging concepts in physics. A good place to start with the second law is to see that it says heat can flow spontaneously from a region of higher temperature to one of lower temperature but heat cannot flow spontaneously from a region of lower temperature to one of higher temperature. For heat to go from a lower temperature to a higher temperature it must be pumped and that always requires high-quality energy (such as electricity) from somewhere. Dolphin (t) 12:13, 18 July 2021 (UTC)

The first source you are quoting is somewhat optimistic. Heating a building with an air source heat pump in extremely cold weather takes considerably more energy than in the average British weather conditions of the last few decades. Properly isolating a house against thermal loss may have more effect. Heat pumps cost money to run because one needs to buy the electricity to make them run, and there is no obvious way of selling the heat energy they deliver. Whether air source heat pumps are truly a lower-cost solution depends on the particulars of the systems being compared.  --Lambiam 11:10, 18 July 2021 (UTC)

Re. However, if heat pumps "deliver more heat energy than the electrical energy they consume" then why can a portion of the delivered heat energy not be converted (somehow, in principle anyway) to the electrical energy required, or the energy required to turn the compressor, leaving "free" heat energy left over? You cannot convert low-level heat energy directly into electricity at high efficiency. Electricity power stations create steam (actually superheated steam) to power a turbine generator. If you can work out a way to use the heat radiated from the back of a refrigerator to power the compressor (somehow, in principle anyway) you will probably be the richest person on the planet in short order, solve global warming and provide free, almost limitless energy for generations to come. Seriously though, heat energy and electrical energy are two very different things. It's easy to turn electricity into heat, not so easy to convert heat into electricity. nagualdesign 11:32, 18 July 2021 (UTC)

The second law of thermodynamics page is a long read. All that you need to know for the purpose of calculation is, that for an ideal heat pump, if it takes in a quantity of heat $Q_{1}$ at a temperature $T_{1}$ and discharges a greater quantity of heat $Q_{2}$ at a higher temperature $T_{2}$, then

${\frac {Q_{2}}{Q_{1}}}={\frac {T_{2}}{T_{1}}}$

and (by the conservation of energy), the energy required to drive the heat pump is $W=Q_{2}-Q_{1}$. Here $Q_{1}$, $Q_{2}$ and $W$ can be in any units - kWh say, but $T_{1}$ and $T_{2}$ are the absolute temperature, which is usually measured in Kelvin - i.e. Centigrade + 273.16°.

An ideal heat engine, whether a steam engine or a thermoelectric generator is subject to exactly the same equations, but now $T_{2}$ is lower than $T_{1}$, and $Q_{2}$ is less than $Q_{1}$, so that useful energy is produced.

Thus in the proposed arrangement of a heat pump supplying a heat engine, the heat pump requires all of the the output of the engine to drive it and vice versa. This is perpetual motion of the second kind. A real, imperfect heat engine produces less energy and real heat pump consumes more. catslash (talk) 15:09, 18 July 2021 (UTC)

A heat pump basically air-conditions the outdoors. It pumps cold air from your house into the environment. That means your house gets warmer. Think of a room A, with an air conditioner whose exhaust goes into another room B instead of the outside. The air conditioner motor consumes electric power and dissipates it as heat: let's say it is set up so the dissipated heat also goes into room B. Meanwhile the air conditioner is making room A colder because that's what they do. So room B is getting heat from two sources: the dissipated electric power, and heat extracted from room A, even though A is colder than B. Looking at that from a heating rather than A/C perspective, room B is being heated more efficiently by also pulling heat from room A, than it would by electric resistance alone.

The basic mechanism and efficiency limits of this process can probably be found at Carnot cycle. In the case of real heat pumps that you install in a home (or also air conditioners), the electricity-in to heat-energy-out ratio is called the coefficient of performance (COP). A household heat pump in a not too frigid environment can often have a COP of around 3, depending on temperature differential etc. COP is sometimes converted into other units like energy efficiency ratio (EER) but COP is imho a more natural measurement. 2601:648:8202:350:0:0:0:2B99 (talk) 21:50, 18 July 2021 (UTC)

Why don't weather reports state the wet-bulb temperature?

I've been reading about how wet-bulb temperature is important in a heat wave, but Accuweather, Weather Channel, and Weather Underground don't report the number. Must we calculate it manually?

AccuWeather has a patented exclusive RealFeel  Temperature, which they really feel is a better measure for how bad it gets in a heat wave. The Weather Channel has a similar FeelsLike temperature.  --Lambiam 10:37, 18 July 2021 (UTC)

Weather reports state the relative humidity and that is more meaningful to most people. If you want the wet-bulb temp you must measure it or use a psychrometric chart. Dolphin (t) 22:56, 17 July 2021 (UTC)

Wet-bulb temperature says 95°F is when sweating stops working which can kill anyone if it's >95°F for more than a few consecutive hours. Eventually this will start happening on Earth and wet-bulb temperature should be more in the news. In theory if dew point stayed above lung temperature the lung will collect dew like cold object until you drown. Sagittarian Milky Way (talk) 00:08, 18 July 2021 (UTC)

Also probably because it's generally no longer measured. In the days before electronic weather stations, the only reliable way to get humidity data was with wet and dry bulb thermometers. Now, electronic humidity sensors are the norm, so it's no longer necessary to observe physical thermometers.--Phil Holmes (talk) 09:34, 18 July 2021 (UTC)

Wet bulb temperature is merely one of the "inputs" needed to calculate relative humidity which is the actual meaningful information. Roger (Dodger67) (talk) 09:41, 18 July 2021 (UTC)

See also Heat index and Apparent temperature.  --Lambiam 10:26, 18 July 2021 (UTC)

Yeasts on fruits

I've read that practically all fruits bear yeasts. Sourdough recipes, for example, call for taking fruit peels or raisins. Some of the fungi spoil fruits, though, and I know not to eat spoilt foods. There are also other diseases, including cancer, caused by certain fungi, I think. How safe, then, are the yeasts on fruits? Imagine Reason (talk) 22:59, 17 July 2021 (UTC)

Within the fungus kingdom, the (unicellular) yeasts are distinct from the (multicellular) molds. Several species of yeast in the Candida genus, including species that are part of the normal oral and intestinal flora of healthy individuals, can under certain conditions cause an infection known as candidiasis. Yeasts are mainly known for their health benefits,^[9] but some species thus far generally considered safe (e.g. Saccharomyces cerevisiae) are now suspected of being potentially pathogenic.^[10] When baking bread, however, all yeast cells in the dough are killed.  --Lambiam 10:19, 18 July 2021 (UTC)

When people try to grow their own yeasts, what are the chances they won't be growing molds as well? Just killing those would not render the food safe, I think? Imagine Reason (talk) 14:59, 18 July 2021 (UTC)

In general, the vast majority of environmental yeasts (and bacteria) are not capable of causing human disease. They are trying to grow and reproduce on/in a very specific plant substrate. In the few cases where there are problems (aflatoxins, for example), public health authorities detect the problem, and step in to require the food manufacturers to do testing and mitigation. See Hazard analysis and risk-based preventive controls for how this is implemented in the US. Abductive (reasoning) 15:13, 18 July 2021 (UTC)

Aflatoxins are produced by molds, not yeasts. There are in fact indications that S. cerevisiae may offer some protection against aflatoxins.^[11]  --Lambiam 19:08, 18 July 2021 (UTC)

I am aware of that, it is an example of something that they caught. Abductive (reasoning) 03:37, 19 July 2021 (UTC)

Bear in mind, Imagine Reason that we (humanity) have been doing this sort of thing for literally thousands of years, and have found by trial and error how to do it properly and what does and doesn't normally give rise to any health problems.

As an aside, you might find the articles Lambic and Noble rot interesting. {The poster formerly known as 87.81.230.195} 2.122.177.31 (talk) 17:13, 18 July 2021 (UTC)

I don't take comfort in traditions. Smoked meats and other food preps may have worked for most of humanity history, when starvation was a bigger worry than cancer risks, but it's not considered safe anymore. Imagine Reason (talk) 20:43, 18 July 2021 (UTC)

According to Smoked meat, "One study has shown an association between the frequency of consumption of smoked foods and intestinal cancer. However, the study was restricted to a small Slovenian population in Hungary, where the local smoke curing process produces levels of contaminants roughly eight times as high as standard processes elsewhere." That's not very convincing. Cullen³²⁸ Let's discuss it 22:03, 18 July 2021 (UTC)

Smoke causes carcinogenic polycyclic aromatic hydrocarbons (PAHs). I don't eat grilled or smoked meat or fish. Thankfully my palate doesn't drive me to seek out such pleasures. Imagine Reason (talk) 23:12, 18 July 2021 (UTC)

I'll give another example of tradition being more harmful than good: Alcohol. I rarely drink it now that the science has swung against it. Third example: Chinese medicine. Imagine Reason (talk) 23:32, 18 July 2021 (UTC)

Also bacon and other processed meats. I rarely eat them at all since reading the report several years ago. nagualdesign 18:43, 19 July 2021 (UTC)

Give me some offals! Or not. Imagine Reason (talk) 00:46, 22 July 2021 (UTC)

July 18

C/2014 UN271 (Bernardinelli-Bernstein) visibility

Are there any estimates on whether or not C/2014 UN271 (Bernardinelli-Bernstein) will become visible to the naked eye? Bubba73 ^{You talkin' to me?} 17:13, 18 July 2021 (UTC)

Despite its massive size Comet Bernardinelli-Bernstein sadly won’t ever get close enough to Earth for us to have a look naked-eye... In fact, it will only reach magnitude +17 as seen from Earth, which is well out of reach for backyard telescopes. [12] Alansplodge (talk) 18:10, 18 July 2021 (UTC)

It is unlikely to ever become visible to the naked eye taking into account its perihelion distance. Ruslik_Zero 18:12, 18 July 2021 (UTC)

That is what I was thinking, with a perihelion of ~ 10AU. Bubba73 ^{You talkin' to me?} 23:04, 18 July 2021 (UTC)

Resolved

July 19

Covering bleeding wound with potato chip bag (or any non-absorbent material)

Are potato chip bags appropriate to cover a bleeding wound? I imagine that's better than nothing, but wouldn't an absorbent material be much more appropriate? I'd also wonder how clean a potato chip bag can be. Hands are a common vector of germs. --Bumptump (talk) 13:39, 19 July 2021 (UTC)

Presumably you're referencing this recent news story.[13] The details indicate the cop had someone get a bag of chips and some tape from a nearby store. Why a chip bag and not something better is hard to know for sure, but the cop probably went with something he could be sure they would have. It's also not like he picked it up off the street, so it was cleaner than it might have been. Sometimes when applying first aid, you have to go for expediency over optimal cleanliness. ←Baseball Bugs ^{What's up, Doc?} carrots→ 15:09, 19 July 2021 (UTC)

Clingfilm is a good alternative. The idea is to stem the bleeding and protect the wound from infection. Removing or disturbing the dressing, if it's absorbent, can cause more bleeding. Absorbent dressings are good if the wound has already been properly treated. nagualdesign 18:49, 19 July 2021 (UTC)

Per Bugs, appropriate compared to what? Are we talking "use the bag or let them bleed to death?" or are we talking "use the bag or use some sterile wound dressings"? The particulars of a situation and the choices available to the person at the time determine the appropriateness of their choices. With only a few seconds to make quick decisions, and with a lack of proper supplies, sometimes people make do. --Jayron32 18:51, 19 July 2021 (UTC)

I can't read the recent news story linked above, but there was a recent one about a polis using a crisp packet on a sucking wound. This is something I've been taught as a makeshift on First Aid courses. DuncanHill (talk) 20:22, 19 July 2021 (UTC)

Yes, same incident, an excellent writeup with details the OP should find interesting. The address given, 290 Lenox Avenue, is at about 125th Street in Manhattan, about 15 blocks north of Central Park and about 2 blocks east of the Apollo Theater. There are plenty of cafes in the area, so getting a bag of potato chips would have been easy - and it would be a good bet that it's not the first time a cop has had to use that trick. ←Baseball Bugs ^{What's up, Doc?} carrots→ 21:04, 19 July 2021 (UTC)

290 Lenox/Malcolm X is about 80 feet from a NYC delicatessen, those are almost guaranteed to have potato chip bags. Sagittarian Milky Way (talk) 04:26, 20 July 2021 (UTC)

• I'm no medic, but I took a 3 week life saver course in the Army years ago, and I know that non-porous bandages are a thing. One technique I remember was packing a wound with gauze and wrapping it in a non-porous bandage to cut down external bleeding (for when bleeding is "tactically ill-advised"), and another use was for sucking chest wounds, where I can be applied in such a way as to form a one-way valve. The main concerns are packing and keeping pressure on a wound to encourage clotting and pinch off any damaged arteries or veins. Note that this is all very much first-aid oriented. It may be very poor advice in an emergency room, and once again, I'm no expert. ᛗᛁᛟᛚᚾᛁᚱPants Tell me all about it. 21:53, 19 July 2021 (UTC)

  @MPants at work: What's that dry powder that you see soldiers pouring into bullet wounds in movies? Reading this it seems that keeping a wound moist is important (although that article is not really about emergency/temporary field dressings). Is it mainly to cause the hero to wince in pain, demonstrating how tough he is, in case taking a bullet and carrying on regardless wasn't enough? nagualdesign 22:31, 19 July 2021 (UTC)

  @Nagualdesign:: the powder is sulfonamide, see end of 3rd paragraph in this section: Sulfonamide_(medicine)#History.Bumptump (talk) 23:07, 19 July 2021 (UTC)

  This is correct, nagualdesign. It's also coagulant, to help stem bleeding. FWIW, we didn't have much in the way of little baggies of powder when I was in (AFAIK, that's a WWII-Vietnam era convention), but there were some for emergencies, and we had lots of different ready-made bandages impregnated with the stuff. ᛗᛁᛟᛚᚾᛁᚱPants Tell me all about it. 12:07, 20 July 2021 (UTC)

As I understand the story, the point of sealing the wound was not to keep the blood in, but to keep the air out.  --Lambiam 23:07, 19 July 2021 (UTC)

Por qué no los dos?--Jayron32 15:18, 20 July 2021 (UTC)

I thought the point of band-aids was to keep bacteria from coming in (as well as keeping blood from coming out), but since you mentioned air, does air bring in bacteria? I thought it was mostly bacteria on the nearby skin that would migrate in. 67.165.185.178 (talk) 06:32, 21 July 2021 (UTC).

Quoting our article Pneumothorax: "Any open chest wound should be covered with an airtight seal, as it carries a high risk of leading to tension pneumothorax." Here, "open chest wound" means a wound that creates a passageway allowing air to enter the pleural space. Tension pneumothorax is a life-threatening condition, as it may lead to cardiac arrest. Keeping bacteria out was under the circumstances not an immediate goal. If you run out of your burning house, it is a good idea not to leave the door open and allow new oxygen to enter. Closing the door may also keep mosquitoes out, but that is not the point.  --Lambiam 07:26, 22 July 2021 (UTC)

Ah, so probably the pressure of air to open-wounds is not suitable, pressure inside blood vessels is likely different? Hence the possibility of cardiac arrest. Body has some mechanism of fighting bacteria in the blood, but different pressures is a very different issue. 67.165.185.178 (talk) 10:47, 22 July 2021 (UTC).

Separate question, but Amoxicilin is a medicine that kills bacteria in the blood. But of course, it requires a doctor's prescription (or dentist). Are there any over-the-counter anti-biotic? That don't require a prescription? I'm also wondering if there are any cases where, a doctor's prescription and non-prescription are the same ingredient, only different concentrations. 67.165.185.178 (talk) 02:33, 20 July 2021 (UTC).

As a general rule, no: while there is some variability across localities, if you are living in an American or Commonwealth nation as your English implies, access to any medical-grade formulations of oral antibiotics designed and manufactured with human consumption in mind is going to be strictly controlled and regulated. If used in an amount or fashion inconsistent with proper medical supervision, all antibiotics have potential for not insignificant harm. Additionally, over use of antiobiotics in a completely unregulated environment would have massive implications for Antibiotic resistance and superbugs. Beyond that, we really cannot comment: your question here buts up against our rule against providing medical advice here. If you have a medical condition you believe would benefit from antibiotics, you should consult a licensed professional, and I recommend that advice if your inquiry is meant only for hypothetical future reference. Snow ^{let's rap} 03:55, 20 July 2021 (UTC)

The cited link is unavailable for European users, for some reason I don't understand. Can someone provide a link to whatever incident this was about? JIP | Talk 01:19, 21 July 2021 (UTC)

Yes, certain European newspaper websites block USA viewers and vice versa. It's to save bandwidth. But the url itself tells you a lot. 67.165.185.178 (talk) 01:24, 22 July 2021 (UTC).

@JIP: This American paper and this BBC link should work for you. DuncanHill (talk) 07:40, 22 July 2021 (UTC)

How gyroscopic forces helps us to keep upright when riding a bicycle

As far as I understand, when riding a bicycle the angular momentum of a spinning wheel creates a small, and not very useful, force that tends to keep the bike upright. This force is negligible, compared to the weight of the rider. Basically, riding upright depends of our ability to ride upright. Is that right? Do modern physic textbooks normally depict it this way? --Bumptump (talk) 13:45, 19 July 2021 (UTC)

Correct. The gyrostabilizing force is negligible. Balancing depends on the skill of the rider. nagualdesign 15:03, 19 July 2021 (UTC)

At the end of many stages in the Tour de France, for example, you'll see the winner peddling while holding his hands up in the air in celebration. Obviously he knows his bike well. But might that force described by the OP be a factor? ←Baseball Bugs ^{What's up, Doc?} carrots→ 15:11, 19 July 2021 (UTC)

Much stronger than the gyroscopic effect is the effect caused by the geometry of the bike. The forward angle of the front wheel makes the bike steer into the direction it leans self correcting. However it does appear to be mostly the actions of the rider Opinion: How does a bike stay upright? Surprisingly, it’s all in the mind -- Q Chris (talk) 15:25, 19 July 2021 (UTC)

I don't think that the force is negligible. It provides information to the rider about the balance. Abductive (reasoning) 16:11, 19 July 2021 (UTC)

We have a fairly lengthy Bicycle and motorcycle dynamics article. DMacks (talk) 23:33, 19 July 2021 (UTC)

Slightly off-topic but readers may be interested. My nephew could ride a push bike the first time he got on one, because he'd previously had a balance bicycle. Apparently, learning to balance is a piece of cake when you don't have to think about pedalling, and pedalling is easy once you've learned how to balance. Highly recommended for anyone with young children. Of course, in my day we got scratches and bruises and we were thankful! nagualdesign 16:30, 19 July 2021 (UTC)

• I still wonder whether my secondary question is true: do modern physic textbooks get this right? Or is it already ingrained into physics that gyroscopic forces are what keeps the bike upright and authors just keep repeating this from other authors? A textbook in conceptual physics, Physics of Everyday Phenomena by Thomas Griffith, at least in the 6th edition, clearly believes otherwise. Bumptump (talk) 23:17, 19 July 2021 (UTC)

I don't read physics books, other than Feynman, but have a look at this http://www.phys.lsu.edu/faculty/gonzalez/Teaching/Phys7221/vol59no9p51_56.pdf Greglocock (talk) 23:37, 19 July 2021 (UTC)

It is not only the action of the rider. When an upright moving bicycle without rider starts to lean to one side, say for concreteness to the right (turning on a horizontal axis), the effect of the gyroscopic force on the front wheel makes it turn right (turning on a vertical axis). While the force is small, not much force is needed to make the wheel turn. This adds to the already substantial effect of the front wheel turning in these conditions because its axis of rotation is slanted with respect to the vertical (as mentioned above). The moving bicycle will then start a turn to the right, resulting in a centrifugal force that uprights the bicycle (if it has enough speed). The faster it goes, the stronger and quicker this automatic correction happens, so that a fast-moving riderless bicycle can travel quite some distance in an almost straight path.  --Lambiam 09:08, 20 July 2021 (UTC)

Gyroscopic effect and trail of the front wheel both contribute to the stability of the bicycle. You can put both in a mathematical model and solve it analytically and find that a bicycle is stable (in the sense that the real parts of the eigenvalues are negative, only at the right speed), but you can't really say which of the two effects causes this. And it's possible to make a bicycle with counterrotating flywheels (to eliminate the gyroscopic effect) or a reversly bent front fork (to eliminate the trail) and it may still be stable. Further, your average bicycle is unstable at high speeds, but only just. Deviations from equilibrium grow exponentially on a timescale on the order of one minute, so that's easy to correct by the rider. A bicycle at very low speed however can fall over in a second, making corrections by the cyclist much harder. PiusImpavidus (talk) 12:26, 20 July 2021 (UTC)

I think wheel size is important. I can ride my "normal" ten speed (26" wheels) no hands, but not my Brompton - it is far too unstable. Of course, this might be due to the frame geometry. --TrogWoolley (talk) 14:18, 20 July 2021 (UTC)

Definitely. Assuming the linear density of rim+tire is the same, the moment of inertia is proportional to the cube of the radius of the wheel, so larger wheels give much stronger gyroscopic effect. PiusImpavidus (talk) 08:58, 21 July 2021 (UTC)

If you stand on one foot while lifting the other high in the air, you might find it hard to keep your balance, but gently touching just one finger to the wall makes it much easier even though very little force is applied. It's similar with the angular momentum of the wheels turning. Balancing a bike without the wheels turning is called a track stand and it is harder, though doable with practice. 2601:648:8202:350:0:0:0:2B99 (talk) 01:17, 21 July 2021 (UTC)

Maps Amhara Region

Greetings, i was referenced here by a theahouse editor, can you help get me maps for the Amhara Region article, a region in Ethiopia?

• Köppen climate classification map of the region, and smaller subdivisions within the region if possible
• Topographic/altitude map
• Drainage map
• Agricultural maps of any kind

Thanks Dawit S Gondaria (talk) 17:16, 19 July 2021 (UTC)

Hey @Dawit S Gondaria: thanks for stopping by. I hope you get some more responses soon, but in the past I have had luck getting maps made at Wikipedia:Graphics Lab/Map workshop. There are several volunteer cartographers who help making maps. They may be able to help out. --Jayron32 18:48, 19 July 2021 (UTC)

@@Jayron32: Thank you so much, i will def try there! If anyone else has a better suggestion please do share Dawit S Gondaria (talk) 05:34, 20 July 2021 (UTC)

July 20

A wind from 4 sides simultanously - possible?

Wind from 4 sides simultaneously - possible? I'm asking this question due to the reading in the bible, the first chapter of Job (1;18-19): "While he was still speaking, yet another messenger came and said, “Your sons and daughters were feasting and drinking wine at the oldest brother’s house, 19 when suddenly a mighty wind swept in from the desert and struck the four corners of the house. It collapsed on them and they are dead, and I am the only one who has escaped to tell you!”. Could it be any scientific explanation for this story?--ThePupil (talk) 02:36, 20 July 2021 (UTC)

The commentaries I've read suggest it would be a whirlwind. Given a "house" in that context it likely mostly just fabric and sticks, more like a tent, a strong swirling wind could easily cause it to collapse in on itself. DMacks (talk) 02:47, 20 July 2021 (UTC)

You must remember that any version of the Bible that comes to you in contemporary English is the result of numerous revisions and translations over the course of millennia, the result of which is a very great amount of idiomatic phrases arising out of anachronism and translational idiosyncrasy. (see Bible#Development, Biblical Canon, Authorship of the Bible, Bible translations, Bible translations into English, and Dating the Bible for just a summary of this immensely complicated subject). Further, often times these outmoded linguistic structures and the accreted peculiarities come to get codified somewhat because they are simply perceived, by way of the liturgical context in which they are learned, as the most appropriate and sanctified wording for the statements. As a consequence, you have every cause (in addition to the question of whether you are looking to take the actual events of the Bible to be considered apocryphal vs. factual) to not view the wording of biblical passages in their most literal possible interpretation here. I am no biblical scholar, so I cannot assist you with tracing backwards through versions of the work to arrive at the likely meteorolgical intent of "struck the four corners", but I rather expect you would find somewhere along that heritage that the phrase that was communicated was meant to convey a rather similar meaning to "shook it to its foundations" rather than "the wind came from four directions at once".

But to answer your query more directly: I know of no principled reason (owing to climatological principle or physical law governing air pressure) why four separate observable winds could not converge on one structure. Mind you, the idea of "a wind" as a unidirectional force pressure moving along a strict vector (as opposed to a rapid contoured change in pressure propagating in a non-uniform matter) is something an oversimplified mental construct. Unfortunately, I cannot supply you with a source which states expressly that it would be possible for four people standing in alignment with the corners of a structure to each perceive the wind to be coming from their cardinal direction, and in such amount that it causes said structure to collapse inward. The best I think you may get here is the observation that it does not seem to be contravened by any particular aspect of atmospheric dynamics. Snow ^{let's rap} 05:19, 20 July 2021 (UTC)

If the air comes in from different directions, converging on a small area, you get a pressure build-up that makes the accumulated air seek a way out. If there is no way out, the air will stop coming in when the local air pressure is equal to the wind pressure. The most obvious way out is in a vertical direction, the opposite of a downburst. But we could also have air flowing in from say 0°, 90°, 180° and 270°, with a horizontal outflow at 45°, 135°, 225° and 315°. This is unlikely to arise naturally, but you never know what a God will do who does not want to lose His bet with the Opponent.  --Lambiam 08:49, 20 July 2021 (UTC)

Similarly, a god (not being subject to the laws of physics) could make some of the air in the house disappear. This would create a partial vacuum which would be filled by wind from all directions and clearly could destroy the structure if the size of the effect was large enough. He could make it a sustained wind by continuing to destroy more air for some time. --184.147.181.169 (talk) 22:11, 20 July 2021 (UTC)

There is nothing in the Hebrew text suggesting the sudden wind coming from different directions. It is a ר֨וּחַ גְּדוֹלָ֜ה (rúakh gədoláh), a "big wind" (singular), coming מֵעֵ֣בֶר הַמִּדְבָּ֗ר (mê'êbér hamidbár) "across the desert". This does not sound like the description of a tornado to me. If we interpret this as a report on an actual event, keep in mind that the reporter is the sole survivor of the collapse of the house, managing to escape from the rubble, so this eye witness then apparently was attending the party inside and presumably had other things on their mind than delivering a accurate report of meteorological observations.  --Lambiam 08:32, 20 July 2021 (UTC)

I'd read "struck the four corners of the house" as "damaged the entire property" e.g not even one corner was left standing. The Living Bible even has: "engulfed the entire house so the roof fell in on them". The outcome is the same. Martinevans123 (talk) 09:03, 20 July 2021 (UTC)

July 21

Density trends in giant-planet moons

The Galilean moons get less dense in sequence as one gets away from Jupiter. This is usually explained by primordial heat from young Jupiter creating a strong radial temperature gradient that vapourised the volatiles especially on Io and Europa, IIRC. (Tidal heating probably helped too, I guess.)

However, the large satellites of Saturn and Uranus don't show any such trend. Is the explanation for this known yet? Double sharp (talk) 03:04, 21 July 2021 (UTC)

Which primordial moons are you considering large?Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Hyperion, Iapetus, Miranda, Ariel, Umbriel, Titania and Oberon? Sagittarian Milky Way (talk) 05:13, 21 July 2021 (UTC)

The ones big enough that gravity should have collapsed them into solid bodies. So, the bodies you mentioned, minus Hyperion. Double sharp (talk) 05:28, 21 July 2021 (UTC)

There are so many variables, scientists can't even agree if Jupiter was once further than modern Neptune or closer than modern Mars. If you very spherical cowedly modeled it as an ideal gas of 1.3333 gram per mole particles that is not hotter in the middle, and the same joules per kg as the planetary gravitational binding energy and the orbits are same as today and the planets were twice as wide (which is the value given for Jupiter in its article) and they're as old as their parent planets' heat peaks then the planet effective surface temperature would be enormous (especially for Jupiter) and the number of square degrees of that in the sky would cause Europa and Mimas be roasted to incandescence even if they were of snow-like Bond albedo. But Mimas and especially Miranda would be less hot than Europa. And the heat wasn't dumped all at once of course and Jupiter had at least ~9 times more of it to get rid of than Saturn but not much more surface area to radiate it away. So I dunno, maybe Mimas wasn't roasted enough to cause the density trend. It's not implausible. Sagittarian Milky Way (talk) 19:46, 21 July 2021 (UTC)

Well, are there any known (stronger) trends for moons as they get farther from a planet? As this density is not a straightforward trend. Good topic! 67.165.185.178 (talk) 00:51, 22 July 2021 (UTC).

July 22

Radio waves and information

So in Layman's terms, what's the basis for how radio waves hold and transfer information? Like for smartphones and Internet. And can it be done for other wavelengths? If the answer is yes, I'm sure it won't be done for a lot due to health reasons, but if radio waves can transfer information, then what about wavelengths as light? Does that mean we can use a flashlight to transfer information as well? 67.165.185.178 (talk) 11:23, 22 July 2021 (UTC).

Light can be and is used quite effectively - see Fiber-optic cable. Wikignome Wintergreen^talk 13:24, 22 July 2021 (UTC)

Very simply you can switch the flashlight on and off and transmit a message via Morse code. That is obviously very slow but it is a form of amplitude modulation, which is one method of transmitting information by radio waves (AM radio). Another method is frequency modulation (FM radio), which is quite a bit harder to do with optical light. --Wrongfilter (talk) 13:43, 22 July 2021 (UTC)

• I'll just point out that Morse code transmission by flashing lights is not just a hypothetical example; it's been used importantly, principally at sea, as seen in the movie The Hunt for Red October, for example. --184.147.181.169 (talk) 00:16, 23 July 2021 (UTC)

In basic terms, waves can be combined and uncombined through a mathematical process called fourier series. The theoretical basis is that literally any arbitrary function can be represented by a superposition of multiple sine waves and that composing and decomposing such functions is done by simple addition of the waves. Information of any kind is embedded in a carrier wave (this is what you "tune" your radio to) and then some electronics in the receiver extracts the information by decomposing the waves from the carrier wave. On paper fourier series is messy, but it takes a relatively few, simple electronic components to perform the process physically and extract information from radio waves. --Jayron32 14:25, 22 July 2021 (UTC)

If you had a top-quality digital sound file of someone tapping the C4 key of a piano once (no pedal), and similar files for each note and you knew which parts of which files should be playing when, then could you also sum them into a graph of any arbitrary sound? i.e. metal or JFK speeches. Sagittarian Milky Way (talk) 16:20, 22 July 2021 (UTC)

Yes, you could. It needn't be a sine wave, you can compose any arbitrary function using any arbitrary wave form or set of arbitrary wave forms and you can also reverse the process (in theory, though it could get very mess in practice). I highly recommend 3blue1brown's videos on the Fourier series process, he takes it from a wide variety of angles (historical development via the heat equation, modeling fourier series using computer graphics, the equations behind it, etc. etc. etc.) If you want a good demonstration of how to create arbitrary plots using the Fourier series process, Here is a video where he uses the concepts of Fourier to tell a computer how to draw any arbitrary picture using nothing but harmonic functions (essentially sine functions). It's quite beautiful. This video is a bit lengthier, but in it he explains how he made those animations, and the math behind it. --Jayron32 16:30, 22 July 2021 (UTC)

edit: I linked to the wrong article. I have corrected my links above. It's the Fourier series I meant to link to, the Fourier transform which I originally cited is a related concept, but not as applicable here. --Jayron32 16:38, 22 July 2021 (UTC)

doi:10.1109/50.54508 is an example of FM of light near the visible range. DMacks (talk) 16:01, 22 July 2021 (UTC)

Okay 1 thing that radio waves cannot be used as light waves, is that light also behaves as particles (photons). So can't use light waves as radio waves since light will not go through a solid object. Can there be the equivalent of photons for radio waves? 67.165.185.178 (talk) 23:40, 22 July 2021 (UTC).

Radio wave radiation IS photons. And wave at same time (wave-particle duality) Sagittarian Milky Way (talk) 00:02, 23 July 2021 (UTC)

July 23

Conversion factor for torque

Repost as suggested from the Mathematics section.

Hi all, I'm trying to find the conversion factor (or constant) to work out the torque of some old engines and their gearboxes. Torque is given in Kilopondmetres (abbreviated in my sources as mkg or kpm), and power is stated in metric horsepower (PS). I realise that max. torque is often reached at lower revs than max. power.
For example:

• Torque (lb.ft) = 5,252 * power (hp) / speed (rpm)
• Torque (N.m) = 9.5488 * power (kW) / speed (rpm)
• Torque (kpm) = X * power (PS) / speed (rpm) — what is X? And if you're feeling charitable, why? I understand that 5252 ≃ 33,000 ÷ 2π.

Cheers, MinorProphet (talk) 02:15, 23 July 2021 (UTC)

GNU Units does this kind of thing automatically:

You have: metrichorsepower/rpm
You want: kpm
* 716.19724
/ 0.0013962634
That is, X=716.19724. 2601:648:8202:350:0:0:0:2B99 (talk) 03:27, 23 July 2021 (UTC)

Wow, that's fantastic! Thanks very much for your help. My PC's hard drive has been developing bad sectors and is likely to die soon, so installing another app wouldn't be good idea: otherwise I would have tried GNU Units. I hunted around a bit for the database/list of supported units, but couldn't find it. Best wishes, MinorProphet (talk) 04:22, 23 July 2021 (UTC)

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