Wikipedia:Reference desk/Archives/Science/2017 April 11

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

April 11

Feynman Lectures. Lecture 51. Ch.51-2 Bore in a channel 2 archive

To understand the problem I have simplified the task. Suppose we have a piston in a tube png. A man acts from the right with force = 2N. An unknown force = 1N acts from the left. I write the energy conservation law:
$\sum E_{t=0}=\sum E_{t=1}$
${\text{Work by man}}=E_{\text{kin}}+?$
$F_{\text{man}}\cdot L=0.5m(v_{t=1})^{2}+?$
$2\cdot 0.5=0.5\cdot 1(1)^{2}+?$
$1=0.5+?$
$\therefore \,\,?=0.5J$
"?" is the work done by the piston against unknown force. Is it correct? Username160611000000 (talk) 07:56, 11 April 2017 (UTC)

There are various outcomes for the extra force provided by the man - it can heat the object by friction, or give it kinetic energy by moving it quickly. Of course if the piston is near weightless and frictionless it will take very little effort or time to push it to the final position where it will have very little energy; F=ma means a very large acceleration in that case from the known net 1 N force to the left. (intuitively, he has to run to catch up, which isn't counted - picture you're pushing a cart that you think is heavy with stuff but you realize it's very light stuff) But the mechanical work done is still F s, force times distance, i.e. this super light piston would fly off with incredible speed if the man could duck out of the way and let it go. That's 0.5 meters (using "m" for the mass value also made this rather confusing) times 1 N = 0.5 J work done against the piston. The rest could be trivial or terrible, depending on the piston. Wnt (talk) 23:39, 11 April 2017 (UTC)

In my example piston has a mass = 1 kg and its motion is frictionless. You mean that I should write:

${\text{Work by Resultant}}=E_{{\text{kin}},t=1}$

But according Feynman (Ch. 13 - 3) we can split the total force into components and total work becomes the sum of works by components. The man may not even know whether there is a force from the left. Moreover, the same amount of chemical energy is burned by man's muscles no matter is the force from the left present or not .

Username160611000000 (talk) 06:03, 12 April 2017 (UTC)

You cannot have both a frictionless piston and a constant velocity from a constant force. That contradicts Newton's second law. There must be a retarding force of some sort to constrain the piston velocity, or else it must accelerate. SpinningSpark 01:48, 16 April 2017 (UTC)

Insanely low calories

Looking at this product it says that per 100g it's 84 calories. The whole pack of six is 312g. Rounding the figures that works out about 300 calories per pack. Are you seriously telling me that I can stuff five packets of this stuff a day and still be in a calorie deficit and lose weight?? 68.190.17.93 (talk) 16:52, 11 April 2017 (UTC)

That depends on how much you burn. If I'd eat 5 packets of Quorn a day, I would probably throw up and lose even more weight. Note that the calories are comparable to lean meat - beef filet has 110-120 kcal/100g, filet of pork has slightly less, skinless chicken breast has a bit over 100 kcal/100g. --Stephan Schulz (talk) 17:19, 11 April 2017 (UTC)

• Yes, if that is the only thing you are eating; but 312g/day is not much (euphemism) - [1] says the average 2011 Somalian adult eats more than one kilogram per day, for perspective. Tigraan^{Click here to contact me} 17:31, 11 April 2017 (UTC)

• And note that eating them plain may be unpleasant. Add in sauces, bread, and cheese, and a drink and the calories go way up. This is fairly common situation. Potatoes aren't very high in calories, until you fry them, and add cheese, butter and bacon. StuRat (talk) 18:01, 11 April 2017 (UTC)

• • I eat a low fat diet, so I do need to get my calories from potatoes, rice or pasta. If I eat potatoes for dinner, then I'll eat about 1 kg of potatoes. I think this is what you must do to eat healthy, you must make sure the balance between calorie intake and nutrient intake is right. Count Iblis (talk) 19:47, 11 April 2017 (UTC)

• Other sources of calories are protein and alcohol. So, low fat doesn't need to mean high carb. StuRat (talk) 00:23, 12 April 2017 (UTC)

It says it is rehydrated egg white (see below), which is notably a low calorie, but unsatisfying, food. There are a lot of low calorie foods - green beans, strawberries, mushrooms, spinach for example. Salad pretty much in general, come to think of it. But even a 160-calorie soft pretzel can be pretty satisfying for the calories; it's hard to say what works best. I would be curious whether there is any risk of egg white injury with this one the way this is prepared, though I highly doubt it. Wnt (talk) 20:52, 11 April 2017 (UTC)

It's less than 15% egg white so what's your point? But maybe you just missed that percentage.--TMCk (talk) 00:21, 12 April 2017 (UTC)

@TracyMcClark: Thanks for the correction! Because the first ingredient listed was "Mycoprotein (tm)" and they said the product was Mycoprotein, I thought that was just a brand name rather than an ingredient. I actually mentioned mushrooms were a low calorie food; this is much the same idea. However, according to our article mycoprotein is from Fusarium venenatum grown in vats. (Hmmm, I wonder if this technology was in any way inspired by previous government efforts to weaponize Fusarium oxysporum for spraying on coca crops, or perhaps incidentally other rebel food sources...?) Wnt (talk) 01:12, 12 April 2017 (UTC)

Omnidirectional vs. tight beam broadcast

How do they send radio waves in a tight(er) beam instead of an omnidirectional broadcast? --Hofhof (talk) 17:44, 11 April 2017 (UTC)

The shape of the antenna makes the difference. A parabolic antenna is good for a tight beam directional antenna. Television antenna lists some omnidirectional antennas, such as rabbit ears or a bowtie antenna. There are also multidirection antennas, which broadcast or receive in more than one direction (typically 2 or 4), but not equally well in all directions. StuRat (talk) 17:52, 11 April 2017 (UTC)

• Wikipedia has an article titled Directional antenna which has some information. It also contains links to other articles which may lead you to find even more information on how they work. this article here seems to go into some more technical details on the general differences between omnidirectional and directional antennas. This article has some basic information on common antenna types as well. --Jayron32 19:06, 11 April 2017 (UTC)
• Only the theoretical isotropic antenna is truly omnidirectional, but no such antenna can be built. Simple antennas consisting of one element, such as the half-wave dipole antenna, are usually referred to as omnidirectional, although they do favour radiation (and reception) toward their sides; that is, they are best at angles that are broadside on to the element. By placing more elements at certain harmonically-related distances from the main element, the beam can be narrowed considerably, focusing more of the signal toward (or receiving more from) the intended target. Thus we have the yagi antenna. It has (usually) one reflector element (slightly longer than the main element) spaced close to one-quarter wavelength behind it. This reflects signal back towards the main element, emphasizing the radiation in that direction. By adding one or more shorter director elements in front of the main element at the same spacing, the signal will be focused even more. It's not unusual to obtain up to 10dB front-to-back gain in this way, meaning that 10 times more power is radiated in the forward direction compared to the back direction. A secondary advantage is that interfering signals and noise from the back direction when receiving are suppressed by the same amount. Also see quad antenna for a different type of construction with similar results. Akld guy (talk) 21:56, 11 April 2017 (UTC)

A spherical antenna is an interesting approach to a directional antenna. Unlike a parabolic antenna, it can be aimed without moving the entire thing, but only over a limited range. See Five hundred meter Aperture Spherical Telescope for one example. The rotation of the Earth does most of the aiming, but from there they can fine tune it.StuRat (talk) 00:46, 12 April 2017 (UTC)

The memorable Hairy ball theorem (animation) can explain why an ominidirectional (isotropic) antenna is impossible for coherent linear polarized radiation. The electric (and magnetic) vector fields would be everwhere perpendicular to wave propagation, forming a continuous tangent everywhere on the surface of a sphere around the antenna. But you can't comb flat the hair on a ball, says the theorem. The limitation does not apply to incoherent radiation which can be omnidirectional, e.g. the flash from an explosion or, cosmologists believe, radiation from the Big Bang. The Arecibo Observatory (305 meter) built in the 1960s in Puerto Rico does use a fixed spherical reflector in contrast to the Chinese Five hundred meter Aperture Spherical Telescope whose primary reflector is actually an active surface that emulates a 300 meter parabola. Blooteuth (talk) 12:49, 12 April 2017 (UTC)

What do "weeds" do in any ecosystem?

I know that gardeners want to "improve" the acidic soil by adding garden lime and "improve" alkalinic soil by adding compost. But I have read that weeds may thrive in an acidic environment. In that case, why not just grow weeds in the soil instead? Will there be an advantage if a gardener makes use of the acidic soil and grows edible weeds, like dandelions and nira greens, and if the soil changes, the gardener grows a different set of crops? 50.4.236.254 (talk) 18:28, 11 April 2017 (UTC)

The problem with weeds like dandelions is that they spread to the neighbor's yard who will then be mad at you. There are some people who believe that a natural garden is better, but you need to live away from everyone else or have like-minded neighbors for that to work. As for farmers, there are some crops that can be grown to improve the soil, depending on the issue. StuRat (talk) 18:35, 11 April 2017 (UTC)

There is no biological definition of weed. A weed is simply any plant that the person who is maintaining the garden does not want for whatever reason. People have their own aesthetic reasons for wanting certain plants in their garden or yard, and farms want to maximize the yields of their crops. If we're just restricting ourselves to gardens and yards and land maintained for aesthetic reasons, which you seem to be, it is "the person who maintains the yard doesn't find the weeds as pretty as the other plants". You'll have to ask that person why they don't grow weeds. We can't answer for you, and no reference will provide any meaningful answer. --Jayron32 18:56, 11 April 2017 (UTC)

Okay, fine. What do dandelions and green nira do? 50.4.236.254 (talk) 19:33, 11 April 2017 (UTC)

Whoo. You're asking a complex question. Dandelions etc are very good to eat but a 'gardener' wants to grow a wide range of veg – not just dandelions and acid soil loving nira. What about the other edible weeds like alkaline loving Sanguisorba minor. Should the gardner deny his family of of the joy of those other-weeds?--Aspro (talk) 18:59, 11 April 2017 (UTC)

• Dandelions have one purpose, to make more dandelions, the more the better. Any 'purpose' alloted to them is a human construct that bears no relationship to the their reproduction. The basic purpose of all living things is to reproduce more of its kind. Richard Avery (talk) 07:12, 12 April 2017 (UTC)

Crop rotation is basically the art and science of doing what you suggest. Alternating crops, sometimes including plants like clover specifically just to let them die on the field and be plowed under as Green manure. Of course clover is only a "weed" if you don't want it to grow, but that's a question of semantics.

As for home gardens, Deciding you want the weeds to stick around can be called Natural landscaping. That article discusses some disadvantages of such an approuch, but it doesn't mention the big one StuRat mentions : Neighbors, you neighbors may be very unhappy if you have local plants growing unchecked on your lawn. They may even have legal power to stop you from doing it, depending on local regulations and especially any Homeowner Association you became a member of when you bought the house.

All of this is more complicated than just deciding to eat dandelions, of course, but you knew it wouldn't be that simple. ApLundell (talk) 19:03, 11 April 2017 (UTC)

I think clovers are actually edible. Why don't people bring clovers to market? 50.4.236.254 (talk) 19:35, 11 April 2017 (UTC)

Because people in that locale don't want to eat them. You can't just force people to behave in a certain way because you don't find the way they are behaving explainable beyond "that's what they like/don't like". Lot more things are edible than what people in a particular location choose to spend their money buying to eat. It does no bit of good to bring those things to market if no one will spend money buying them to eat. --Jayron32 19:38, 11 April 2017 (UTC)

APL provides some very good additional perspective. To expand on his excellent reading material APL has provided: As far as HOAs and private property usage in the U.S., many housing developments in the U.S. have covenants attached to the property you are buying; when you purchase a house and its associated land in the U.S., the covenant is attached to the land and requires you, as a homeowner, to maintain your land and dwelling to certain standards. These covenants are a prerequisite of buying the land from the developer, and become attached in perpetuity to the property, since you are not required to buy that property, you are free to not buy land with such covenants attached to it. Why do such lands have such covenants? Because there is a lot of value tied up in Real Estate; as an individual homeowner a substantial portion of your total net worth is tied up in your home. If your neighbor makes it hard for you to sell your house because he doesn't maintain it to a certain standard, that costs you A LOT of money, a prospective buyer may refuse to buy because his yard is in below standard, and that means you have to lower your sale price. Basic economics. So HOAs and covenants exist to protect the investments of everyone who owns property in the development by ensuring that all other owners keep their property up. --Jayron32 19:15, 11 April 2017 (UTC)

Aside: Blanched dandelions don't produce seeds, for to which, spread to your neighbours garden. They (your neighbours) may look upon you as a bit odd at first, until you invite them to taste real home grown produce. Tastes better than anything they can by at the supermarket. Covenants usually restrict nuisance weeds. Before the automobile, land owners had to remove any Jacobaea_vulgaris#Poisonous_effects because of it detrimental effects to horses. Now, nobody takes any notice of this legal requirement and thus it is allowed to grow everywhere. --Aspro (talk) 19:51, 11 April 2017 (UTC)

Fine. Then, the only way to plant stuff is to buy a patch of farmland and live on the farmland in a tent. Though, one probably needs to find fresh water and a way to go to town every now and then. 50.4.236.254 (talk) 19:38, 11 April 2017 (UTC)

Finding a way to go into town should not really be a problem. If you're average, you may well find you have two lower appendages. By moving them back-and-forth you can perambulate at some three mile per hour. Fast enough to get you into town and back home again before nightfall. Make sure your tent is properly water proofed – otherwise you will be bathed in a mist of fine water droplets when it rains. Then, cut turf and built yourself a sod house. A six dollar colt revolver may come in handy too. Oh. And don't be afraid of the natives. They are very friendly, it was Hollywood that portrayed them in a bad light.--Aspro (talk) 20:20, 11 April 2017 (UTC)

I'm pretty sure there is a middle ground between living in a planned community with very strict gardening rules, and living off-the-grid in a tent. ApLundell (talk) 20:38, 11 April 2017 (UTC)

That's quite true. In New England, for example, housing covenants and HOAs are uncommon. I'm sure they may exist in a few places, but they are mostly unknown. --Jayron32 10:51, 12 April 2017 (UTC)

I just remembered I started a Draft:Weeds as soil indicators a while back, meaning to fill out a massive table based on some of the sources there and probably others... and something came up, and it's set there ever since. The history has some semi-relevant material I'd taken out. I was meaning to learn how to read the soil pH by looking at the plants... but there are similar data for garden plants, some of which like more acid pH. Wnt (talk) 23:46, 11 April 2017 (UTC)

In the most basic sense, the "weeds" are the ecosystem, and whatever crops or gardens we grow are just a temporary aberration. StuRat (talk) 00:37, 12 April 2017 (UTC)

Not really. I mean, yes, there are native organisms, but the best "weeds" are invasive species that thrive in disrupted environments, such as gardens and roadsides. There are many weeds which are present nearly anywhere, which is what made them attractive to me as a potential sign to read the soil by. Wnt (talk) 01:14, 12 April 2017 (UTC)

Yes, while dandelion is a common weed in NZ [2] (but not Allium tuberosum AFAIK, some garden shops sell suicides so I assume it isn't considered a risk given NZ's climate), if you look at lists e.g. [3] [4] [5] [6] [7] [8], you'd note stuff of concern can be things like Tradescantia fluminensis, various Hedychium, Solanum mauritianum, Privet. While some of these are perhaps are most concern in native bush or on reserves, I selected examples I've probably dealt with before, and I don't live that near any native bush. Nil Einne (talk) 01:42, 12 April 2017 (UTC)

Salvia are often used in gardens to keep insects at bay. Garlic is a famouse antidot against many sorts of fungi. So "weeds" complement eachother by keeping specific harms in check in their ecosystem. Thats also the key reason why monoculture farming exessively needs so many chemicals. To compensate for the exclusion of weeds that otherwise make mechanical harvesting a challenge, take away a fraction of the plant area and need some direct farmer attention, because some weeds are a also ofcourse a danger to humans. --Kharon (talk) 04:04, 12 April 2017 (UTC)

Beginning of Period R

(Note: the E is not a real E, but an approximation of the C with a horizontal line through it because of its similarity to an E)

We know about periods E, O, S, D, C, P, T, J, K, Pg, N, and Q. Do any scientists have theories on when period R will start?? (The R doesn't stand for anything; it's simply the combination of the fact that it's the next letter after Q and that no other period has that letter as its name.)

The periods can be grouped into ages as follows: E and O are the Age of Trilobites; S and D are the Age of Fishes; C and P are the Age of Amphibians; T, J, and K are the Age of Reptiles; Pg and N are the Age of Mammals; and Q is the Age of Humans. (Most of the known dinosaurs lived during periods J and K.) Do any scientists have any theories on either when period R will start or what life on earth will be like during period R?? Georgia guy (talk) 21:08, 11 April 2017 (UTC)

Technological singularity in less than 25 years. --Digrpat (talk) 22:14, 11 April 2017 (UTC)

According to Stephen J. Gould, those "Age of XXX" labels are very misleading and shouldn't be used. However the division into periods is mostly based on when mass extinctions occur. Since the sixth mass extinction is currently in progress, the next period probably is starting right now. CodeTalker (talk) 22:53, 11 April 2017 (UTC)

The relevant term is Anthropocene, but I couldn't find a cite for an abbreviation. The use of "A" seems obvious, but I can't tell you it's fact. Also note that it is still not that universally accepted. Wnt (talk) 23:52, 11 April 2017 (UTC)

At least partly because homocene isn't inclusive and would confuse many creationists? Sagittarian Milky Way (talk) 00:30, 12 April 2017 (UTC)

It might confuse evolutionists also, since Homo has been around much longer than the very late proposed start dates. I have to admit some skepticism because really the whole Holocene has been shaped by human activity, say the extinction of Megatherium. A variant of this is given in the "early anthropocene" version of events. There's a case to be made that the Holocene is just too short to chop off yet. Wnt (talk) 01:23, 12 April 2017 (UTC)

The term Homogenocene is also used for the same concept, the book 1493: Uncovering the New World Columbus Created uses that term when discussing the ecological changes associated with globalization. --Jayron32 10:47, 12 April 2017 (UTC)