Wikipedia:Reference desk/Archives/Science/2016 January 3

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January 3

Air quality index and highest takes all

According to Air quality index, most of the AQI systems used by governments uses the "highest value takes all" approach, where out of each weighted pollutant concentration, the worst one is selected as the overall AQI and all other pollutants are ignored.

Is there any governmental/supranational institution that doesn't use this approach? I.e. they factor in the lesser pollutants in their AQI instead of dropping them.731Butai (talk) 12:23, 3 January 2016 (UTC)

There have been a wide variety of academic proposals for combined indices, most of which are unsatisfying in one way or another. As far as I know, the only national organization to adopt an approach that is not winner take all is the Canadian AQHI. On the subnational level, Hong Kong also has an approach that combines components [1]. Dragons flight (talk) 12:56, 3 January 2016 (UTC)

Thank you very much again! You've been an awesome help.

I read through the Hong Kong page([2]), but unfortunately they don't cite any references nor do they clarify exactly how their "Air Quality Health Index" is calculated. Is the Hong Kong AQHI formula the same as the Canadian AQHI formula , which came from this study[3]?731Butai (talk) 13:45, 3 January 2016 (UTC)

That seems to be just a general information page on the general HK government site. Nearly all the links there direct to the main HK government AQHI page, http://www.aqhi.gov.hk. On the main AQHI page, while the "About AQHI" section which is listed under "What's AQHI" tab on the page is somewhat unhelpful [4]; the method for calculation seems to be listed in the FAQ [5] which also listed under the "What's AQHI" tab on the page. There is also a link to the study they used to arrive at the health risk [6]. Nil Einne (talk) 14:31, 3 January 2016 (UTC)

Thank you! Seems like the Hong Kong formula is based on the Canadian one, and has various additions. This is great stuff. 731Butai (talk) 14:51, 3 January 2016 (UTC)

BTW, the HK study mentions some studies and proposals where amongst other things, combined indices were I think developed but appear to be (from what I can tell, and also what the HK report suggests) largely unused like that for the Cape Town "dynamic air pollution prediction system project" and the Provence Alpes Côte d'Azur Aggregate Risk Index. I presume these are some of the academic examples Dragons flight may have been thinking of. Nil Einne (talk) 15:00, 3 January 2016 (UTC)

Manhunt for suspect in recent TA shooting: Were tracking dogs used? If not why not?

I've seen some criticism of the way police handled the search for the suspect in the recent shooting in Tel Aviv. But one thing I did not see was any mention of tracking dogs. Yet the suspect left a bag in the store next door which I suppose would carry his scent. (You can see the relevant surveillance videos on numerous news sites). So I have two questions: 1. Was this a case where tracking dogs can do the job? (Assuming you know something about tracking dogs and/or have got some sources) 2. If this was such a case, have you seen anywhere any discussion of whether dogs were actually used, and if not, why not? The purpose of this question is more to use a real-life case in order to learn something about the capabilities of tracking dogs than anything else. Contact Basemetal here 17:04, 3 January 2016 (UTC)

I could not find any evidence of tracking dogs being used in this case. Maybe they did use them, but could not get new clues, so it was not worth reporting. Maybe they just assume from the beginning that it was useless. I know that it's easier for a dog to track a human in open nature. A crowded city implies many other human scents as background, which appears to be confusing. It is also useless if the human uses any vehicle to escape or a lift. --Scicurious (talk) 01:01, 4 January 2016 (UTC)

Which is more comprehensive regarding exercise problems? The Feynman Lectures on Physics or Course of Theoretical Physics by Landau & Lifshitz?

75.75.42.89 (talk) 21:15, 3 January 2016 (UTC)

Do you mean Feynman's text itself or including the complementary book of exercises (Exercises for The Feynman Lectures on Physics)? Feynman's Lectures have some exercises for beginners, but they are not enough. And there is also the Feynman's Tips on Physics.

Landau and Lifshitz is a series of rather short works and poor on exercises. There are legal free downloads of them available. So, take a look at them. --Scicurious (talk) 23:30, 3 January 2016 (UTC)

Biology

My biology teacher told the class that he could sit in a bowl of water and suck the water up through his anus via "reverse peristatis" to clean his intestines instead of using an enema. Is this remotely plausible? GrailKun (talk) 22:27, 3 January 2016 (UTC)

Ask him to demonstrate Such a technique is used in yoga and is called jala basti (see this). However it seems you have to use a tube (a bit like you use a straw). If your biology teacher says he doesn't have to use a tube he might be bragging although I can't imagine why anyone would want to brag about something like that. Contact Basemetal here 22:35, 3 January 2016 (UTC)

A case of bragging is one possibility. It's not the first time I hear someone bragging about this ability. Apparently this is something you can be proud of. The wiki article is Basti (Panchakarma)--Scicurious (talk) 23:22, 3 January 2016 (UTC)

I think he probably just wanted to reserve a place for his picture in the dictionary, right next to "over-share." - Nunh-huh 06:47, 4 January 2016 (UTC)

Flatulists draw in air through their anus (prior to expelling it for entertainment purposes), so presumably they can suck in water too.--Shantavira|^{feed me} 10:00, 4 January 2016 (UTC)

In yoga this is called sthala basti. In this case you don't use a tube. Even more impressive. Contact Basemetal here 10:58, 4 January 2016 (UTC)

For all I know next week he'll tell you he can suck the chrome off a trailer hitch. You sure he isn't looking for a date? :) Wnt (talk) 16:19, 4 January 2016 (UTC)

Also, it's a total misconception that intestines need to be "cleaned". Vespine (talk) 22:08, 4 January 2016 (UTC)

I will mention that User:Vespine has linked to "colon cleansing" while saying that the need for clearing one's intestines is a misconception. As an IBS sufferer who has had diverticulitis and a resection and hernia surgery as sequelae I can assure the OP that talking to a good gastroenterologist, who might prescribe something, is a better idea than assuming that medical attention is a mistake. See our WP:Disclaimer. μηδείς (talk) 21:50, 5 January 2016 (UTC)

I'm having trouble unpacking your comment, I certainly wasn't trying to suggest that medical attention is a mistake. I was refering to the failry common misconception that typical people without diagnosable medical conditions need to regularly "clean" their bowles or intestines, or that there is some "benefit" to doing it. Vespine (talk) 23:45, 5 January 2016 (UTC)

I am loathe to offer medical advice, my point was to specify that there are medically valid reasons why some people do need to use purgatives under medical supervision; it's not all hoakum. But basically I think that the phenomenon you are referring to, Vespine, is indeed a pseudo-scientific fad. μηδείς (talk) 03:50, 6 January 2016 (UTC)

Mercury bubble time-of-use meter

I can't find it in the archives, but within the past few months I remember a question about the identity of an electronic component consisting of a small mercury-filled tube with a visual gap in the mercury. While searching for somethign else, I came across our Mercury coulometer article, which does not appear to be linked from the ref-desks. If anyone is still interested in this topic, there's the article. DMacks (talk) 22:29, 3 January 2016 (UTC)

[7] -- Finlay McWalterᚠTalk 22:33, 3 January 2016 (UTC)

Thanks DMacks. I edited that archived section, adding a link to our article. -- ToE 03:13, 4 January 2016 (UTC)

Are you sure it's not a mercury tilt-switch? Those are quite common in all sorts of applications. SteveBaker (talk) 14:20, 4 January 2016 (UTC)

No I remember it too, by "small mercury filled tube", the OP means like that in an old mercury thermometer (the part with the scale). Vespine (talk) 21:59, 4 January 2016 (UTC)

Sharpness of cutting device a physical unit

Is there a physical unit for expressing the sharpness of a knife or it's ability to cut? The article Sharpness is just about a an English port in Gloucestershire and not of much use here. Is the ability to cut just a question of angle and hardness of the blade? --Scicurious (talk) 23:21, 3 January 2016 (UTC)

Lots of many different factors. Here's some:

1) A hard cutting edge.

2) A softer backing material to absorb the force and prevent the cutting edge from cracking (ceramic blades often lack this protection).

3) Thin blade, to reduce drag as it passes through.

4) Weight, to increase the force applied.

5) Length, to allow cutting longer items and get more leverage.

6) Proper care, such as keeping it sharpened and oiled, to prevent rust.

7) A good handle for proper grip.

So, hard to combine all those into a single rating. See Japanese swordsmithing for an example of how many of these factors are maximized. StuRat (talk) 02:07, 4 January 2016 (UTC)

I think most of that list kind of misses the point and specifically, I think the width of the cutting edge is perhaps one of the primary factors to actual "sharpness". Weight, length, care, handle etc are secondary factors to how WELL something can cut perhaps, but not 'sharpness', like a diamond grain is still the HARDEST natural substance, regardless of other factors like how big a scratch it can make, or something. For pure sharpness, you might find Obsidian interesting. In that article it describes the edge as just a few nanometers thick, i suspect if there was a unit for expressing sharpness, it would have been used in this article. Vespine (talk) 03:15, 4 January 2016 (UTC)

And it occurs to me also, just because something is very complex and multifaceted, would do nothing to hinder us making an arbitrary pragmatic scale, such as: "how much force you need to apply to a blade for it to slice through a 10cm cube of ballistic jelly." Not really much different to saying "how much energy it takes to heat up 1 cubic cm of water by 1 Degree". Vespine (talk) 03:21, 4 January 2016 (UTC)

The Japanese used tests just like this. A hair (or was it paper?) dropped on the sword from as close as possible and gets cut with only it's own weight in force is a good sword. They also used things that looked like a reed(?) fasces without the ax about 2 feet wide. Cutting it off in one chop was a sign of sharpness and skill. A rubber band getting cut by the weight of a metal arrowhead without the arrow is a test for deer hunters. Christopher Columbus' men tested sharpness with Indian kid legs and when they got bored they bet on who could cut through a Native American torso with one stroke. Sagittarian Milky Way (talk) 04:19, 4 January 2016 (UTC)

Quite a bit different, actually. For example, how ever much energy it takes to heat 1 cubic cm of water by 1 degree, you can bet it will take twice as much to heat 2 cubic cms, and that the source of the energy won't matter. Not so with a knife cutting through a cube. Also, the optimal angle of attack will vary. For some cutting devices, you push straight into the cube, in others you drag the blade almost parallel to the face, with others in between. The temperature of the blade and cube would also matter, as would how the blade had been sharpened, etc. Testing knives to decide "which is sharpest" would be more like testing cars to see "which is fastest" (where it would depend on track conditions, etc.). StuRat (talk) 04:23, 4 January 2016 (UTC)

You could make a scale as suggested, but I think only for a specific application. I have a friend who is a chef, and his favorite knife of choice is usually a tomato knife as it's better for general purpose cutting than a chef's knife (yes, i was surprised by that), even though it isn't sharp in the way a chef's knife is. So the particular part of the question regarding the ability for a knife to cut must depend on the material you want to cut, and the blade you're going to use, which I think would make it very difficult to specify a definitive unit for sharpness. A bread knife, for example, can easily cut a loaf of bread, and scissors are good for cutting paper, but you couldn't use either any of those tools effectively to perform the other jobs. The 'sharpness' depends entirely on the application. 95.146.213.181 (talk) 05:07, 4 January 2016 (UTC)

You're obviously right in saying that some tasks require a serrated blade, others smooth - and scissors provide a shear force rather than acting as a wedge like a knife. But I don't think that changes the OP's desire to measure sharpness. We're not being asked to measure "cutting power" - which would obviously be unreasonable given the differences in cutting styles. But it doesn't seem unreasonable to ask how narrow the cutting edge is or perhaps what the angle of the wedge close to the cutting edge is...and a handy scale to represent that might be the kind of thing that someone shopping for knives might want to use.

If I was a buyer of knives - it would be nice to know that blade X has a sharpness factor of 5 where blade Y has a sharpness factor of 6 but dulls easily and can only maintain a sharpness of 4 over the longer term.

However, there doesn't seem to be such a scale - so our OP is out of luck here.

SteveBaker (talk) 14:17, 4 January 2016 (UTC)

Microtome might be interesting, and it points out that of course the thinnest blades are only suitable for slicing soft objects so there is a definite difference between sharpness and being able to cut. 14:42, 4 January 2016 (UTC)

This question was asked a a few years ago. ISO 8442.5 is the international standard for kitchen implements. See also this device, a commercial sharpness tester. Tevildo (talk) 17:01, 4 January 2016 (UTC)

Ah yes thanks, should have thought of them - they have standards for everything so you can give a number to how slippery your floor is for instance. They're normally based on some human agreement of what's good for the purpose rather than anything universal you'd find in a physics book. Dmcq (talk) 11:39, 5 January 2016 (UTC)

Interesting enough, the comment mentioning the history behind the ISO standard, and also the testing device was only added about 8 months later [8] [9]. A good reminder that belated additions whether by IPs or whatever, should be welcomed if on topic and not in violation of any editing restriction or general policy. BTW I've now added the unsigned template [10] to help clarify. Nil Einne (talk) 18:24, 6 January 2016 (UTC)