Wikipedia:Reference desk/Archives/Science/2007 August 30

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.

August 30

EFFECT OF ATMOSPHERIC PRESSURE ON BODY WEIGHT

I'M ON A DIET. SOMEDAYS I KNOW THAT I HAVE EATEN NOTHING AND YET MY WEIGHT HAS STILL GONE UP --ONE TIME IT WAS 5 LBS OVERNIGHT. A FRIEND SAID THAT ATMOSPHERIC PRESSURE OR BAROMETRIC PRESSURE CHANGES CAN AFFECT THE RESULTS OF MY STEPPING ON THE BATHROOM SCALE. IS THIS TRUE? AND IF SO, WHY? —Preceding unsigned comment added by Gtigue (talk • contribs) 00:37, August 30, 2007 (UTC)

If atmospheric pressure goes up, your apparent weight will be slightly less, because of buoyancy. Emphasis on slightly -- there's no way this is the real issue you're seeing. But your scale will be somewhat variable, so it could show your weight as higher when it's not. Or you might have drunk water. Or you might have forgotten about a snack. --Trovatore 00:47, 30 August 2007 (UTC)

Slight differences in foot positioning can affect cheap scales. Try weighing yourself 5 times per session (just step off and back on), and take the average. If you are still seeing a significant overnight increase in your weight, you might be a somnambulist.  :) --Sean 01:19, 30 August 2007 (UTC)

There are a lot of factors that can change the reading on scales. You can weigh different at different times of day, depending on how much water you've drunk recently, whether the scale is placed on a hard or soft surface, what you're wearing, whether the planets are aligned (ok that's only going to have an effect of a fraction of a gram, no matter what the astrologers say), and - this might be what your friend was thinking of - humidity. If you have a mechanical scale, especially a really cheap one, it may be possible for high humidity to affect the inner workings in such a way that the reading changes. So, the best way to get reliable weighings is to (a) buy a good quality, possibly digital scale, that is fully enclosed, (b) weigh yourself at the same time of day every time, wearing a similar amount of clothing, (c) try to make sure your eating and drinking habits are regular enough that you won't have drunk a litre of water before your weighing one day and not the next. Confusing Manifestation 01:41, 30 August 2007 (UTC)

I agree with humidity. I had just such a cheap spring scale that would vary by 5 lbs depending on the humidity. One clue that it was "sticking" was that the needle would go back and forth maybe 10 times before it settled on a reading when the humidity was low, but only once or twice when humid. Also, a change in barometric (atmospheric) pressure frequently goes along with a change in humidity, so your friend wasn't wrong. StuRat 02:03, 30 August 2007 (UTC)

The weight of air pushing down on the top of the scale is absolutely huge, 14.7 pounds on every square inch - that's probably more than a ton on the whole top surface of the scale! However, that same force is pushing on the bottom of the scale's cover too. And it's there even when you're not standing on the scale, so any effect from air pressure should simply be zeroed out. Even though you may not often think about it, the air weighing down on you from above is actually very heavy! --Reuben 02:16, 30 August 2007 (UTC)

No, not quite zeroed out. The air around your feet is under slightly higher pressure than the air around your head. This creates a slight net upward force, equal to the weight of the chunk of air that would have occupied the space where your body is, if your body weren't there. See Archimedes' principle for details -- haven't looked at that article but that's where it should be. --Trovatore 02:21, 30 August 2007 (UTC)

Right - that's a small but nonzero effect too. According to atmospheric pressure, air at sea level is about 0.08 lb / cubic foot. A human is a few cubic feet [1], so each of us displaces a weight of air of order 1/4 pound. Not big, but that's far more than I expected! --Reuben 02:33, 30 August 2007 (UTC)

According to this site: [2] "The lowest sea level air pressure ever recorded was 870 mb (25.69 in.) in the eye of Typhoon (Tip) over the Pacific Ocean, whereas the highest sea level air pressure ever recorded was 1084 mb (32.01 in.) at Siberia associated with an extremely cold air mass." That's about 24.6% higher at the maximum air pressure using the min air pressure as a base. If we multiply that by Reuben's 1/4 pound figure, we get around 1/16 pound differential in your weight from the highest atmospheric pressure to the lowest. The air pressure in your bathroom won't vary nearly as much, so air pressure alone can't explain a 5 lb diff. It's off by well over a factor of 80. StuRat 02:47, 30 August 2007 (UTC)

On another note, eating nothing at all for a day is not generally considered an effective way of losing weight, for a variety of reasons. Skittle 14:11, 30 August 2007 (UTC)

Gecko feet

Just read about Geckos and their sticky feet. Since their feet are so sticky, why don't they get totaly clogged up with dirt and lose their stickyness? -OOPSIE- 02:40, 30 August 2007 (UTC)

Perhaps they use the slug method and constantly ooze more slime, leaving the old "clogged up" slime in a trail behind them. StuRat 02:53, 30 August 2007 (UTC)

Gecko feet do not use a sticky slime to generate their stickiness, instead they have "a network of tiny hairs and pads on their feet which produce electrical attractions that literally glue the animals down. With millions of the hairs on each foot, the combined attraction of the weak electrical forces allow the gecko to stick to virtually any surface." [3] [4] I expect small dirt particles do not interfere with the Van der Waals forces between the gecko's seta and the surface of adhesion. Rockpocket 06:12, 30 August 2007 (UTC)

The setae somehow self-clean, but the mechanism is still a subject of current research [5]. --mglg_(talk) 21:08, 30 August 2007 (UTC)

Thank you! -OOPSIE- 03:48, 31 August 2007 (UTC)

Physics

What is Radio Frequency Identification? —Preceding unsigned comment added by Beta alpha (talk • contribs) 07:32, August 30, 2007 (UTC)

Just look it up! Radio-frequency identification. —Bromskloss 07:52, 30 August 2007 (UTC)

Hunger related

What is the scientific term used to describe the condition when a person chooses not to eat and their body comes to a point where it no longer triggers us to let u snow we are hungry? - Pastor Jon--JacksonParkBC 12:18, 30 August 2007 (UTC)

symptomatologically, this is known as anorexia. Tuckerekcut 12:46, 30 August 2007 (UTC)

That's a more general term, though. I'm more familiar with what the OP's asking in terms of thirst. At first, you experience sensations of thirst, but after a certain level of dehydration, your body "gives up" on trying to notify you that you need water. I was always taught that if you're not thirsty (and circumstances suggest you should be), that can mean that you're extremely dehydrated. Anyway, I can imagine a similar situation with hunger. jeffjon 13:28, 30 August 2007 (UTC)

Spider Web

In regards to this link,

http://www.cnn.com/2007/US/08/30/spider.web.ap/index.html

What species of spider is this? Are there any spider species that work together with other species? --WonderFran 13:52, 30 August 2007 (UTC)

If the entomologist in the article can't identify them through the pictures, I doubt anyone here can. However, there is a social spiders section in the Spider article. -- JSBillings 17:07, 30 August 2007 (UTC)

This past spring there were quite a number of smaller versions of this phenomenon covering hedges alongside roads to the north of Salisbury, (UK). Those webs were made by the larvae of some species of moth that lived on the leaves of the plant it had covered as a form of protection from predators. I think they might have been tent moths, Lasiocampidae, although several other species spin webs. But in whatever case that is one huge infestation. Richard Avery 17:39, 30 August 2007 (UTC)

Time period for the deposition of calcium carbonate in Italy.

I have written the following footnote for a book I'm working on using information gleaned from Wikepedia.

The pure-white stone mostly associated with the word “marble” is the result of the metamorphism of pure limestone (calcium carbonate, whose origin was the deposition of the shells of marine organisms 65–100 million years ago). The heat and pressure in the metamorphic process usually destroys the fossil remains. This marble was used in Roman interiors and works of art. The most common Roman “marble” was travertine, a precipitate of carbonate minerals often aragonite but at times calcium carbonate. It has not undergone metamorphism and is often yellowish with visible embedded fossils. The stone blocks of the Coliseum are of travertine. The Latin name for the rock was lapis tiburtinus because of the large deposit quarried at Tibur (Tivoli).

The info for the time period of the deposit of calcium carbonate was from an article on the formation of chalk in England. Would this period also include the deposition of calcium carbonate in Italy? 69.201.141.45 14:04, 30 August 2007 (UTC)Linnaeus Shecut

(I just wanted to pick you up on a sentence you gave above - "..travertine, a precipitate of carbonate minerals often aragonite but at times calcium carbonate.." - aragonite is a form of calcium carbonate so.. you could just omit the "..but at times calcium carbonate.." - did that make sense.?

as for your main question - until someone else answers - I think it's best to say not neccessarily - the cretacious period is named after the chalk that formed at that time - but that doesn't mean that carbonates weren't produced in earlier aeons - that said - I've no idea.87.102.14.233 15:15, 30 August 2007 (UTC)

HOWEVER from the article "The Cretaceous is justly famous for its chalk; indeed, more chalk formed in the Cretaceous than in any other period in the Phanerozoic" - so it's a good bet that the chalk is cretacian - but I suppose it depends where it's found and at what depth...213.249.232.26 19:14, 30 August 2007 (UTC)

NOTE a web search for "marble formation italy" suggests that liassic is a common age quoted for the date of formation. —Preceding unsigned comment added by 213.249.232.26 (talk) 19:02, August 30, 2007 (UTC) UK chalk I believe formed in the cretacious - however it's up to you to discover whether the liassic date refers to the laying down of the sediment OR the partial metamorphosis to marble (disclaimer I'm not a profesional geologist)213.249.232.26 19:05, 30 August 2007 (UTC)

As suggested, the age range of various marbles and limestones in Italy can be large. The Carrara marbles are Jurassic but there are probably plenty of other ages as well. The lapis tiburtinus travertines are quite young - around 100,000 years [6]. The mineral name for the other crystalline habit of calcium carbonate (apart from Aragonite which as mentioned is also calcium carbonate) is calcite. Also note the correct spelling of Cretaceous. Cheers Geologyguy 19:37, 30 August 2007 (UTC)

geography/cartography issue, example included

I needed coordinates of the topmost easternmost, etc., points of a country, I used those specified in the article on the respective country. Here's what I get.

OK, there's an offset or something of some 40 km.

But, I get that it's not uncommon to specify this (i.e., coordinates of the northernmost esternmost, etc., points of a particular country) in literature dealing with this. I was wondering if anyone knows of some website or something that would have reliable information. I'm actually not sure how do I google this. Alternative methods of putting territories into boxes and getting the coordinates of the sides of the box would be even greater appreciated. 354d 16:09, 30 August 2007 (UTC)

Is it possible that one source is including the terratorial waters around the country and the other isn't? If the numbers in the article included the oceanic terratory and the map that you laid into that box did not - then there would be an error of around this much Perhaps there is some teeny-tiny island that belongs to Latvia that you didn't include in your map. This seems a very 'iffy' way of getting things to line up right. SteveBaker 19:01, 30 August 2007 (UTC)

I doubt there are territorial waters included cause they're like 200 km or something idk and it has no islands. Actually as I look at it there's some distortion - the top part's offset less than bottom - the overlain map appears to be somehow bigger although google maps is supposed to scale it.

The solution might be pulling the contours of the country from a snapshot of google maps (since it apparently has different projection) and somehow fitting the original map in there. Actually yea I thought earth is round and google maps have cylindrical projection - there has to be distortion.

No I'm wrong - there has to be a built-in "google-maps-pptimization" cause specifying a polyline for instance involves specifying real life coordinates and google maps dispalys it correctly so that's def not relevant. 354d 06:31, 31 August 2007 (UTC)

but anyway the coordinates in the wikipedia article are very crude or something. It would be cool if there was some google-maps-optimization function somewhere, but guess I have to ask this on google earth forums.

anyway any authoritative source on "country lies between ... and..." coordinates would be appreciated. 354d 05:01, 31 August 2007 (UTC)

Tell me - is the error much worse for a country further from the 0 meridian? If you pick a place like (say) India, is the error in the result worse? What about for a country closer to the equator? This could be to do with the map projection that Google uses - or because you are failing to correct for WGS84. SteveBaker 14:29, 31 August 2007 (UTC)

I just made a test (could've done it earlier :D) I specified coordinates that I took myself manually (arbitrarily or whatever) and the map gets positioned pretty much where it should be except for the eastern part where it is clearly visible that there's something wrong with the overlay map (and I'm pretty sure there is something wrong with it) and the second conclusion is that the coordinates in the article are very robust or whatever. Coordinates they (Google) use in their demos work pretty much to a zoom level where you can see cars so I'm really not sure that it's a projection issue or something. I should review sources rather, it might be a very error prone process using something they made back in USSR, I'm pretty sure they didn't abide by any WGS84 rules. 354d 10:20, 2 September 2007 (UTC)

Bachelor of Science in Engineering vs Bachelor of Engineering

I have a BSc in Engineering, my work colleagues have a BEng. We are trying to figure out what is the difference between the two and if one is better than the other. —Preceding unsigned comment added by Pebbles82 (talk • contribs) 16:12, August 30, 2007 (UTC)

If the degrees are from different schools, there may be no direct comparison. Schools may choose their own nomenclature. You might look under accreditation guidelines to see if both schools are accredited - if so, they must meet certain minimum requirements. If it's boasting rights you seek, why don't you compare number of classes you had to take? That will clearly settle which of you had more requirements in college; but of course there is always room for subjective interpretation. You might also look into Philosophy of education - our article details some of the different viewpoints about quantity and quality of coursework. Nimur 16:33, 30 August 2007 (UTC)

We also have Bachelor of Engineering and Bachelor of Science articles, but you should be aware that regional and school variations may supersede information in those articles. Nimur 16:35, 30 August 2007 (UTC)

I don't think there is any kind of formal distinction. Lots of British universities award 'BA' (Bachelor of the Arts) degrees for science subjects - whilst others award the 'BSc'. There is no significance in that beyond tradition. I would expect the same to be true of a BEng versus a BSc. To know which is 'best', I think you'd have to look at the quality of the institution that awarded it. In the minds of most employers, a 'B-anything' from Harvard, Yale, MIT or whatever trumps a 'B-whatever' from some of the lesser colleges - even if the name is the same. SteveBaker 18:35, 30 August 2007 (UTC)

Not certain here, but I think that by 'lots of British universities' you mean 'Oxford and Cambridge': certainly that isn't common practice here in Britain. Algebraist 23:28, 30 August 2007 (UTC)

Are those really the only ones? Well - I guess it could be - my two friends who have BA's in science subjects both went to Oxford and my BSc comes from Kent - so that fits the facts. But anyway - there is no special significance to that beyond tradition. SteveBaker 01:20, 31 August 2007 (UTC)

Time, space and the universe.

Hi there, my question is simply this :- Assuming the universe is exactly 13.7 billion years old, if you had a telescope capable of looking back 13.7 billion yrs and 1 minute, would you actually see the big bang happen? ( Also assuming you were pointing it at the right point in space ). I can fully relate to " looking back 10 billion yrs " but seeing the big bang happen ( all be it with a super telescope ) is perplexing me. I hope someone can answer my question. Many thanks. Symon. —Preceding unsigned comment added by Gun dog 1 (talk • contribs) 17:20, August 30, 2007 (UTC)

The problem is that the big bang wasn't just a teeny-tiny dot somewhere in an infinitely empty space that kinda exploded and tossed out matter in all directions. It was the fabric of space and time itself that was a dot (a 'singularity' to be more exact). So the curvature of the universe was infinite and there was literally nothing other than the singularity itself...no 'outside'. So you couldn't be looking from 'outside' of the big bang and watch it going off like a stick of dynamite. No matter which way you look, you are looking towards the big bang. Worse still, time itself started with the big bang - so there is no 'before'. You could look back to (say) one nanosecond after the Big Bang - but space would be so screwed up that I'm not at all sure there would be things like photons flying out from it. Using specialised instruments mounted on satellites, we can see the Cosmic microwave background radiation which formed about 300,000 years after the Big Bang - that's pretty amazingly close to the beginning of time and space (compared to 14 billion years anyway). We could theoretically look back to within 2 seconds of the Big Bang by examining the Cosmic neutrino background - but neutrino's are insanely difficult to detect and a typical neutrino "telescope" is something that lives in a deep mineshaft and has a couple of swimming pools worth of dry cleaning fluid in it and can detect a couple of neutrino's every month. This is not the kind of instrument that's going to give you much in the way of a photograph! So, no - you can't watch the Big Bang going off. SteveBaker 18:17, 30 August 2007 (UTC)

Further to SteveBaker's response, the reason we need neutrinos or something to see further back is that before c. 380,000 years after the big bang, the universe was so hot and dense that it was opaque to photons (see Cosmic background radiation#Features) Algebraist 18:22, 30 August 2007 (UTC)

We can see artifacts of the expansion and the inhomogeneity of the early universe - see W map. Nimur 19:19, 30 August 2007 (UTC)

A question as this, you may enjoy an authors take on timeline and understanding of beginnings and ends. Gabriel García Márquez' One Hundred Years of Solitude. Note the studying of the Melquíades Parchments. --i am the kwisatz haderach 19:50, 30 August 2007 (UTC)

Stephen Hawking wrote 'A Brief History of Time' - it is a very approachable book on this subject and was actually a best seller when it was first published. SteveBaker 01:17, 31 August 2007 (UTC)

Colored water on plants

What is the effect of colored water on plants such as daisies or roses? —Preceding unsigned comment added by 71.166.60.75 (talk) 19:35, August 30, 2007 (UTC)

This is commonly used to give artificial color to flowers. Some dyes will be absorbed by the plant and show up in the flowers, others will not. -- Kainaw^(what?) 22:59, 30 August 2007 (UTC)

And some dyes spread relatively evenly through the flowers while other remain concentrated around the veins. StuRat 03:28, 31 August 2007 (UTC)

Looking back in time

Hi, i wanted to know that if you had a powerful enough telescope could it look back in time to places on Earth, more specificly could it be programed to look back to a certain time period on Earth, e.g may 2000. If so how long would it take and how would it work.

Thanks Bailster2k7 20:30, 30 August 2007 (UTC)

If you had a sufficiently powerful telescope, yes, you could look at Earth circa 2000, although you would have to be 7 light years from Earth. It would take at least 7 years to get 7 light years from Earth, so even if you left right now, by the time you got there you'd only be able to look at right now. Further, the telescope would only be able to see the light coming in, "at-the-moment." So, if you have situated it 7 light years from Earth, it will always be seeing what happened on Earth exactly 7 years prior. Someguy1221 20:47, 30 August 2007 (UTC)

Check out the discussion on the space-time continuum just up the page. — Lomn 21:15, 30 August 2007 (UTC)

Of course, if there was a mirror 3.5 light years away from Earth, sufficiently large and facing the right way, then you could use the super-telescope to view things in the mirror. --Anonymous, 21:57 UTC, August 30, 2007.

I suspect that nicely polished large mirrors in space are unlikely...although I suppose that it wouldn't be beyond the bounds of possibility to find some light path that passed close to two or more massive objects, say neutron stars or black holes, whereby you could receive some photons in your telescope that originally came from the Earth in the past. The problem being that this path would likely be short-lived due to the changing relative positions of these objects and the Earth - and I suspect that you'd receive only a very tiny proportion of the photons due to the accuracy needed to get any to return back to the Earth. Also, I suspect the nearest things massive enough to deflect the light sufficiently would give a very long light-path of at least several thousand years.Richard B 23:07, 30 August 2007 (UTC)

We had this in answer to another question a few days ago. Basically, there is no way to see further back in time than the day you launch your mirror - and even then, you'd have to launch it at the speed of light. Of course if you launched your mirror - waited a few hundred years to get a few lightyears out there - then you'd be able to look back to a point a few years in the past...but not before the day you launched the mirror. There would doubtless be practical problems with making a large enough mirror and steering it so it would still be looking at the earth as the earth goes around the sun...but I guess this is a thought experiment. It would be cheaper and easier to stick a bunch of cameras into orbit with video recorders hooked up to them...the result would be pretty similar. SteveBaker 01:12, 31 August 2007 (UTC)

Think about this - "seeing into the past" is going to be a lot like using Google Maps (many of those photos are a decade old). SteveBaker 01:14, 31 August 2007 (UTC)

You are separated from your past self on earth by a time-like interval, so there is no mechanism consistent with modern theory that would allow you to view your past self (regardless of where you travel to before setting up your hypothetical telescope). Nimur 01:31, 31 August 2007 (UTC)

Hmm, aren't I looking at my past self when a look in the bathroom mirror (albeit very recent past self). Richard B 13:25, 31 August 2007 (UTC)

Oops! SteveBaker mentions an important correction below (which I thought was implicit, but should have been clearly stated). Nimur 16:21, 31 August 2007 (UTC)

Yes, you are...and you could set up a mirror on Alpha Centauri (4 lightyears away) and see yourself as you looked 8 years ago. What you can't do (even with a brand new mirror) is look back at yourself at a time in the past before you placed the mirror there. But as I've pointed out - cameras and VCR's make for an easier-to-understand version of this problem. You don't need mirrors the size of a planet and telescopes of mythalogical power in order to explore this concept - it's actually very familiar to us. When you watch a TV show that you recorded on your VCR - you are watching something from the past. SteveBaker 14:22, 31 August 2007 (UTC)

Flies in the dark

What happens if you switch off the light while a fly (or some other insect which is usually active during daylight) is still flying around? Do they crash into the nearest wall? Can they instantly switch to some type of "night vision" (if so, how?)? At least some insects do seem to see rather badly at night. Or let's just assume it's really pitch black. Do they fly more cautiously? Do they have a way of "extending their legs, so they'll land instantly on whatever they touch"? Or what happens? ... Thanks, Ibn Battuta 23:02, 30 August 2007 (UTC)

• It's an interesting question, but I note that houseflies will routinely fly at full speed into a glass window pane with no obvious ill effect. --Sean 01:19, 31 August 2007 (UTC)

• Agreed, I see no sign that they watch where they're going even during the day, they just bounce off the walls until they find something they can eat or something eats them. Sort of Roomba logic. StuRat 03:22, 31 August 2007 (UTC)

I can imagine that they could sense a difference in air movement near a surface. So maybe they slow down very quickly (flies can change course extremely fast) and don't quite hit the wall/window at full speed. However, being very small makes them light, which means the momentum and therefore the force exerted on their bodies can't be very high. Being small also means they can withstand more force. So maybe they wouldn't even need to slow down. DirkvdM 06:40, 31 August 2007 (UTC)

Houseflies are naturally diurnal and do most of their foraging during the daytime. They have apposition eyes and navigate by using polarized skylight. The ommatidia of apposition eyes have a light-absorbing screening pigment, so the only light that reaches the rhabdom enters through the small lens. Thus by dusk their eyes capture insufficient light to allow foraging, and most of their activity ceases for the day. In contrast, nocturnal insects such as moths, beetles, cockroaches and locusts have superposition eyes, a design based on the superposition of light rays entering hundreds, if not thousands, of ommatidia. This permits them "to see in the dark" and forage at night.

The bees with poor vision which you refer to have apposition eyes, yet despite this, tropical sweat bees (Megalopta genalis) are nocturnal and appear to see rather well at night. Scientists have found unusual interneurons in the optic ganglion, of these nocturnal bees, that have significantly wider lateral branching. The scientists hypothesize these neurons permit "a strategy of photon summation in time and in space" thereby enhancing night vision.

As it happens houseflies also have an very interesting neural modification: each ommatidia has seven rhabdomeres. This permits signals from adjacent ommatidia to interact within the neurons of the fly’s brain system. The result is fast directionally selective motion detection, which enables the fly to maneuver perfectly in three-dimensional space. This type of adaptation is called a "neural superposition eye" (even though it is a type of apposition eye). There is some debate over whether having a neural superposition eye provides significantly better night vision also [7]. It is generally thought that neural superposition is better than a regular apposition eye, but not as good as a regular superposition eye. [8] So its likely flies are not completely nightblind, but are still restricted. I don't know what a fly would do if the lights were suddenly turned off, but I expect they would probably settle on a nearby safe surface, in a similar manner as we navigate in the dark (relatively gingerly, with "arms" extended), until their eyes have suitably adjusted. Then they may fly around if need be, but because their natural behaviour does not support the requirement for night foraging, I expect they would probably take it relatively easy until the lights came back on. Rockpocket 08:42, 1 September 2007 (UTC)

Ahem. --Sean 12:43, 1 September 2007 (UTC)

Ah. DirkvdM 08:08, 2 September 2007 (UTC)

OK, follow-up question: What if there is a bird flying around inside a large room such as a gymnasium, and you switch off the lights? — Michael J 01:53, 5 September 2007 (UTC)

Titration and pH meter/indicator

What did chemists use to determine pH of chemicals before the electric pH meter was invented? Seeing how determining the pH of chemicals are very important, the instrument that measures it has to be accurate. —Preceding unsigned comment added by 128.163.224.103 (talk) 23:30, 30 August 2007 (UTC)

Why, litmus paper! I remember it well :D. I didn't even know there was an electric pH meter SGGH ^speak! 23:48, 30 August 2007 (UTC)

I've never seen an electric pH meter, but when I did A-level chemistry, most titrations were done with phenolphthalein: it doesn't have the range of litmus, but it's not bad for locating the acid/base changeover fairly precisely. Algebraist 00:19, 31 August 2007 (UTC)

See pH indicator for more useful chemicals. Algebraist 00:22, 31 August 2007 (UTC)

But aren't those methods not accurate in terms of getting the exact number? In other words, they're merely approximations. In acid/base calculations, such as the Henderson Hasselbach equation, and in doing titrations doesn't the exact pH need to be known? 128.163.224.103 00:54, 31 August 2007 (UTC)

There was no way to get an exact (accurate) number, before the electronic pH meter. All you could do was approximate based on calculations. Even using the most sophisticated derivations of the Henderson Hasselbach Eqn, there is a level of error that can't be accounted for. Ionic shielding and activity coefficients try and compensate, but there is no replacement for using a pH meter.Mrdeath5493 01:25, 31 August 2007 (UTC)

Mythbusters

On Mythbusters last night they tested using a grappling hook shot from a moving car onto a stable structure to make fast 90 degree turns. When they used a steel cable rated for 15,000 pounds, it broke. How much pressure would actually be on a cable going from a car to a structure at 30 miles per hour making a 90 degree turn? 68.231.151.161 23:40, 30 August 2007 (UTC)

By pressure I assume you me force? F=M*A?? At the time the cable became taught, acceleration was zero to thirty miles an hour (I think) which is 48 280.32m/s/s if the cable goes taught over one second. But without the mass of the car, and the time it took for the cable to get caught, I don't know if it can be worked out. I predict that I am about to be proved wrong though! :D SGGH ^speak! 23:47, 30 August 2007 (UTC)

I think they said the weight of the car was 7000 pounds. —Preceding unsigned comment added by 68.231.151.161 (talk) 00:41, 31 August 2007 (UTC)

We'd also need to know the length of the cable. Someguy1221 00:46, 31 August 2007 (UTC)

Yep - we sure would. We need the centrifugal force - but to calculate that, we need the length of the rope...which we don't know. F=mw²r - but we don't know 'r' (the radius) and to calculate w (the rotational velocity) given only the 30mph linear velocity, we also need the radius. Since w is inversely proportional to r, the final force is proportional to 1/r. So if we guess the radius wrong by a factor of two, we'll be off by a factor of two when we work out the force on the rope. Can't calculate it...sorry! I'm kinda surprised the force was that great though - if a 7,000lb car broke a 15,000lb rope - that's a lateral force of 2g's...this surprises me because my tricked out MINI Cooper takes turns like that at around 30mph without resorting to ropes and it can only manage about 1g of lateral. I guess that turn was tighter than it looked. SteveBaker 01:03, 31 August 2007 (UTC)

No need for angular momentum formulas; for circular motion a = v²/r. If a 7,000-pound car (Is that right? What was it, an SUV?) exerts 15,000 pounds force on the rope, we have a = 15/7 gees = 21 m/s². And v = 30 mph = 13.4 m/s, which gives r = 13.4²/21 m = 8.55 m = 28 feet. The radius needed to produce the same force in the rope is proportional to the car's mass, so if a the car weighed say 3,000 pounds they'd have to have tried a rope only 12 feet long.

I haven't seen the episode; could the turn they tried have been that tight? If not, I'd guess that the rope was not capable of supporting its rated force when it was applied suddenly. That seems possible intuitively but I don't know if it's likely in terms of materials science.

--Anonymous, 01:34 UTC, August 31, 2007.

It was a tight turn, probably well under 2 car lengths, meant to be tighter than one could do with a car alone. Also, since they were doing it without the aid of the steering system, there would be an additional substantial force created by the friction of trying to drag the tires sideways. 76.225.157.167 07:28, 31 August 2007 (UTC)

A critical factor that you have to know is the elasticity of the cable. The cable must stretch as it goes from no load to full load, lest the loading (and change in the car's velocity) take place instantly and the force on the cable rises to infinity (owing to F=MA). In a successful experiment, the load on the cable as it stretches won't exceed the breaking strain of the cable.

Atlant 12:44, 31 August 2007 (UTC)

The force is not infinite, and neither is the acceleration; they are mv²/r and v²/r respectively. The quantities that do become infinite if there is no stretching of the cable are the yank and the jerk. As I said, it seems possible intuitively that the cable could break under a big yank, but I don't know if it's likely in terms of materials science. --Anon, 22:30 UTC, August 31, 2007.

And that's precisely why the cable must stretch. Consider: The car is speeding away and the slack in the cable is being taken up. Suddenly, the cable draws taut. What happens? If the cable is not elastic, either it instantly stops the car (which, by the way, did represent infinite acceleration and exerts infinite force on the car and so is unlikely) or it instantly breaks (also unlikely). In the real world, the cable is elastic and stress and strain both begin building in the cable. Eventually, either the cable exerts enough force to constrain the car's motion or the cable snaps.

Atlant 00:02, 2 September 2007 (UTC)

I was assuming that setup had the car running tangentially to the circular arc at the moment the cable was attached — in other words, that the cable was attached while extending at right angles to the car's initial path. In that case a non-stretching cable would produce an infinite jerk but not an infinite acceleration. But of course that would be difficult to arrange in practice; it'd be much easier to make it at least a bit diagonal, and no doubt they did. My apologies for missing that point. --Anonymous, 05:30 UTC, September 3, 2007.