Wikipedia:Reference desk/Archives/Science/2008 June 11

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June 11

Does MOND have problems with Newton's third law of motion?

I read carefully the MOND article in Wikipedia, and it seems to me that it would be difficult to reconcile the idea with conservation of momentum, or at least that only some $\mu (x)$ functions would be consistent (and I cannot figure out which).

Consider an isolated star with a single planet like Pluto, small, distant, and in an elliptical orbit. As I understand MOND, the planet will always be in a gravitational field such that $\mu (x)$=1, while the star will be in a field that will be close to $\mu (x)$=1 when the planet is at perehelion, but far from $\mu (x)$=1 at aphelion. Thus at perehelion, the change in momentum of the star will balance that of the planet, but at aphelion, the momentum change in the star will be significantly greater.

This will cause the combined system to accelerate slowly but continually in the direction from the star to the planet at aphelion. This acceleration will continue until the system enters the gravitaional field of a galaxy.

I suppose MOND enthusiasts could even argue that this provides a new mechanism for the growth of galaxies, but the whole thing strikes me as counter-intuitive.

Have I understood this correctly? John Blackwell (talk) 01:42, 11 June 2008 (UTC)

According to Bekensteins paper, Millgrom's original formulation of MOND does indeed have a problem with conservation laws and some other problems as well. However, Bekenstein goes on to give a Lagrangian/Relativistic formulation that does not (he claims) have these problems. Follow the link in the references section of the article to read more. SpinningSpark 07:19, 11 June 2008 (UTC)

Thanks - I guess I'm back where I am with many things - the accurate theories are beyond my understanding (or perhaps I'm just too lazy to put in the work to understand them) - but I can't see past the flaws in the simplified descriptions. —Preceding unsigned comment added by Johnblackwell (talk • contribs) 01:21, 12 June 2008 (UTC)

Beavers

I'm trying to find out how long it takes a beaver to fell a tree like an Aspen. The photo with the article refers to cutting a 10 inch tree overnight. I would like to know if there is any more specific information on the time, assuming that 10" overnight would be 10" in 8 hrs? or 10 hrs? or 12 hrs? —Preceding unsigned comment added by 65.255.187.5 (talk) 02:03, 11 June 2008 (UTC)

I remember seeing a very good nature documentary film on beavers; I think it was by Rein Maran, an Estonian cinematographer, if my memory serves me well (it was some 20 years ago...). If you can find that documentary, you will have the answer to your question. If you find this film on the web, please give a link. All the best, --Dr Dima (talk) 06:08, 11 June 2008 (UTC)

A quick google search finds a cached article [1] saying 6" in 15 minutes. Scaling that to 10" is three times longer, although the more mature tree would also have more hard wood. Don't know if that helps... Franamax (talk) 15:18, 11 June 2008 (UTC)

I believe the time needed scales with the cube of diameter, not the square, so it would take about six times as long. In order to fell a tree, a beaver chews a wedge-shaped ring, not a cylindrical ring, so the amount of material removed increases as the cube of the diameter. Also, you need to take into account non-linear factors such as fatigue. --Carnildo (talk) 21:45, 11 June 2008 (UTC)

Yes, it is the cube. Just for fun, I tried working out the formula for the volume. for a 90° wedge, chewing 3/4 of the diameter, I get .344D³. I can do that geometrically right, I don't need an integral? My math is a little rusty :) Franamax (talk) 01:38, 12 June 2008 (UTC)

Phthalate

Though controversial, phthalates are still being used in a variety of household applications (shower curtains, adhesives, perfume), modern pop-culture electronics and medical applications such as catheters. Notable recent examples include Apple Inc.'s iPhone and iPod, and personal computers. The company has been criticized by environmental supporters claiming that tests on a commercially-purchased iPhone returned "toxic" levels of the chemical, prompting public declarations for change due to its associated hazards.

— ., http://en.wikipedia.org/wiki/Phthalate

Are iPod nanos included?21:49, 6 June 2008 (UTC)68.148.164.166 (talk)

Wow, ophthalmology (and its few derivatives) isn't the only english word with a "phth" in it? – b_jonas 10:17, 9 June 2008 (UTC)

What difference would it make? That is, what possible path is there by which a (hypothetically) phthalate-loaded iPod could poison you?

Atlant (talk) 12:20, 10 June 2008 (UTC)

I'm concerned about phthalate's estrogenic effects.68.148.164.166 (talk) 03:22, 11 June 2008 (UTC)

Unless you grind up your iPod and sniffed all the powder the amount of phthalate exposure from your iPod is negligible, and if you did sniff the powder your biggest concern wouldn't be phthalates either. --antilived^{T | C | G} 04:14, 11 June 2008 (UTC)

The problem are plastics which are in contact with fat rich food. The people who suffer most are the people needing dialysis, because for a long time the tubing and the memranes in the dialysis machine contained high amount of phtalates doi:10.1002/1097-0274(200101)39:1<100::AID-AJIM10>3.0.CO;2-Q, but for the rest a phthalate rich iphone has no chance to poison you, because the skin is not very permeable.--Stone (talk) 07:23, 11 June 2008 (UTC)

What if you alternately fiddled with your iPod and ate potato crisps/chips all day every day? From my observations some people do exactly that. Some phthalate would dissolve in the fat. You might have to worry more about the oestrogenic/estrogenic effects of the soy/soya oil the chips/crisps were fried in though. Itsmejudith (talk) 11:39, 11 June 2008 (UTC)

Phthalates are of most concern in the developmental stages, so don't let your baby play with your iPod... Franamax (talk) 17:22, 11 June 2008 (UTC)

So how do you dispose of your old iPod without putting phthalates into the environment? —Pengo 20:57, 14 June 2008 (UTC)

custom parts

does any one know a site where i can order custom metal parts? like say i wanted all the parts of a car engine...but i wanted them one third the size,where could i get that for cheap? —Preceding unsigned comment added by 76.14.124.175 (talk) 07:06, 11 June 2008 (UTC)

Hmm, not really a science question, but it might help to get an answer if you said what country you were in. SpinningSpark 07:24, 11 June 2008 (UTC)

One magazine I receive includes ads from custom metal fabricators, who can mill or make castings based on electronic CAD (machine drawing) files you email them, with quick turnaround. But unfortunately you said "cheap," which it certainly will not be. Wait a few years and "fabbers" may become as common as computer printers and scanners, and you can create your own miniature car engine via Desktop manufacturing. Edison (talk) 20:51, 11 June 2008 (UTC)

Fabbers usually work with low-melting-point materials, and occasionally with sintered metal powders. Neither is very good for making a car engine, which requires high temperature resistance and decent mechanical strength. --Carnildo (talk) 21:48, 11 June 2008 (UTC)

Opening Boxes of Tablets

There are 2 ends to my boxes of tablets (hayfever, paracetemol, aspirin, whatever). If I open it at one end I can get straight to the tray of tablets. If I open it at the other I have paper folded over the tray which contains warnings/instructions. I never look which end i'm opening so sometimes it will be (if you have the front facing you) the left-side and sometimes the right. Why is it that a good 80% of the time I get the paper-end? I would expect it to be 50/50 (2 ends, 1 end is 'clear' the other isn't) but I virtually always open it what is (to me) the 'wrong' end. I've asked a few other people and they find the same thing.

My guess is that it is down to the additional weight from the folded-paper, but wondered if A) do other people experience this too? and B) Does anybody know 'why' this might be? 194.221.133.226 (talk) 08:19, 11 June 2008 (UTC)

Here's two explanations:

1. The way the box is designed. The manufacturer wants you to look at the warnings, if only to avoid a lawsuit. Therefore, they may design the box so that it subconsciously encourages you to open it at the end with the paper.
2. It really is more like 50-50, you just don't notice it. Have you been keeping a log somehow? If not, there's a good chance that the percentage of the time you open it on the paper end is lower than you think, but you only notice a "pattern" when you open it on that end.

Or maybe you're just psychic. =) « Aaron Rotenberg « Talk « 08:42, 11 June 2008 (UTC)

I've not kept a log so it could well be that my brain is finding a pattern - i'll start a log (yeah i'm that kind of person) and see. The idea of the design sounds interesting though, I used to read a great blog that was called 'architecture of control' or something (it looked for designs that pushed people to do stuff/had a second purpose that people might not realise was actively intended). —Preceding unsigned comment added by 194.221.133.226 (talk) 09:36, 11 June 2008 (UTC)

This is related to the phenomenon I've observed whereby if a microwave oven has a turntable, and you follow the advice I've given elsewehere and set the food "off-center" on the turntable, when the cooking cycle is done, the turntable always ceases rotation with the food located at the back of the microwave oven. Seriously, I'd also suggest that the weight of the package insert instructions may be cueing you to more-often hold the box one way rather than the other. But I have no explanation for the microwave oven phenomenon except that maybe the peculiarities of the timer and turntable drive motor lead to the turntable always turning n.5 rotations (for those cooking-time values that I routinely use).

Atlant (talk) 11:41, 11 June 2008 (UTC)

How are you orienting the package? Main label side up, text reading left-to-right? That would be your first clue to unconscious positioning. There is also the possibility of the observer preferentially registering adverse events. On an unrelated note, I recall reading about a study which showed the probability that if you dropped your toast, it would land upside-down. so weird things do sometimes happen. Franamax (talk) 17:39, 11 June 2008 (UTC)

Franamax - the toast thing is to do with the average height of work-surfaces/table-tops and only having enough time for it to rotate from butter-side-up to butter-side-down (or at least that's the explanation i've been given and it sounds plausible). ny156uk (talk) 17:47, 11 June 2008 (UTC)

I'm just hazarding a possible guess here....are you right-handed (i.e. do you open the right-hand flap of the box most of the time)? Do you most of the time hold the box upright with text reading left-to-right and the flaps at either side? Zunaid©® 14:56, 13 June 2008 (UTC)

Many people will be opening these boxes. Most people will open either end about 50% of the time. However, they will not report it to anyone or even remember it. A small proportion of people will find, purely by chance, that they open one end much less/more than 50%. They may remark on it, as you have done. So you've just been lucky. If this is the true explaination, then reversion to the mean suggests that you probably will open up either end about 50% of the time in the future. 80.2.203.46 (talk) 10:52, 16 June 2008 (UTC)

Neutrino oscillation and conservation

Suppose I measure that an electron is an electron neutrino at a given time, and I also measure that it has a certain velocity (subject of course to uncertainty). Now assume that later I measure that it's oscillated into a tau neutrino, with a different velocity. How does this work with conservation of mass/energy and conservation of momentum? I mean, you can make the velocity uncertainty arbitrarily small in both cases, so I don't see how the observer effect could account for this. Veinor ^{(talk to me)} 08:57, 11 June 2008 (UTC)

There is an explanation at neutrino oscillation. As far as I understand it, the hypothesised neutrino mass eigenstates have different masses and so propogate at different velocities (because of conservation of momentum). The mass eigensates are, in turn, combinations of flavour eigenstates, and so the mix of flavour eigenstates varies along along the neutrino's path. Therefore the probability of observing a particular flavour eigenstate varies according to whereabouts along the neutrino's path you make your observation. Gandalf61 (talk) 09:24, 11 June 2008 (UTC)

To put this more directly, flavor and momentum form an uncertainty relation, so if you had made the uncertainty on momentum arbitrarily small then you'd have no way of determining what kind of neutrino it was and vice versa. Dragons flight (talk) 17:28, 11 June 2008 (UTC)

Blow job pregnancy

Can I get a girl pregnant if I cum in her mouth when having a blow job, cause it goes into her but does it just get digested or is there a risk it might find its way to her uterous? —Preceding unsigned comment added by Milkly man (talk • contribs) 09:25, 11 June 2008 (UTC)

No, there is no connection between the digestive system and the uterus. — QuantumEleven 09:34, 11 June 2008 (UTC)

However there is a risk of the transfer of STDs. There is also a (minor) risk of pregnancy if your semen goes near the vagina somehow. Probably the greatest risk for pregnancy is if you do somehting like kiss your girl after the act and then perform unprotected oral sex on her. All in all, unless you are in a committed relationship and both of you have been checked for STDs since you entered into that relationship, any form of unprotected sex is simply a bad idea. Nil Einne (talk) 10:20, 11 June 2008 (UTC)

Angela Ermakova got pregnt from Boris Becker after transfering semen from a blow job, so it is possible if somebody wants it to happen. [2]--Stone (talk) 12:24, 11 June 2008 (UTC)

So did JD's girlfriend in Scrubs. Quipquip (talk) 20:12, 11 June 2008 (UTC)

If memory serves, that wasn't a blow job, it was a poorly aimed premature ejaculation. --Tango (talk) 21:31, 11 June 2008 (UTC)

Indeed I probably should have said, unless you are in a committed relationship, have both been checked for STDs and are both ready to and have talked about how you will deal with an unexpected pregnancy Nil Einne (talk) 22:02, 11 June 2008 (UTC)

Hi. I'm not really qualified to answer this, but I think I once read somewhere (I forget where) that a girl in Lesotho who was unable to reproduce and gave a BJ, then her boyfriend got angry and stabbed her in the stomach thus allowing seminal fluid to seep into her uterus and she became pregnant. Thanks. ~AH1^(TCU) 23:34, 11 June 2008 (UTC)

That sounds unlikely. If the knife pierced the uterus, it would probably be unable to support a fetus. The semen may get in and fertilise an egg, but it wouldn't be able to go to term. I suppose the knife could have hit a fallopian tube, and the semen got in that way, and the uterus was left unharmed, but that still seems unlikely. Also, if she was unable to reproduce, the semen getting in via a knife wound is very unlikely to make any difference. She probably got pregnant the old fashioned way and then got stabbed, and the whole infertility thing was just incorrect and has made for an interesting, but erroneous, story. --Tango (talk) 23:55, 11 June 2008 (UTC)

Hypothetical Questions about Gravity and the Sun

Question 1: What exactly would happen if the sun moved 500 miles away from earth?

Question 2: What exactly would happen if the earth slowed its rotation speed by, say, 20 miles?

Please note that I am a 35 year old man and these are not homework questions. I would like to know exactly what I would experience when these phenomonas happen. --Vincebosma (talk) 13:03, 11 June 2008 (UTC)

You wouldn't notice a 500 mile change in the orbit of the Earth. The variation in distance caused by the orbit not being a perfect circle is about 5 million kilometres, so 800km would be insignificant. Question 2 doesn't make sense, do you mean 20 miles per hour? And by rotation speed do you mean the Earth rotating on its axis, or orbiting the Sun? --Tango (talk) 13:16, 11 June 2008 (UTC)

On its axis. And 20 mph. Also what would I experience if it slowed down say 200 mph? Forgive me for my serious lack of science knowledge. Just trying to answer questions my nephew is asking me. He's only 6. --Vincebosma (talk) 13:30, 11 June 2008 (UTC)

Assuming you mean the speed at the equator (which is the fastest - the poles don't move at all), the actual speed is about 1040 mph. Slowing that down would result in a longer day. 20mph is about 2%, so the day would be nearly half an hour longer, 200mph is 20%, corresponding a day nearly 5 hours longer. Lengthening the day would have all kinds of effects. Most would be fairly minor for just 30 mins longer, although some animals and plants may be confused. 5 hours would confuse pretty much everything. It could also affect the climate. One of the causes of prevailing winds is the Earth's rotation, so slowing that rotation could result in slower wind speeds. It would also result in a slight increase in gravity due to reduced centrifugal force (which acts to cancel out part of the gravitational force), but that would be negligible (it would just reduce the difference between the poles and the equator). --Tango (talk) 13:55, 11 June 2008 (UTC)

(After edit conflict) This is a good opportunity to introduce your nephew to orders of magnitude. The circumference of the Earth is roughly 25,000 miles. So a point on the equator travels a distance of 25,000 miles every 24 hours due to the Earth's rotation on its axis. This is a speed of about 1,000 mph - the speed of a supersonic jet. If you were flying in a supersonic jet which slowed down by 20mph, would you notice the difference ? If every hour was 61 minutes instead of 60 minutes, would you notice the difference ? The Earth is about 93 million miles from the Sun. Compared to this distance, 500 miles is about 5 parts in a million. A millionth of a mile is about 1/16 of an inch. If you had to walk a mile, and then someone increased the distance by 5/16 of an inch, would you notice the difference ? Gandalf61 (talk) 14:03, 11 June 2008 (UTC)

Thank you, and congratulations, for trying to answer his questions. Too many parents/uncles/aunts give the "Because God said so." or "Because it is, stop asking." To help you explain the answers by Tango and Gandalf, it may be helpful (and instructive, and get you two some valuable play time, and get you outside) to perform this exercise. Go outside with a basketball and a marble (or, if you're not that commited to scale, which isn't quite as important at his age a golf ball or ping pong ball will do). Have him put the basketball down where he wants the sun to be. Then, hand him the Earth ball (the smaller one) and tell him to put it where the Earth is. It should be 534 feet away, assuming the basketball is the size of the sun (Note that at this scale, the earth is 1/4 of an inch in radius). You may need to go about a block away...its not quite a backyard exercise. Now, to answer his question of what would happen if the Sun moved 500 miles. Have him move the basketball 1 inch, or have him roll the ball once around or whatever. (The actual amount of movement would be .03 inches, but thats not particularly visual for a 6 year old). Ask him if he thinks its made a big difference. EagleFalconn (talk) 14:21, 11 June 2008 (UTC)

As for question 1, the distance of the Earth from the Sun is determined by the orbital speed. If you moved them apart by 500 miles without increasing the orbital speed, this would result in the Earth "falling" back those 500 miles, in about 3 months, and then another 500 miles closer to the Sun, in the next 3 months, then it would turn around and go back out to the original point in the next 6 months. So, if you had started with a circular orbit, this would make it slightly elliptical. However, since the orbit of the Earth is already more elliptical than that, this would either increase or decrease the eccentricity of the orbit, depending on whether the 500 extra miles were added when the Earth was closest or farthest from the Sun. StuRat (talk) 14:44, 11 June 2008 (UTC)

To really screw with his head, explain the Milankovitch cycles which dictate that we're all gonna die even if the Earth stays in its current orbit. Ice age! EagleFalconn, thanks for your excellent outline of a practical, understandable demonstration. I'll be able to use that in another 4 years or so :) Franamax (talk) 14:51, 11 June 2008 (UTC)

Hi. Well, as for the sun moving 500 miles away, that depends on which way it moves. If it moves horizonal and along Earth's orbital plane, it would be 500 miles closer than normal 6 months later. If it moved verticly (north-south), it might not be exactly 500 miles as from the Earth. However, distance from the sun can affect the climate. For example, currently the Earth is closer to the sun in the southern hemisphere summer than in the northern hemisphere, so the southern hemisphere gets more summer sun intensity. I'm not sure, but if the opposite were true, as can happen with cycles lasting tens of thousands of years, the Sahara might green, for example, but things like Global warming may disrupt these cycles. However, if the centre of the Earth were only 500 miles from the sun, then part of the Earth would be in the sun, and if the surface of the Earth were that far, we'd be in the sun's atmosphere and be past its Roche limit, and fall into the sun. The speed of our orbit around the sun is determined by several factors, which I'm not too familiar with, but is most likely dictated by distance from the sun. However, if our orbit speeded up dramaticly for whatever reason, the Earth would fly away from the sun due to centrifugal force, then slow down because the sun's gravity there is weaker. If you need articles that are written in Simple English which might be easier to explain, you can check the languages box in the article to see if there is one. Thanks. ~AH1^(TCU) 23:30, 11 June 2008 (UTC)

As a clarification, yes, distance certainly as an absolute value has an effect on the amount of energy recieved by the earth. However, at the distance the earth is currently at, the amount of time and the directness of the striking of the sun's rays on the earth's surface as measured by the angle of insolation are much more significant factors.EagleFalconn (talk) 16:04, 12 June 2008 (UTC)

Rancid meat

Why can't you eat rancid meat if it is cooked? Surely if it is cooked really, really well then the heat will kill any bacteria, viruses or other germs that would make you sick. Pob The Plumber (talk) 13:41, 11 June 2008 (UTC)

Cooking the meat may sterilise it. But some toxins that were produced by the putrifactive bacteria survive the cooking process. Escherichia coli produces one such heat stable toxin. So dont eat last years, raw,forgotten Christmas Turkey now, cooked or not :-) Fribbler (talk) 14:05, 11 June 2008 (UTC)

Got it! Thanks. Maybe my mother-in-law would like a turkey sandwich..... Pob The Plumber (talk) 14:43, 11 June 2008 (UTC)

Just to clarify, rancid and putrid are not the same thing. One is oxidative decomposition, which makes things smell bad (rancid butter, rancid tuna), the other is a bacterial process. Nevertheless, if it's meat and it smells bad, throw it away. Franamax (talk) 15:01, 11 June 2008 (UTC)

One of the reasons why various cuisines developed highly-flavored/highly-scented/piquant sauces, of course, is...

Atlant (talk) 17:22, 11 June 2008 (UTC)

Why didn't all English gourmets and hunters die in centuries past from eating "high game" as discussed by Dickens [3] or in this 1889 book [4] or this recent publication [5] "The pheasant was not as well hung as the staff had told us and lacked the real oomph of high game." Apparently some gourmets still seek a "well hung" pheasant. Note: I strongly discourage eating rotten meat. Edison (talk) 20:44, 11 June 2008 (UTC)

Oh yumm, hare soup and make sure every drop of blood goes in it! Many meats are hung, for instance beef carcasses for 2-3 weeks, to allow the existing enzymes to partially "digest" the meat and soften it. Bacteria will grow only on surfaces, so the toxin load would not be high, and washing or boiling would handle it anyway (contrast with contaminated ground beef). Plus you need the right unlucky combination of toxin-producing bacteria and conditions under which the bacteria will be producing the toxin. I'm thinking that not all, but quite a few gourmets of ages past actually did die from eating spoiled food, they probably called it dropsy or something back then, before they figured out what was causing it. Franamax (talk) 00:37, 12 June 2008 (UTC)

Some mycotoxins in particular are nasty stuff. While they don't tend to be much of a problem on meat (well other then by meat from animals feed contaminated grains), they are a problem on grains and also fruit. Aflatoxin is one good example. As with bacterial enterotoxins and endotoxins, cooking does not destroy them so if you come across fruit or grains that look like they are moldy, throw them out. Particularly for anything soft as often what you see is only a small percentage of what is there. Nil Einne (talk) 22:00, 11 June 2008 (UTC)

Why don't we have 4 seasons every 27 days?

Hello giant minds! The Earth's 23.5 degree inclination pushes points on its hemispheres a few thousand miles either closer/farther from the sun causing seasons throughout the year. Shouldn't the Earth's massive "wobble" as it holds the Moon spinning around it cause the same effect? Earth orbits every 27 days or so (just like the Moon) around the barycenter of the Earth-Moon system, located thousands of miles from Earth's apparent center. Why am I not expecting snowstorms followed by beach weather every few weeks? Sappysap (talk) 15:06, 11 June 2008 (UTC)

The seasons happen because of tilt - the angle of the sun's rays to the Earth - not the distance. Those few thousand miles are insignificant for seasonal purposes. — Lomn 15:21, 11 June 2008 (UTC)

The reason is multi-fold. To address a common misconception you mention: The distance between the sun and the earth has almost no effect on the seasons. Between summer and winter, the change in the distance is about 3.1 million miles. However, the northern hemisphere's winter occurs when the Earth is closest to the sun. See [[6]] for more.

The first is the angle of insolation, which is the angle at which the sun's rays, directly striking the Earth's surface, strike the earth. The greater the angle, the less energy is transferred to the surface of the earth and its atmosphere. That is why summer occurs when the hemisphere you occupy is tilted toward the sun.

However, the Earth does not 'wobble' as the Moon revolves around it. The direction of the gravitational force between the moon and the earth does cause a center of rotation between the two of them, but that does not change the angle of insolation. There is no angle of insolation change due to the barycenter, though there is a slight distance change. However, if 3.1 million miles isn't making a difference, this certainly won't. EagleFalconn (talk) 15:27, 11 June 2008 (UTC)

For effects that are significant over the period of a month, see tide. --Prestidigitator (talk) 16:17, 11 June 2008 (UTC)

Sweating

A little background: I went for a bike ride the other day, it was about 15 degrees out, so I wore a t-shirt and some shorts to keep cool. During the bike ride, as is normal for me, I started sweating. This isn't much of a problem, but then this guy pulls up next to me on his bike wearing jeans, a sweater and a vest, and he's not sweating a bit. This blew my mind, as we were doing the same work--riding down the same street for essentially the same ammount of time, he on a mountain bike and me on a road bike, so I was actually doing a little less. What gives? Why was I sweating like a pig, and he not even uncomfortable? 142.33.70.60 (talk) 16:22, 11 June 2008 (UTC)

Just because he wasn't sweating doesn't mean he was comfortable. Some people just don't sweat much, which can make them overheat more easily. Also, weight makes a huge diff, as extra fat both provides thermal insulation and extra mass to move, requiring the burning of more calories. Cardiovascular fitness also makes a diff, as some people will be seriously stressed by that level of exercise while others "won't even break a sweat". StuRat (talk) 16:39, 11 June 2008 (UTC)

Assuming you meant it was 15°C, or 59°F, that seems cool enough that most people wouldn't sweat noticeably during a relaxing bike ride. StuRat (talk) 16:44, 11 June 2008 (UTC)

Also, maybe he hadn't been riding for as long as you? It takes a while after you start exercising for the body to produce enough excess heat to trigger sweating. —Ilmari Karonen (talk) 16:54, 11 June 2008 (UTC)

And if you were truly sweating "like a pig" you wouldn't be sweating at all, since they don't. Matt Deres (talk) 15:21, 12 June 2008 (UTC)

Cameras in rough times

Why do cameras often temporarliy show just noise when subject to violent vibrations? Example at 0:35. —Bromskloss (talk) 17:59, 11 June 2008 (UTC)

Theres a slight difference here between digital and film cameras, but the explanation is essentially the same. On a digital camera, when you press the button a sensor is triggered which takes a time average of (essentially) what color light is hitting the sensor at that particular pixel. If the camera is shaking, the average is going to be over a range of colors, usually resulting in a gray or other odd color, what you might call noise. On a film camera, its the same effect except instead of a detector you've got chemicals on a piece of film. The chemicals, being exposed to several different colors of light, will report all of them, which we typically see as white. EagleFalconn 18:20, 11 June 2008 (UTC)

Like motion blur, you mean? I don't think that's it. What I'm talking about looks more like the camera is about to fail completely, like if a signal cable is not properly plugged in, but spuriously loses contact. Mabye that's it – the signal cable losing contact? —Bromskloss (talk) 18:42, 11 June 2008 (UTC)

The sort of noise you see there is caused by a bad connection being vibrated so it isn't always transmitting a signal. --Carnildo (talk) 22:02, 11 June 2008 (UTC)

How come radio waves aren't stopped by walls and such?

You gotta ask to learn! If, for instance, AM waves have a wavelength of 100m to 1km (from Radio frequency), how come they can pass through walls and such? The way I see it is obviously wrong, so I'd be thankful for a few pointers, although I think that I've looked through the main articles. -- Aeluwas (talk) 18:05, 11 June 2008 (UTC)

The quantum mechanical view on it would be that for (1) photon to interact with an atom, that atom/molecule/whatever needs to have a valid energy transition available to it at the same energy as the photon or (2) the possibility for a Stokes collision.

For case (1), if the energy levels in the atom do not correspond to the energy of the photon, no interaction is allowed because no electron is available for promotion to a higher energy level. This rarely (I'm prepared to say never) happens with radio waves because the photons are of such low energy (a quantity which is inversely related to wavelength) that there are no electronic transitions available. Nuclear energy state transitions do occur in those areas, however those transitions are very difficult to achieve by inputting electromagnetic radiation and is better done with a magnet, as in NMR.

Case (2) There is a probability that an electron will 'collide' with a photon and remit the photon at the same wavelength or a different one. See Stokes shift. This is a very difficult effect to observe with radio waves, more so than with other types of light. In general, these collisions are very improbable and any experimentation with them has to be done with a laser to generate sufficient intensity so as to be able to collect data. EagleFalconn (talk) 18:33, 11 June 2008 (UTC)

I'll give a very general, non-technical answer. Because the walls aren't dense enough. Now if the walls were made of thick lead, then they would stop the radio waves. If you are in the center of a large building with lots of walls, or underground they would stop the radio waves too. ScienceApe (talk) 18:59, 11 June 2008 (UTC)

None of the above. There are two ways, both correct, to think of radio waves: either as oscillating electromagnetic (EM) fields or as photons. At the long wavelengths that you are talking about, the photon view is not helpful, and it is better to think of EM fields. An EM field is partly electric and partly magnetic, hence its name. In order to stop an EM wave, you need either an electrical or a magnetic barrier, or both. An electrical barrier needs to be an electrical conductor, like a sheet of copper or aluminium. A magnetic barrier is harder to achieve, but a layer of soft iron would work as both an electric and a magnetic barrier for low-frequency waves. Walls are generally made of neither electrical conductors nor magnetic materials, so they don't stop radio waves at the frequencies you are talking about.

Higher-energy radio waves (shorter wavelengths, like microwaves) behave more like light and are stopped by walls, but that's another subject. --Heron (talk) 20:04, 11 June 2008 (UTC)

I don't believe that is entirely true. Or at least saying I'm wrong isn't true. Putting a sufficient amount of matter in between you and the radio waves will stop them. If you were surrounded by say a kilometer of ice on all sides, it would block any EM radiation from getting in. ScienceApe (talk) 20:49, 11 June 2008 (UTC)

It is true that the shorter the wavelength is, the more like visible light the radio wave will behave, since the only difference between visible light and radio waves is that radio waves have a longer wavelength. I'm not sure microwaves are completely blocked by walls, though - if memory serves, the frequencies used by Wi-Fi are in the microwave range, and they certainly can go through walls, although the signal is noticeably weakened. --Tango (talk) 22:36, 11 June 2008 (UTC)

Going off what Tango said, I can confirm that microwave is not always stopped by walls since cell phones are run off of microwaves (hence the whole cancer/cooking your brain scare). However, using a material of a sufficient density/thickness is simply taking advantage of the Stokes collision effect I mentioned above. It works better with shorter wavelengths because shorter wavelengths tend to exhibit more particle like behavior (hence why walls are not transparent). Another more satisfying way to think about it (for me at least) is that shorter wavelengths are more likely to be in the correct range of energy states to be able to interact with the wave function defining the translational motion of the nucleus/electron cloud of the atom. EagleFalconn (talk) 16:12, 12 June 2008 (UTC)

Actually, the absorption scale for radio waves in ice is many tens of km. For example, radar reflections are used to measure the shape of the bedrock under the ice at Antarctica. Also, in line with Heron, the limiting factor is still conductive impurities (mainly sulfates and H+ ions) and not bulk matter per se. Dragons flight (talk) 22:30, 11 June 2008 (UTC)

More like 5 KM actually. Nope, it really is bulk matter. You can take any matter and if you surround yourself with enough of it, it will block EM radiation. Denser material is better at stopping EM radiation than less dense material. ScienceApe (talk) 23:34, 11 June 2008 (UTC)

Strictly speaking denser is better, but free electrons (e.g. metals and ions) have a much, much greater effect on absorption/scattering than density. Dragons flight (talk) 00:10, 12 June 2008 (UTC)

A thin layer of iron, steel. or even iron hardware cloth will interrupt most AM or FM radio broacdasts by acting as a Faraday cage. By thickness, ferrous metal provides far better radio shielding than brick, rock, or concrete. Edison (talk) 20:31, 11 June 2008 (UTC)

Um, doesn't the thin layer of this stuff have to be somewhat enclosed? You can't really have no charge on the inside if there is no "inside". --Wirbelwindヴィルヴェルヴィント (talk) 04:13, 12 June 2008 (UTC)

Yes, it does need to be enclosed. EagleFalconn (talk) 16:12, 12 June 2008 (UTC)

You might enjoy our article about TEMPEST, a U.S. military standard that is deeply involved with exactly how well walls stop radio emissions.

Atlant (talk) 12:25, 12 June 2008 (UTC)

Uh, yeah, if it is not a closed surface it is not a Faraday cage. But a large sheet of metal could provide some directional degree of shielding, as could a metal surface with some holes in it, or a curved metal surface. A metal surface or rod could also increase the field strength, if it happened to be where it was a director or reflector, depending on the location of the transmitter and receiver, like in antenna design. Edison (talk) 18:59, 12 June 2008 (UTC)

The Radius of the Earth

Is it possible to calculate the radius of the earth simply by measuring the time between two sunsets, one observed by lying down and the other observed by standing up just after the sun (apparently) goes down while we were lying down?? If so, how?? —Preceding unsigned comment added by 117.194.226.115 (talk) 18:18, 11 June 2008 (UTC)

Have you tried drawing a picture of the scenario? -- Coneslayer (talk) 18:33, 11 June 2008 (UTC)

Yes. For an observer standing on the surface of the earth, his line of vision in both cases are tangents to the earth's surface. The sun covers 360 degrees in 24 hours, so supposing the time interval between the two sunsets is x seconds, we can calculate the angle covered by the sun in that time. But does that really help? I'm completely lost as to what to do after this. —Preceding unsigned comment added by 117.194.226.115 (talk) 18:38, 11 June 2008 (UTC)

This webpage explains the experiment: http://astronomy.nmsu.edu/nicole/teaching/ASTR110/lectures/lecture10/slide05.html . The experiment can be performed either at sunrise or sunset. If you measure the height of the standing person, denoted h (cm), and the time between the sunsets, denoted ΔT (s), then the radius of the Earth, denoted R (cm), can be found by the equation:

$R=h\cdot {\frac {\cos(\triangle T/240)}{1-\cos(\triangle T/240)}}$.

Convert R to more sensible units of meters or kilometers by dividing your answer by 100 or 100,000 respectively. Jdrewitt (talk) 20:48, 11 June 2008 (UTC)

Keep in mind that you should do this only where you have a true horizon (like watching the sun set over the ocean). Mountains, trees, or other large and relatively close obstacles are going to mess up the experiement completely. --Prestidigitator (talk) 21:06, 11 June 2008 (UTC)

Also keep in mind that atmospheric diffraction distorts the apparent positions of the Sun and the horizon. I don't know if this will have a noticable effect on the experiment, but when dealing with things as small as the timing differences involved here, it's worth thinking about. --Carnildo (talk) 21:59, 11 June 2008 (UTC)

I think you mean atmospheric refraction, not diffraction. -- Coneslayer (talk) 14:25, 12 June 2008 (UTC)

Thank you so much! I get it now! —Preceding unsigned comment added by 117.194.225.178 (talk) 05:46, 12 June 2008 (UTC)

Note that I'd expect the margin of error to be absolutely huge, so consider yourself lucky if you get the answer within an order of magnitude. Also, you'd need to consider that the time between two consecutive sunsets isn't exactly 24 hours anyway, depending on whether the days are getting longer or shorter. StuRat (talk) 04:35, 13 June 2008 (UTC)

Good point -- so it's better to observe the same sunset twice. Lie down, watch it set and start your stopwatch, then quickly stand up and do it again. If you do it at sunrise you can quickly drop to the ground instead of quickly standing up. Or have two people do it together, one in each position. --Anonymous, sitting down, 05:45 UTC, June 13, 2008.

Yeah, it's probably impractical with a height of 6 feet or so, but it's really cool from an airplane. I once watched the sun set while were taxiing, then we took off and I watched the sun come back up over the horizon, then it set again. It's like that old ad (for Life Savers?) with the kid saying, "Do it again, Dad!" -- Coneslayer (talk) 11:18, 13 June 2008 (UTC)

And even cooler, if you are in a plane going West fast enough, you can apparently make time go backwards and make the Sun rise in the West and set in East. This would require supersonic speeds at the Equator (over 1040 MPH), but much less at the Arctic or Antarctic circles. StuRat (talk) 12:24, 13 June 2008 (UTC)

Hottest time period in Earth's history

What was it? I have heard it may have been the Middle Cretaceous or the early Eocene, (Paleocene–Eocene Thermal Maximum), but from climate graphs of the earth, it is hard to tell because many periods are warm and I can't seem to find an exact answer. Thanks 142.150.72.199 (talk) 18:23, 11 June 2008 (UTC)

Hi. Well, I'm not sure, but I think it may have been sometime around the Hadean eon in the Precambrian, when the earth just recently formed, the crust was not yet solid, and the Earth was experiencing the Great Bombardment. However, if you include the future, there may be periods hotter than some mentioned above. Hope this helps. Thanks. ~AH1^(TCU) 23:08, 11 June 2008 (UTC)

Thanks, but I was more so asking about the climate a time period so far where there was life and ecosystems, particularly, animal life. 192.30.202.21 (talk) 22:35, 12 June 2008 (UTC)

Hi. Does this this graph help? It graphs temperatures throught the time periods but with the present at the left and the far past at the right. Also note that it goes to the beginning of the Cambrian 542 million years ago. Thanks. ~AH1^(TCU) 00:52, 14 June 2008 (UTC)

I'm guessing that just when the Earth was formed, and was still molten magma all over would have been a pretty hot time. —Pengo 20:30, 14 June 2008 (UTC)

Planet orbit question and the formation of our solar system

Are all the planet in our solar system revolving around the sun in the same direction? Are they all rotating/spinning in the same direction? If not, how is this possible? During the formation of our solar system, shouldn't they be revolving and rotating in the same direction due to the conservation of angular momentum? ScienceApe (talk) 19:02, 11 June 2008 (UTC)

All of the planets revolve the sun in the same direction: from the north pole of the sun, counter-clockwise. All the planets except for Venus also rotate counter-clockwise (again looking from the sun's north pole). See solar system, Venus, and formation and evolution of the solar system for more details. Jkasd 19:28, 11 June 2008 (UTC)

With regards to angular momentum and the formation of the solar system, how or why is Venus spinning clockwise? ScienceApe (talk) 20:25, 11 June 2008 (UTC)

Nvm, it seems like an impact event caused it. ScienceApe (talk) 20:26, 11 June 2008 (UTC)

Also, Uranus's axis is inclined at 98 degrees. And also see retrograde and direct motion. Jkasd 19:40, 11 June 2008 (UTC)

Perhaps yours is, butt not mine ... StuRat (talk) 04:27, 13 June 2008 (UTC)

I've often wondered why they would say the axis is rotated 98° and it's rotating in the normal direction, instead of saying it's axis is rotated 82° and it's rotating backwards. StuRat (talk) 04:27, 13 June 2008 (UTC)

I would guess it's because chances are it started rotating in the normal direction and then got knocked over by more than 90 degrees, rather than it reversing its direction. --Tango (talk) 13:00, 13 June 2008 (UTC)

That is surely how it happened, but it doesn't explain why people use that way of describing it. Really it's just an arbitrary choice. --Anon, 22:15 UTC, June 13, 2008.

Yeah, so depending on how you look at it, Uranus can have retrograde motion or not. Jkasd 01:08, 14 June 2008 (UTC)

Oh, it definitely counts; it's just a question of how you describe it. By the way, Pluto also has retrograde rotation; although of course some people don't think it's a planet any more. --Anonymous, 00:54 UTC, June 15, 2008.

Weight of human hair

How much does one's head hair weigh? More precisely, is there a formula that can be used to estimate the weight of head hair based on length(and accounting for differences in hairline, bald spots, and such)? 69.111.189.55 (talk) 22:20, 11 June 2008 (UTC)

You can always shave it off and put it on a scale. Paragon12321 (talk) 21:11, 12 June 2008 (UTC)

If you ask a hairdresser very nicely and say it's for a science project to give it cred, they will let you collect the day's hair takings. Wear surgical gloves and bag it neatly to impress, Julia Rossi (talk) 01:04, 13 June 2008 (UTC)

My guess is that given the differences you've already mentioned (in hairline etc) combined with other differences (like in thickness of the hair), any general formula will be pretty useless Nil Einne (talk) 03:42, 14 June 2008 (UTC)

Flammable cleaning products

(No, I'm not a vandal.) Hello. What are the most flammable cleaning (or otherwise) products that a janitor could use in his work? (No, I'm not a janitor either.) Thanks in advance, Kreachure (talk) 23:15, 11 June 2008 (UTC)

Toluene would rank right up there, or any other solvent the janitor might be using for some purpose. Most chemicals and commercial products have a safety data sheet, googling the name plus "msds" will usually get you some good data. Franamax (talk) 00:50, 12 June 2008 (UTC)

Toluene has a very low flash point, but alcohols such as ethanol or isopropanol have much lower autoignition temperatures. However, the most flammable, common solvent is probably diethyl ether. Not sure if this would be found in many janitor closets, though. --Russoc4 (talk) 03:28, 12 June 2008 (UTC)

I suspect that among the more dangerous solvents a janitor might try to use would be plain old gasoline. In addition to being rather toxic, it's acutely flammable and its vapors are easily ignited by spark or open flame. Other flammable chemicals likely to be found in a janitor's closet might include various solvents used as paint thinners: acetone, turpentine, xylene.

If you're writing a story and need ideas, you might just walk down to your local hardware store—find the products with the scariest warning labels. TenOfAllTrades(talk) 05:10, 12 June 2008 (UTC)

You're right on the money. Thanks for the tips! Kreachure (talk) 19:11, 12 June 2008 (UTC)