Wikipedia:Reference desk/Archives/Science/2008 September 4

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September 4

Zoom H2 Handy Recorder

Sometime ago I saw a link (may have been on a blog) to a page on the audio chips used in the Zoom H2 Handy Recorder. I cant find it now, can anyone help.

Its the Texas Instruments TLV320AIC32 low power stereo audio chip according to someone who took a zoom H2 apart.

You can look at this page : [1] or, better, but in german : [2]

Going through walls

Someone told me this once: "Say you had your on a wall. Eventually your atoms will line-up perfectly that your hand will actually go through the wall. But the changes of that happening is so small, it would take millions of years. But even after that it probably wouldn't happen."

Is this true? Is there anything on Wikipedia on this? I think it was from a book or something -- Coasttocoast (talk) 01:36, 4 September 2008 (UTC)

This is the sort of thing someone who is into quantum mysticism would say. Scientifically its crude. Your description is close but even if you described it perfectly its a misapplication of quantum mechanics. The capacity of electrons and protons to quantum tunnel is important to biology and life. It really happens. Atoms (other then hydrogen) are generally too heavy to tunnel in a meaningful way. The larger molecules formed by atoms certainly don't tunnel past each other. Chemical reactions would occur well before any tunneling happened. The person that is made of molecules won't tunnel ever. At least not without the assistance of technology that has yet to be invented or meaningfully imagined. Hope that helps.--OMCV (talk) 02:08, 4 September 2008 (UTC)

That depends on your definition of "in a meaningful way". The diffusion of molecules through solids is in many ways a tunneling process (though there are more factors involved than the tunneling of a single electron). We know from ice cores that something as large as methane will move 10s of centimeters per 100,000 years in ice. Dragons flight (talk) 19:40, 5 September 2008 (UTC)

Roughly speaking, I believe that is true, but it's not millions of years, it's far far more than that. I don't know how to calculate it, but I imagine it's far far longer than the universe will exist (in a state comparable to how it is now, anyway). --Tango (talk) 02:17, 4 September 2008 (UTC)

From what I recall, Brian Greene discusses this in his documentary The Elegant Universe. - 202.168.20.29 (talk) 03:11, 4 September 2008 (UTC)

Gee, Mr. Wizard, I've noted cases where helium or hydrogen passed right through a rubber balloon. Ya fill the balloon, tie it off, and it goes limp in a day. What is the process in those cases? Edison (talk) 04:31, 4 September 2008 (UTC)

That's caused by the rubber being slightly porous, ie. there are lots of tiny holes in it. Helium atoms are very small, so can fit through those holes far easier than the nitrogen and oxygen molecules in air could get out of a regular balloon, which is why helium (not hydrogen unless you were in doing a lab experiment or something - people are not in the habit of button explosive gasses in children's toys!) balloons deflate faster than air ones (you often get them made of foil to stop that). --Tango (talk)

I have had hydrogen in balloons, (yes, lab experiment) and it seems to pass through the rubber faster than helium does. Edison (talk) 18:38, 4 September 2008 (UTC)

H₂ has a molecular mass of 2 grams/mol while He has an atomic mass of 4 grams/mol. Check Kinetic theory#RMS speeds of molecules. Since hydrogen moves faster it collides with the walls more often Kinetic theory#Number of collisions with wall also finding the gaps more often. This principle is the bases of effusion experiments described by Graham's law that are sometimes preformed in pchem classes.--OMCV (talk) 19:16, 4 September 2008 (UTC)

Penis errect in the morning.

Why penis sometime errect in the early morning203.116.34.194 (talk) 05:00, 4 September 2008 (UTC)

See Nocturnal penile tumescence. Kenjibeast (talk) 05:11, 4 September 2008 (UTC)

Drinking water from a bottle of Clorox

Alright, I'm not going to go into why, but I just want to know if there's any way of making an empty bottle of clorox safe to drink water out of. Would a really thorough wash do, or would there still be some sort of nasty residue? Kenjibeast (talk) 05:09, 4 September 2008 (UTC)

Multiple rinses should do the trick. Its the chemical equivalent of washing a nalgene bottle with bleach. In fact bleach bottles used to be choice container for hauling water while climbing El Cap. furthermore adding a small amount of bleach or iodine to stream water is a method killing many of the microbes that might be living in the stream that you don't want living in your GI track. Chances are you will never get the flavor all the way out but you bottles of water will be less likely to grow bacteria. Good luck.--OMCV (talk) 05:45, 4 September 2008 (UTC)

One recommended method of disinfecting water is to add a small amount of household bleach. For details, you might look at the website of the Environmental Protection Agency of the USA.

http://www.epa.gov/ogwdw000/faq/emerg.html

Based on the advice on this website , it seems that an empty bleach bottle, without any further cleaning, is a safe container in which to store drinking water.

(Scented bleach is not recommended.)

Wanderer57 (talk) 05:40, 4 September 2008 (UTC)

I dunno. There are so many other plastic containers out there, I can't imagine why you would be so dead set on a clorox container to bargain with your health. The EPA website is talking about an EMERGENCY, a situation in which the risk of ingesting harmful parasites and bacteria from your water outweighs the dangers of a mild dose of bleach. Also, given that the container is designed to hold bleach and not potable water, the plastic itself might not be optimal (if you're not a fan of BPA anyway). For general drinking purposes, I'd say just find a different container. --Shaggorama (talk) 05:56, 4 September 2008 (UTC)

Allow me to explain myself. It's for a piece of transgressive/postmodern performance art, the legality of which I will have to do some more looking into. However, the main two concerns I know for sure are 1. This act must be safe. as well as 2. A prop is unacceptable, only a legitimate bottle of clorox or similar bleach will suffice. —Preceding unsigned comment added by Kenjibeast (talk • contribs) 06:44, 4 September 2008 (UTC)

How about putting a small amount of water into a clear plastic bag of about the same size as the container, using the weight of that water to slip the bag through the mouth of the container, then filling the bag partway, and anchoring it around the mouth of the container with a rubber band or glue or something? Then, you could drink from it without having the water touch the container? 10:44, 4 September 2008 (UTC) —Preceding unsigned comment added by Scray (talk • contribs)

I think a lot of this is bordering on and surpassing medical advice. We're telling you something is safe based on highly suspicious reasoning about analogs. We're making assumptions about the plastic that bleach bottles are made out of and about the concentrations of bleach. I would seriously take all answers above with some heavy grains of salt. These people are not qualified to make this sort of judgment and their reasoning is extremely dubious. --98.217.8.46 (talk) 12:44, 4 September 2008 (UTC)

Providing the bottle has been thoroughly cleaned and rinsed (just as you would clean and rinse any container used for drinking water) there will be no problem at all. You will need a way of ensuring that only the correct bottle is used in the performance, as a mix-up could have undesirable effects. DuncanHill (talk) 12:49, 4 September 2008 (UTC)

Bleach containers were not designed for holding drinkable beverages. Without checking exactly what the plastic is I'm not sure we can say it's the same as "any contained used for drinking water." I know that in the cases of other chemicals a variety of speciality rinses are used before a given container can be considered to be free of the chemical (in the case of chemical transportation, for example). Again, I would be wary of off-the-cuff advice and analogies with familiar territory unless they are strongly backed up by experience or evidence. --98.217.8.46 (talk) 13:06, 4 September 2008 (UTC)

Household bleach is widely used in the cleaning of food and drink containers and preparation surfaces, and it is inevitable that traces of it remain on those surfaces. The traces which would remain in a thoroughly cleaned and rinsed (I am assuming the questioner has access to ample supplies of potable running water here) would be likely to be insignificant. The container (which is one designed and manufactured for household use by persons having no special training or equipment) is to be used in a performance, not for the long-term storage of water, so the likelihood of harmful quantities of any substances leaching out of the plastic is extremely low. This is not medical advice by any stretch of the imagination, but the OP is welcome to take it as the basis of a risk assessment from someone who has to do loads of the things. The principal risk associated with the activity as far as I can see from the information presented is that the stunt bottle could be confused with other similar bottles in the performance venue, and the props mistress will need to provide adequate controls to prevent this happening. DuncanHill (talk) 13:14, 4 September 2008 (UTC)

Also of interest is the MSDS for Clorox bleach. It provides the concentration of the bleach, the exposure limits, and the appropriate response to ingesting pure bleach. Combined with other reliable information, like the EPA reference above, the OP should be able to reasonably assess the safety of the intended performance. -- Coneslayer (talk) 13:45, 4 September 2008 (UTC)

I just checked the bottom of a Chlorox bottle to see what type of plastic it is. It was labeled with recycling number #2 and HDPE both of which indicate high density polyethylene. In terms of the bottle's plastic it should be no more dangerous than drinking out of a milk bottle and will contain fewer hormones (from the milk). The chemical in bleach are fully water soluble and thus require no special solvent rinses. Besides the chemicals are only toxic at high concentrations (similar to most acids and bases) so the small amount of oxidant left in the plastic to leach into the stored water should do no harm.--OMCV (talk) 13:53, 4 September 2008 (UTC)

Bleaches come in different makes, so I think the plastic bottles can differ as well. – b_jonas 22:16, 6 September 2008 (UTC)

epilepsy / impact on brain

hi! i heard that not only alcohol can damage brain cells but also other abnormal conditions (like orgasms(sic!) and epileptic situations). i further read that psychiatric electro shock (it creates a grand mal situation artifically) not only causes no structural changes in the brain[3] but even lets new brain cells grow[4]... can somebody solve that contradiction for me? thx. ps: there they suggested to ask here... bye. --Homer Landskirty (talk) 05:23, 4 September 2008 (UTC)

Let's hope orgasms aren't an abnormal state of the brain :). The only orgasms that cause brain damage are those associated with erotic asphyxiation, otherwise if someone's had a natural orgasm so strong it caused them brain damage....well, I mean... damn. Way to go! Ask them how they did it and spread the knowledge. As for your comment on EST, if a therapy stimulates new brain cells to grow, that IS a structural change to the brain because new connections (i.e. pathways) are being forged. It's not my understanding that seizures cause brain damage outside the risk of the individual hitting their head or obstructing their airway during the seizure. --Shaggorama (talk) 05:50, 4 September 2008 (UTC)

:-) hm... ok... "structural change" was most likely meant as "structural damage" and they were talking of CT and MRI scans which don't give detailed information on neuron-level... i was always bad in biology, but the high electrical energy in the brain during a seizure seems to be a potential cause for electrolysis (when i held a 9V direct current source into a glass of salt water, it changed its color, although salt and water r quite simple compounds...) and thereby necrosis... but i don't insist that it must be like that... --Homer Landskirty (talk) 06:42, 4 September 2008 (UTC)

My understanding is that prolonged seizure, i.e. status epilepticus, causes brain damage metabolically (through accumulation or depletion of noxious or essential substances, respectively). The electrical energy generated is not extreme and I am pretty sure much lower than that needed to damage human tissue. Scray (talk) 10:35, 4 September 2008 (UTC)

As the amount of elecctrical activity in the brain increases, the voltage present in the brain stays the same. Action potentials are all-or-nothing phenomena. --Shaggorama (talk) 15:30, 4 September 2008 (UTC)

twin universe hypothesis

i have a hypothesis which our spacetime is double sided, while a Big Bang and Big Crunch singularities will be avoided, because my hypothesis is that while one side is empty, the other side is full, and therefore, matter will flow to the other side. which is the Big Bang. And like the inflation theory, the stuff flowing through will accelerate, and in later stages, will start to decelerate. at this time, the other side is shrinking, and this is my Big Crunch. Please give comments. —Preceding unsigned comment added by Superwj5 (talk • contribs) 13:05, 4 September 2008 (UTC)

The reference desk is not a discussion forum, and entries to that effect may be removed. Is there a question here? — Lomn 13:17, 4 September 2008 (UTC)

As Lomn say, this isn't really the right place to ask for comments on an original theory. Even despite that, your description is too vague to make any meaningful comments on. --Tango (talk) 13:24, 4 September 2008 (UTC)

It's very, very easy for anyone to come up with random hypotheses like these - but unless they explain absolutely all of known physics AND explain some unexplained phenomena AND predict some testable thing - they are just useless. Unless your hypothesis is grounded in and backed by an IMMENSE amount of physics, it's about as interesting as your next door neighbors holiday photos. SteveBaker (talk) 19:10, 4 September 2008 (UTC)

"explain absolutely all of known physics" is too high a bar - I think that typically, science settles for "does not contradict any existing known physics and either explains a piece of physics that was previously unexplained or is simpler than other available explanations for what it covers" --Random832 (contribs) 19:20, 4 September 2008 (UTC)

Well, it needs to explain at least as much as the theory it is replacing. It can contradict existing theories, but only if it can explain all the same observations. --Tango (talk) 22:01, 4 September 2008 (UTC)

Yes - exactly. Perhaps I should say that it has to provide convincing explanations for all of the currently observed phenomena for which we already have convincing explanations. Doing that alone would be a pretty amazing thing for a radical new theory of the universe - but the bar to replace current theory has to be that it provides something new - preferably an experiment that could be done that would produce a result that the present state of science would not predict. Perhaps explaining something that's not currently understood - or (I guess) being vastly simpler than existing explanations. But making a testable prediction that turns out to be true is the gold standard.

So, for example, nobody took much notice of Einstein's special relativity until he predicted that the sun's gravity would bend the light of a distant star and that this should be visible in a total solar eclipse. When that experiment was eventually carried out (an interesting story in itself) - and it produced the result that Einstein predicted - the world of science was overturned pretty much overnight. So the theory itself (good though it was) didn't attract much interest until it predicted something amazing.

To produce a counter-example, string theory does a good job of providing an explanation for everything we currently know - but all of the predictions it makes have proven impossible to test experimentally. Some claim that the Large Hadron Collider (LHC) will provide some sort of proof - but the jury is still out on that one. Hence we don't usually treat string theory as "The Truth" in the same way we treat relativity.

SteveBaker (talk) 22:43, 4 September 2008 (UTC)

White hole transformation

today, i read that if the Higgs Mechanism or Superstring Theory is correct, then above a magic energy, matter will become massless. Suddenly, I was reminded of that massless black holes become white holes. I then thought when a black hole mass becomes very small, the Hawking Radiation will set it above the magic energy which it will become a white hole. After some time, the energy will decrease below the magic energy. and in supermassive black holes, the accretion disk will also pull the black hole over the magic energy, and the same thing will happen, except that the accretion disk flies away.Please give comments. —Preceding unsigned comment added by Superwj5 (talk • contribs) 13:13, 4 September 2008 (UTC)

There's a lot of unsupported guesswork in this. Can you cite "magic energy" or this rather unusual definition of a "white hole"? Otherwise there's nothing to this. — Lomn 13:16, 4 September 2008 (UTC)

Massless black holes don't become white holes. White holes have mass just like black holes. Also, white holes don't repel matter, they attract it just like black holes. The classical distinction was that white holes emitted matter and shrank, while black holes absorbed matter and grew; but since Hawking noticed that quantum black holes seem to do both, it's no longer clear what the difference between a black hole and a white hole is.

I don't know much about the Higgs mechanism, but I think it's not governed by a magic energy so much as a magic temperature. It's a phase transition; we're in a "frozen" state of the Higgs field right now, and above a certain temperature it "melts" again. Tiny black holes have very high Hawking temperatures, but I don't think you'd notice weird Higgs effects just because of that. Small sizes and large temperatures are related, and there's probably something weird happening with the Higgs field (and all other fields) at close enough distances to any pointlike particle, like an electron, but we don't see those effects at ordinary distance scales. -- BenRG (talk) 15:08, 4 September 2008 (UTC)

Name of interesting microorganism

I took a really great parasitology course in undergrad but i can't remember for the life of me the name of one of the organisms we went over. I remember the professor was talking about opportunistic infections and fungal pneumonia, and he then proceeded to mention another creature that he said was very distinct in that it was comparable in its morphology to (i think?) a stinging cell on a jellyfish or something along those lines. Does this sound familiar to anyone? Thank you so much!

129.252.70.53 (talk) 13:14, 4 September 2008 (UTC) Timmy

The commonest fungal pneumonias are aspergillosis, coccidioidomycosis, histoplasmosis and blastomycosis. From your description, it sounds like aspergillosis is the most likely. Axl (talk) 19:16, 4 September 2008 (UTC)

I think it possible you're thinking of Pneumocystis jiroveci, previously called Pneumocystis carinii, first thought to be a protozoan but now known to be a yeast-like fungus. On the other hands, stinging cells in jellyfish are "nematocysts", and if you were discussing animal diseases rather than human ones, the most likely candidate is Myxobolus cerebralis, which infects fish by piercing them with polar filaments ejected from nematocyst-like capsules. - Nunh-huh 06:25, 5 September 2008 (UTC)

Human breasts-a store for fat?

How large do breasts have to be to produce the required amount of milk? And if they larger than that minimum, then why? Are they also used as a store for fat? —Preceding unsigned comment added by TrapdoorTrevor (talk • contribs) 13:34, 4 September 2008 (UTC)

Breasts must produce approximately 8 ounces of milk every 3-4 hours during breastfeeding. That is pretty much a single glass of milk. Spread among the two breasts, the required size is noticeable, but not great. It is common for breasts to enlarge beyond the required amount. As for fat, breasts are mostly a store of fat. Men have mammory glands as well, but do not store fat in the breasts as usual as women do. Men usually only store fat in the breasts when fat stores throughout the body are already very full. -- kainaw™ 14:35, 4 September 2008 (UTC)

Sociopaths

Are most sociopaths males? If so, what is the reason for this? --Anilmanohar (talk) 19:02, 4 September 2008 (UTC)

Probably yes. This research thesis gives answers to your second question. Axl ¤ [Talk] 19:50, 4 September 2008 (UTC)

How do you get 'probably yes' out of that abstract? - Lambajan 02:37, 8 September 2008 (UTC)

It's not in the abstract. It's from page 15 of the thesis: "Hare (2003) recommended that a cut score of 30 (out of 40) be used in classifying individuals as psychopaths, and this cut score has been used in research on psychopathy for years. As compared to the 25 to 30% typically found in male offender samples, research indicates that females are significantly less likely than males to be classified as psychopaths when this cut score is utilized. For example, Salekin et al. (1997) found only 16% of 103 female jail inmates met this cut score. Similarly, Warren et al. (2003) reported only 17.4% of 138 female prison inmates met the recommended cut score. Several possible explanations exist for this discrepancy, including an actual gender difference in the prevalence rate or possible biases present in the assessment measure for psychopathy." Axl ¤ [Talk] 18:03, 8 September 2008 (UTC)

That's interesting, thank you. Sorry if I was too harsh. - Lambajan 04:38, 9 September 2008 (UTC)

Not at all. :-) Axl ¤ [Talk] 07:24, 9 September 2008 (UTC)

growing biodiesel in your back garden

I drive a diesel car which does about 40 miles to the gallon, about 100 miles a week - so I need 2.5 gallons of diesel every week. Can you grow that in your back garden? Judging by Jatropha oil, that means you would need one quarter of a hectare in your back garden to run my car completely on jatropha. That is if "A hectare of jatropha produces 1,892 litres of fuel" means per year.

Can anyone tell me if I'm right or not? Is it practical to grow jatropha in your garden?89.240.139.65 (talk) 19:37, 4 September 2008 (UTC)

How long is the growth cycle for jatropha? Assuming you can get only one crop a year, you'd have to produce all the oil you need for the following year at once. So you'd need storage. How is jatropha oil recovered? I know with some other veg oils there significant processing, beyond just the extraction/expellation, to remove impurities, moisture, etc. You might be able to skip these, but excess moisture might lower your yield, and impurities might gum up your engine. I think you could do it for personal satisfaction, but it would be a lot of trouble and not necessarily benefit the environement (if that is your goal). ike9898 (talk) 19:45, 4 September 2008 (UTC)

The storage problem isn't a huge one... he needs only 130 gallons/year. I have 2×275 gallon oil tanks in my basement for heating oil, which is a pretty common setup around here. -- Coneslayer (talk) 20:05, 4 September 2008 (UTC)

You would also need to grow something else like corn to make ethanol or buy methanol. Regardless you need an alcohol to transesterification your bio-oils and produce biodiesel. Next you need means to heat/cool/electrify your home. Then after that we need to do that for our work places. Neat stuff to think about.--OMCV (talk) 20:22, 4 September 2008 (UTC)

I seem to recall hearing that your need a federal permit to grow Jatropha in the United States as it is potentially invasive non-native plant. No idea on whether that is easy to get or hard. You might do better with a locally available alternative of some kind. Dragons flight (talk) 21:31, 4 September 2008 (UTC)

Thanks for the warning about permits - I'm English, so I'll get that checked out with somebody. And I'm sure something like an oil tank could keep the rest of it. Anyone know how long it takes the stuff to grow, if it is one year or not?78.144.169.2 (talk) 22:33, 4 September 2008 (UTC)

Our article Jatropha curcas says "While Jatropha curcas starts yielding from 9–12 months time, the effective yield is obtained only after 2 - 3 years time". yield is also affected by planting pattern, and of course you want a preponderance of female plants. DuncanHill (talk) 22:36, 4 September 2008 (UTC)

Hmmm - interesting! I have 1.2 acres (which is about half a hectare) at the back of my house - and my car also does ~40mpg and I run it 100 miles a week...this sounds rather interesting! I guess the other considerations we need to answer are:

• What amount of water and fertilizer does this stuff need to grow year after year on the same land?
• Would you need to do "crop rotation" and thereby have some years in which you'd get no biodiesel because you'd have to plant nitrogen-fixing plants instead?
• What equipment would you need in order to prepare the soil, plant, weed, fertilize and harvest the stuff? Can you rent it? Borrow it? Share it with neighbors who have the same idea as you?
• How much fuel do those machines require in a typical year?
• What "economies of scale" would you fail to achieve by growing so little of the stuff? Are the numbers you're getting based on planting hundreds of acres?
• Does it grow well in your local climate and soil conditions?
• What pests attack your crop and how can you deal with them?
• What waste products remain after extracting the biofuel (eg bits of the plant that aren't used in the production - left over chemicals - that kind of thing)?
• 130 gallons of Diesel costs maybe $650 in the USA...What is the "opportunity cost"? In other words, what other things could you do with that land and whatever capital you have tied up in equipment that might earn you more than $650?
• How much value to you place on your own labor? Seems like you might spend a LOT of hours ploughing/planting/fertilizing/killing insects/weeding/watering/harvesting/brewing - are you committing yourself to manual labor at less than minimum wage? If so, maybe taking a second job for a couple of weeks a year would be a more profitable use of your time?

Lots to think about...but at first sight, it doesn't sound so attractive. The lack of economy of scale is the problem...if you were planting hundreds of acres then the cost of the equipment and perhaps much of your time/effort would be amortized over a larger amount of product...but at this small scale, you can't afford to buy even a small tractor because it would be idle 50 weeks of the year and wipe out your potential savings for MANY years. Without a tractor, ploughing, planting and harvesting 1.2 acres sounds like back-breaking work for at least several weeks a year.

SteveBaker (talk) 23:18, 4 September 2008 (UTC)

10 inches of rain a year, grows well on poor soils, needs little if any feeding, life expectancy of 40 years so not a lot of ploughing or planting to do, resistant to disease and pests. It sounds, from our article, to be a remarkably easy plant to grow. That said, lots of good points from Steve. As usual! DuncanHill (talk) 23:23, 4 September 2008 (UTC)

Why even grow your oil? Everyone I know who operates converted biodeisel cars get their grease from restaurants. They actually get PAID for their fuel because their fuel source doubles as a waste disposal! And they get to feel good because they're recycling used cooking oil. Just another option to consider. --Shaggorama (talk) 04:36, 5 September 2008 (UTC)

It's OK to a point - but a typical small town can't support more than a few dozen cars that way...there just aren't enough deep fryers being emptied often enough. It's OK as an "I'm alright" solution for an individual - but it's not the way forward for mankind in general. Also, I suspect it's only a matter of time until the McDonald's and the Wendy's of this world start to realize that they could actually charge for this stuff. Right now, they don't because they usually have to pay to have it hauled away - and if someone will take it for nothing - it saves them money. But once there are two or three people hanging around begging them for their left-over oil every day, they may realize that they have a valuable commodity and start selling it. What I suspect MIGHT happen is for them to install the equipment necessary to filter and dry the oil on-site and start filling the tanks of their delivery trucks with the stuff - it would be a GREAT PR thing ("We're going green!") and the whole business of vehicles that burn the stuff smelling like fries could be put to considerable advertising advantage. So while this is a great gimmick - it's hardly going to save the planet and it's only a matter of time until that particular well runs dry for the average motorist. SteveBaker (talk) 05:47, 5 September 2008 (UTC)

OK everybody, thanks for your answers to my questions. You need a big garden to run a car on your own jatropha and at the end of the day, it isn't really practical on a domestic scale for non-green warriors because of processing. If someone was paying you for the right to harvest your garden and process the stuff, it would be great, and then you could buy refined jatropha back from them. Then whoever harvests your garden needs LOTS of gardens to do, and you need to persuade people to give up their lovely gardens as well. Alternative fuels is a fringe business (here at least). In my experience, that means don't jump for it right away, use it in blends. And yes, you can get oil from restaurants, but every little helps (pardon my tesco advert). Imagine the effect if everyone kept just a few jatropha plants, and that would avoid the controversy that the biofuel industry takes precious land from food cultivation.78.149.56.198 (talk) 22:39, 5 September 2008 (UTC)

On another tack, have you thought of approaching your local takeaways offering to remove their cooking oil to recycle as diesel? In my country this perk was proscribed by government bodies only if it were being sold on to others, but okay for personal processing and use. Julia Rossi (talk) 22:35, 11 September 2008 (UTC) Oops, Shaggorama got it. Julia Rossi (talk) 22:38, 11 September 2008 (UTC)

How do a student weight the moon

I'm thinking about how to weight the moon using nothing that a teenager student cannot get hold of. 202.147.44.87 (talk) 23:15, 4 September 2008 (UTC)

Let's assume that you already know the mass of the Earth. From that, Newton's Law of Universal Gravitation, and the equation for uniform circular motion (I'm making the simplifications that (1) the moon's orbit is circular, and (2) the Moon orbits the Earth, rather than both orbiting a common centre of mass), you should be able to get an equation relating the mass, orbital velocity and orbital radius of a satellite. Finding the orbital radius of the moon is a little tricky, but you could probably work it out via a couple of sextants and a parallax argument, and then the sextants will also help you work out its orbital velocity. Not the easiest of projects, but not impossible, either. Confusing Manifestation(Say hi!) 23:48, 4 September 2008 (UTC)

Well, if you knew how much the gravity of the moon affected things on Earth, you'd be able to calculate the mass (not, technically, the "weight"). At first sight, I think you'd need to look at tides - or perhaps the amount the earth wobbles each lunar month because the earth/moon system orbits around a common center that isn't at the center of the earth.

• If the Earth and the Moon weighed the same amount - we'd both be orbiting around a point that would be halfway between the two. But the earth is heavier - so the point about which we both orbit is closer to the center of the earth than the center of the moon. Hence the earth doesn't just rotate on it's axis once a day - it also orbits around that common center once every lunar month. There are probably things you could measure with a telescope that would tell you that...but that's going to require a really accurate telescope - which doesn't sound like something you'd obviously have. This document [5] describes the math - but doesn't explain how you measure the numbers you'd have to plug into the math.
• Measuring the moon's gravity using the effect it has on the tides. You have to be very careful to control for the effect of the Sun's gravity - but in theory, if you know the height of the tide - and you know how far away the moon is, how heavy the earth is...you can figure out the gravity due to the moon - and hence it's mass. The math is in Theory of tides - and it's kinda horrible.
• You'd think (at first sight) that your weight would be less when the moon is directly overhead than when it's beneath your feet - because when it's overhead, it's pulling you upwards and reducing your weight. But sadly, the centrifugal force due to our orbiting around that common center handily cancels that out - which is why we get two high tides each day - one when the moon is overhead and another when it's underfoot. However, you should find a difference between when the moon is directly overhead and when it's at the horizon. It's that difference that accounts for the tides - but perhaps this is a more direct way to measure it - if you have an accurate enough measuring device. Remember, you can't use a beam-balance to do it because the weight of your standard weights is also changing. You'd need a spring balance - or some kind of electronic pressure gauge.

I sure hope there is an easier answer!

SteveBaker (talk) 23:49, 4 September 2008 (UTC)

For what it's worth, Isaac Asimov's book The Double Planet has a chapter on how the moon was weighed. That might be available in used book stores. — Lomn 13:06, 5 September 2008 (UTC)

How about this method:

• Measure the orbital velocity, ω, of the Moon. Its orbital period is about 28 days, so ... let's see ... that's about 2.6x10^-6 radians per second.
• Measure the acceleration due to gravity at the surface of the Earth, g. That's about 10 ms^-2.
• Estimate the radius of the Earth, r_E, using Eratosthenes' method. Let's say that's 6.3x10⁶ m.
• We know that gravitational attraction decreases with the square of distance, so if the Moon is a distance d_M from the centre of the Earth we must have:

$d_{M}\omega ^{2}={\frac {gr_{E}^{2}}{d_{M}^{2}}}\,\Rightarrow \,d_{M}^{3}={\frac {gr_{E}^{2}}{\omega ^{2}}}$

• Plug and chug and we get d_M = 3.9x10⁸ m.
• Measure the angular diameter of the full Moon - that's about half a degree.
• If the Moon subtends half a degree at a distance of 3.9x10⁸ m (neglecting a small correction for the radius of the Earth) then its diameter is 3.9x10⁸ x π / 360 = 3.4x10⁶ m, and so its volume is about 2.1x10¹⁹ m³.
• Assume that the Moon has a similar make-up to the Earth. Measure the density of an average lump of rock, and round this up a bit to allow for higher density towards the centre of the Moon. Let's take an average density of 3,000 kg m^-3. That gives us a mass for the Moon of about 6x10²² kg.

Estimating the radius of the Earth from first principles might stretch a typical teenager, but the rest of the measurements are quite straightforward. Gandalf61 (talk) 13:14, 5 September 2008 (UTC)

You were doing great right up to the point where you assumed the density of the moon. There is no reason to assume that moon rocks have the same kind of density as earth rocks or that the density profile is anywhere near similar to what we know for the earth. The earth has a liquid nickel/iron core - the moon almost certainly doesn't. (And if we cheat and look it up - we find that the earth is nearly twice as dense as the moon...but that's cheating because we're not doing an experiment). The guesstimate you came up with happens to be within 10% of the true number - but that's just luck. If you'd used the same guess to estimate the density of the earth - you'd have been off by a factor of around 2. The problem with weighing the moon is precisely that we DON'T a-priori know it's density. We have to figure it's mass some other way (tides - for example) and use that with our size measurement to deduce a density. SteveBaker (talk) 15:59, 5 September 2008 (UTC)

Yes, the tidal method avoids making assumptions about the make-up of the Moon. However, it only gives you the ratio of the Moon's mass to the Earth's mass. To derive the Moon's mass you need to know the Earth's mass, or equivalently its average density. Asking a student to measure the Earth's mass or density to an accuracy of better than a factor of 2 is quite a tough challenge - they would need to perform a backyard version of the Cavendish experiment. Gandalf61 (talk) 09:34, 6 September 2008 (UTC)

Why is measuring the mass of the earth so tough? We can easily measure g to high precision (stopwatch, rock, tall building) - so we can figure the mass from Newton's law of Gravitation - all we need to know for that is the radius of the Earth. That's obtainable by Eratosthenes method. (And your proposed approach requires us to do that too!)...you can get pretty good precision with Eratosthenes using two tall buildings, two students in different cities with cellphones and tape measures. SteveBaker (talk) 13:37, 6 September 2008 (UTC)

Well, yes, it's easy to determine GM for the Earth by various means. The hard part is determining G at all accurately. If you just give that to the students, then you've done most of the work for them. Algebraist 14:04, 6 September 2008 (UTC)

Indeed. Measuring g and r_E only gives you the product GM_E. It wasn't until the 18th century that G or M_E (or, equivalently, the average density of the Earth) could be measured separately with any degree of accuracy. In 1774 Nevil Maskelyne measured the gravitational attraction of Schiehallion and estimated the average density of the Earth to be 4.5 g cm^-3 - and this took seven weeks of detailed surveying [6]. In 1797/8 Cavendish used John Michell's torsion balance method to get a more accurate value of 5.45 g cm^-3, but this required making very precise measurements. Gandalf61 (talk) 14:54, 6 September 2008 (UTC)

Portion of agriculturally usable land on the surface of the globe

It is said that 29% of the earth's surface is covered by the land mass. What portion of the total surface is usable for agriculture for growing food? 86.33.247.155 (talk) 23:30, 4 September 2008 (UTC)

Agriculture says "Livestock production occupies 70% of all land used for agriculture, or 30% of the land surface of the planet." - so, if I read that right, livestock agriculture covers 30% of all land - and that's 70% of agricultural land. Overall then, there must be more like 43% of all land under agriculture...which is maybe 11% or so of the earth's surface. That number seems high to me - but that's what the article says. It references a UN report - but the link has gone bad - so I can't see the original source. SteveBaker (talk) 00:05, 5 September 2008 (UTC)

That quote says "used", the OP says "usable". There is presumably some unused but usable land (although perhaps not much). --Tango (talk) 00:27, 5 September 2008 (UTC)

Yep. I think it's pretty debatable as to what's "usable" - I mean we COULD erect giant pressurized, heated greenhouses over the summit of Mt Everest - bring in a few feet of dirt and plant stuff right on the top - it's technologically quite possible - but you wouldn't normally say that the summit of Everest was "usable agricultural land". So where do you place your limits? Usable "with what degree of difficulty" is the real question. When you see how (for example) the Dutch reclaimed ocean-bottom with relatively crude windmill pumps - and now they intensively farm it. That's arguably a lot harder than farming Mt Everest so it would be rash to disallow any region on the grounds of technological difficulties - and the way people are steadily mowing down the Amazon rainforest for crops - you can't even ask what we sensibly should farm. We even do agriculture out in the oceans with fish farms - so it's impossible to know what limits to place on such a question. But 11% is evidently what we actually "do" farm. If we're asking what could 'easily' be farmed or 'easily and sensibly and legally' - then I suspect we're already beyond that limit. SteveBaker (talk) 05:06, 5 September 2008 (UTC)