Wikipedia:Reference desk/Archives/Science/2011 March 18

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March 18[edit]

Breast milk and iron[edit]

According to this site: [1], human breast milk is almost devoid of iron, and many other vital nutrients. So then, how do babies which are exclusively breast fed get their iron, and those other nutrients ? StuRat (talk) 00:40, 18 March 2011 (UTC)

They store it before they are born. Especially non-c section babies get filled with extra blood from the placenta by the contractions (sometimes causing jaundice when the extra blood is broken down). The iron in blood is almost completely recycled, and the store of iron lasts them till they start eating solids. Ariel. (talk) 00:44, 18 March 2011 (UTC)
Where is this "excess blood" stored ? Even with perfect recycling, they would need more iron to account for their growth. And, if babies can recycle iron so well, why can't adults (especially men, most of whom don't menstruate :-) )? Why do adults need dietary iron ? StuRat (talk) 03:32, 18 March 2011 (UTC)
(edit conflict)Two things:
  1. When reality seems to be in conflict with "experts", trust reality. In this case, since humans for millions of years fed their infants breast milk, and yet we as a species survived; and since today, still, the majority of humans still feed their infants breast milk, and those babies keep growing into healthy adults, there must be something right about breast milk. Regardless of what the "label" says, or the list of ingredients or anything else, its not that breast milk is deficient, it must be that what you are presupposing to be necessary OR what you are presupposing to be actually in breast milk, per that label, is really there, that is what is deficient. Breast milk is not deficient, obviously, so we need to figure out what is going on here.
  2. Read the label again. The % of iron is the % of the daily requirement an adult eating a balanced 2000 calorie diet would get out of taking in 172 of those calories in breast milk. Instead what you have is an infant, who is eating much less (say 500 calories or less), and getting all 500 of those calories from the breast milk. In that case, the infant may be getting 100% of their daily requirment for iron, since a) their iron requirements are likely different b) American food labels are rounded somewhat, so what might be, say 0.44% for an adult would still show up as zero. That doesn't mean there is no iron, just a small amount when compared to an adult requirement, but likely suffient for what an infant needs. And that is all dependent on the fact that you can trust what that website is telling you...
That's how I read the situation. (post EC extra stuff) what Ariel said makes sense regarding how infants work out their iron situation. Makes sense. I knew it had to work out, because of point 1.--Jayron32 00:54, 18 March 2011 (UTC)
To your point 1, if a baby has only 1/4 the calorie requirements, and also only 1/4th the iron requirements, then 0.44% of an adult RDA would become 1.76% of a baby's RDA, still grossly insufficient. And I suspect that more iron is needed, proportionally, when growing, than as an adult. StuRat (talk) 03:28, 18 March 2011 (UTC)
Apparently not; again your assumption that small babies need, proportionally, the same amount of iron as adults is not borne out by the fact that breast fed babies aren't all dead, and are perfectly healthy, even with regards for the kinds of reasons that, you, as an adult, need iron. Breast milk is empirically not deficient, so either a) that "nutritional facts" label is wrong or b) babies need a lot less iron in their diets than adults. Ariel, above, provided ample evidence that the real answer is b, though I have my suspicions that there may be some option a at play as well. Again, you can't start with the supposition that breast milk is deficient when there's ample empirical evidence (nearly all of humanity for all of history; indeed even more than that, we have nearly all of mammalia for even longer). The experiment has been run trillions upon trillions of times and milk works. So we have to rephrase the question: What in your supposition is wrong, or what is it about how babies need iron that is different than adults? Ariel answered that succinctly: they don't need dietary iron during their first several months because of the massive iron boost they get during normal birth. --Jayron32 03:50, 18 March 2011 (UTC)
It's patently obvious that it has to work out somehow, so stating that over and over doesn't help much. I want to know HOW it works out. The only assumption I made in my initial post is that the nutrition site I listed has correct info. If you have another site which disputes that info, then that would be useful. If babies are born with "excess blood", I'd like to see a site which supports and describes that. Also, I believe some cultures have babies who are exclusively breast fed for years, so how can the iron last that long ? StuRat (talk) 07:11, 18 March 2011 (UTC)
The extra blood is stored as rather red-looking babies (at first, till the excess red blood cells are digested in the spleen). Iron is also be stored in the liver and other places from before they are born. Normally elements diffuse from a higher concentration to a lower across the placenta, but iron is actually pulled against the gradient if necessary (potentially leaving the mother anemic). This iron is then stored. The primary loss of iron is mucousal irritation of the bowels (red blood cells are recycled basically perfectly in the spleen), but that doesn't happen to breastfed babies, so their need is lower. That's one of the reasons their feces have such a strange (light) color. This page says more or less the same things: http://www.kellymom.com/nutrition/vitamins/iron.html It also mentions that breastmilk is not as deficient as it might appear - the iron is in a form that is extremely well absorbed. Normally an adult will need 20-100 times as much iron in their food as they actually need since it's so poorly absorbed, but in breast milk you only need twice as much. The net result is that breastmilk effectively has 10 (or more) times as much iron as it might appear. Ariel. (talk) 11:53, 18 March 2011 (UTC)
Thanks. It looks like human breast-milk is still deficient in iron, even after the better absorption is considered, since babies do gradually use up their excess stores. Do children in societies that breast-feed longer suffer from anemia ? Has anyone attempted to create iron supplements in the more absorbable form, similar to breast-milk ? Do any other mammals create such milk ? StuRat (talk) 17:44, 18 March 2011 (UTC)
Yes, they do eventually run out, but by then they are eating solids. I don't think even in those societies that it is exclusive, but rather it's a supplement. Presumably they would get iron from the other foods they eat. It looks like Iron Glycinate (or any iron attached to an amino acid) is best absorbed. It's also important to eat the iron together with vitamin-c. Ariel. (talk) 18:27, 18 March 2011 (UTC)
There's an advert for a toddler follow-on milk on British TV which goes on about how little iron breast-milk has and how much their product has, conveniently ignoring the fact that toddlers will (or should) be eating a varied diet including several sources of iron, and not relying only or mainly on milk. DuncanHill (talk) 12:08, 18 March 2011 (UTC)

Thanks, everyone. I'm glad I can trust the nutritional info on human breast milk I've found, as I've never been able to find a nutrition label directly on the natural package. :-) StuRat (talk) 02:48, 22 March 2011 (UTC)

How the following time dilation senario in three frames of reference can be explained?[edit]

Let we have A and B transparent spaceships such that B is inside back of A as the size of A is greater than B. Also there is an observer at the middle of A who is stationary for B.

Now both spaceships ignited and set out at same time (with speed close to the speed of light) for space journey. The front of A is open so that B may leave A if want. The reading of speedometer of B may or may not be the same with that of A. Light clocks installed in each ship with top and bottom mirrors,  is also starts automatically with ignition.  

We have following three reference frames

1-  within B (with light clock LC2)

2- within A (with light clock LC1 and moving spaceship B in which LC2 is installed)

3- for outside observer who is not co moving on asteroid

Now

1- LC2 is making time dilation triangle for (a) for observer at the middle of A who is stationary for B (b) for outside observer who is on asteroid.

2- LC1 is making for (a) for outside observer on asteroid (b) may be for onboard observer B.74.198.150.213 (talk) 02:48, 18 March 2011 (UTC)Eccentric Khattak#1

I think I understand the setup, but what is the question? You only asked "how can it be explained?", which is not specific enough. -- BenRG (talk) 03:25, 18 March 2011 (UTC)
Then you understand more than me. The explanation above about the setup is quite confused in my opinion. Dauto (talk) 03:48, 18 March 2011 (UTC)
I don't understand what the point is of making the spaceships transparent or allowing one to enter the other. Couldn't they simply be normal spaceships that pass by one another? More to the point, can't you just say that A is the surface of the Earth and only look at B (i.e. usual twin paradox?) But I don't understand the acceleration that A and B might undergo. Wnt (talk) 05:34, 18 March 2011 (UTC)
I don't know what it is that you want to know or you have trouble understanding, but maybe you should have a look at Bell's spaceship paradox. If you are asking whether 2 spaceships, initially at rest, and starting at the same time with the same acceleration, are moving relative to each other while they are accelerating, then the answer is yes (specifically, in the reference frame of an observer at the position of one spaceship with the velocity of this spaceship at any point in time while they are accelerating, as we only look at non-accelerating reference frames). Icek (talk) 08:07, 18 March 2011 (UTC)

I want to make it a little more clear if all above is incomprehensible. Let Spaceship B is at rest  in A while  A is moving with speed close to the speed of light. When A is about to pass by an asteroid,  B take off in A in the same direction. Thus an observer at middle of A will see a ship B is moving with light clock LC2 in his frame of reference. An outside observer will see B is moving in moving reference frame  (with light clock LC1)  of A. The direction of both A and B is same. Thus there are three reference frames

1- Within B (with LC2) 2- Within A (with LC1 and moving ship B) 3- For outside observer (with moving A inside which B is moving).

My question is how the aforementioned clocks (two time dilation triangle) can be analyzed w,r.t both onboard observers, observer at the middle of ship A and outside on asteroid. 

I don't know how to post a question with diagram therefore it's a little confusing but I hope I have explained things clearly enough for you all  to understand74.198.150.213 (talk)Eccentric Khattak#1 —Preceding undated comment added 23:09, 18 March 2011 (UTC).

Is spaceship A accelerating? Anyway, you can always analyze the situation by applying the Lorentz transformation to the time and space coordinates (and if you are specifically asking what the velocity of B is with respect to the asteroid if you only know the velocity of A with respect to the asteroid and the velocity of B with respect to A then you can use the velocity-addition formula). Icek (talk) 12:37, 19 March 2011 (UTC)
It makes no different whether one spaceship is inside the other. The analysis is the same as if they were side by side. Does that help? -- BenRG (talk) 20:31, 19 March 2011 (UTC)

No A is not accelerating. The speed of B may be  1- less than A  2- equal to A 3- greater than A

An observer on asteroid will see his local clock is correct as compared to the moving one (LC1of A) that is slow.

Similarly an observer at the middle of A will see his clock is correct as compared to that of the moving one (LC2 of B) that is slow.

So for observer on asteroid would LC2 = LC1 or LC2 slower than LC1 or vice versa OR..... 

Thus, how the following  clocks can be described in terms of slower and faster w.r.t each other while keeping in mind the aforesaid different speed of B.

  1- of an outside observer on asteroid  2-  of LC1 of A 3- of LC2 of B

74.198.150.213 (talk) 01:03, 21 March 2011 (UTC)Eccentric Khattak#1

For quantitative details, see Special relativity#Time dilation and length contraction and Time dilation. But qualitatively, the rule is that "moving clocks run slow," and the faster a clock moves, the slower it runs. So the clocks on both rockets A and B will be observed to be running slow as observed in the asteroid's frame of reference, with the clock on the fastest rocket (as measured in the asteroid's frame of reference) being observed to be running the slowest. If A and B are traveling at the same velocity, then their clocks will agree with each other.
Of course, there's nothing special about the asteroid's frame of reference, just because it happens to be the largest object of the three objects whose comoving frames are of interest (assuming the asteroid isn't so large that its gravity starts becoming significant to the problem). So for example, in A's frame of reference, a clock on the asteroid will be observed to be running slow, and A will observe B's clock to be running slow as long as B is moving relative to A. And which of the asteroid's clock and B's clock is observed by A to be running the slowest depends on which of the asteroid or rocket B is moving at the highest speed, as measured in A's comoving frame. Red Act (talk) 12:37, 21 March 2011 (UTC)

Three more simple questions

1- For outside observer: would B be touching/ hitting the end/ tail of A if B is hovering inside A or it's speed is less than A

2- For both onboard observer A and outside on asteroid

"Would B be escaped through the front of A at the same time (or different) if it's speedometer reading is greater than A

3- is it possible for outside observer to see

Speed of A = 0.8c and that of B greater than 0.20 c ? "74.198.150.220 (talk) 02:37, 22 March 2011 (UTC)Eccentric Khattak #1-420

1- If B is inside A and only the front of A is open, then relative to the asteroid, B must be traveling at least as fast as A, or B will soon be resting on the back of A and hence go the same speed as A.
2- You haven't specified how B's "speedometer" measures speed. It's possible to create an accelerometer, which can measure acceleration independent of any outside object. But in contrast, there is no such thing as absolute speed. You can only measure the speed of an object relative to some other object. For example, a car's speedometer measures the car's speed relative to whatever the car is driving on, which is usually the car's speed relative to the Earth's surface, but might be different if for example the car is on a ferry at the time. But I will take your question as meaning that B's speed relative to the asteroid is greater than A's speed relative to the asteroid.
All observers will agree that there exists an event at which the front of rocket B passes the front of rocket A. Whether all observer's clocks that are present at that event read the same time or not depends on how the observers go about setting their clocks. That specific event may as well be the arbitrary event that all observers use as defining what the "zero" point of their clocks is. In other words, that event may as well be the event that all observers use as defining the origin of their coordinate systems. When dealing with two different events that are separated by a space-like interval, relativity of simultaneity makes it such that which of the two events is considered to have occurred first depends on the observer. But when there's only one event involved in a question, whether or not observers agree on the time that the event occurs depends only on an arbitrary choice as to how the clocks are zeroed out.
3- Yes to the question as asked, but I think you aren't phrasing the question to mean what you are intending it to mean. As asked, an example situation that meets your criterion would simply be rockets A and B both traveling at 0.8c as observed in the asteroid's reference frame.
What I think is more along the lines of what you're intending to ask would be a situation in which rocket A is traveling at 0.8c as measured in the asteroid's frame of reference, and rocket B is traveling at, say, 0.9c as measured in A's frame of reference. It's certainly possible for that situation to occur. However, in that situation, B will still not be measured to be going faster than c in the asteroid's (or any other) frame of reference. Instead, B's speed as measured in the asteroid's frame of reference will be as per the velocity addition formula. Red Act (talk) 23:39, 24 March 2011 (UTC)

Efficiency of nuclear reactor[edit]

Hello, What % of the heat generated by a nuclear reactor is converted to electricity?(whats the efficiency of the turbine-cooling circuit i mean) TY DST DSTiamat (talk) 08:53, 18 March 2011 (UTC)

See the list at List_of_BWRs. For example, the Fukushima Daiichi reactor 1 had a MWth of 1380 and a MWe of 460, or 33% efficency. F (talk) 10:22, 18 March 2011 (UTC)
The efficiency is usually in the range of 30 to 35 percent which is the same efficiency of a coal power plant since they work the same way, only with a different source of heat. Dauto (talk) 12:44, 18 March 2011 (UTC)
Some of the newer designs are above 40%. Dragons flight (talk) 16:25, 18 March 2011 (UTC)

Radiofrequency ablation for typical atrial flutter[edit]

Is there any term which is addressed by "drag and burn" and it is related to cavotricuspid isthmus. Cavotricuspid radiofrequency ablation for typical atrial flutter is a proceudre and something on the lines the term "drag and burn" gets mentioned. Is "drag and burn" the right term, or there is something else related to the radiofrequency ablation for typical atrial flutter and Cavotricuspid. aniketnik 10:27, 18 March 2011 (UTC) — Preceding unsigned comment added by Aniketnik (talkcontribs)

I'm not sure I understand the question. The term "drag and burn" is widely used in the literature for a method of pulling a probe across the atrium in a linear series of steps and creating a lesion at each point. My understanding from a Google search, though, is that the technique does not work very well and is not widely used. Looie496 (talk) 17:13, 18 March 2011 (UTC)

Resource starvation or Maximum throughput scheduling for doing office paperwork?[edit]

I spend nearly all my working hours doing non-routine paperwork. The paperwork varies in importance. Doing the most important things (which also require the most time to do and are the most difficult) would result in not being able to do anything else for months, as described in the Resource starvation article. Currently I do Maximum throughput scheduling where I do the quick easy jobs. However this leaves the big difficult jobs undone.

What method of scheduling should I use, that does not have the diadvantages of the two extremes above? Thanks 92.15.2.23 (talk) 11:54, 18 March 2011 (UTC)

This reminds me of the Time Manager scheduling system, used by an IT department I used to work for. It involves setting goals, identifying key tasks identified with each of these goals along with milestones and deadlines, and scheduling tasks associated with each of the goals every day until the deadlines/milestones were met. (As far as I understand - you see I was a mere untrained secretary...) You might like to investigate this further. --TammyMoet (talk) 12:46, 18 March 2011 (UTC)
Which tasks, if left undone, will prevent other employees from doing their work? Which bits of paperwork are likely to have the most (and least) serious consequences for your employer? Is there a reason why you can't break up your day or week into blocks of time which can be allocated to different tasks, instead of your described all-or-nothing approach? (Do long-term projects in the morning while you're fresh, and do the quicker tasks in the afternoon. Or make Monday through Wednesday 'major project' days, and do small stuff Thursday and Friday. Or whatever.) Which tasks do the supervisors who evaluate your performance think are most important? Seriously; the people who pay your salary get the ultimate say in your work priorities—what happens if you ask your boss? TenOfAllTrades(talk) 13:05, 18 March 2011 (UTC)
There is nobody else - just me. No other employees, no supervisor, no employer, but only me. 92.15.2.23 (talk) 13:53, 18 March 2011 (UTC)
Ah, so your question is really "How do I do twelve hours' work in eight hours per day?" The answer, of course, is "You can't." At least, not forever. Either find a way to reduce/streamline the workload, hire additional help, or subcontract some tasks. It's difficult to advise you further without knowing what line of work you're in. TenOfAllTrades(talk) 16:55, 18 March 2011 (UTC)
Maybe the single machine/total tardiness sections of Scheduling: theory, algorithms, and systems By Michael Pinedo might be of interest. Sean.hoyland - talk 13:48, 18 March 2011 (UTC)
I had a similar situation, for writing computer programs, although I did have a boss. They gave me so many assignments they knew I couldn't do them all, and they realized I couldn't do all of them, but they did allow me to set my own priorities. I decided to write those programs which I found most enjoyable, and leave the rest to fester. StuRat (talk) 17:30, 18 March 2011 (UTC)
I think you already answered your own question by saying certain tasks were "most important". The only alternative consideration is whether you value getting more tasks done or more important tasks done. But as long as the less important (though perhaps more numerous) ones don't block other work (even your own) then I say let 'em fester, since by definition they are not as important as the other ones to accomplish (and as others noted, you apparently cannot get it all done). DMacks (talk) 17:53, 19 March 2011 (UTC)
One way which I have seen is for all the managers (you in this case) make a big pile of all the projects and decide which are the most urgent, every week. Do those first. Eventually the jobs that weren't regarded as important will become urgent, if you are gaining on the stack of work. Greglocock (talk) 03:07, 23 March 2011 (UTC)

How much warning of the Japanese tsunami was given?[edit]

Did the coastal Japanese get any prior warning of the tsunami, or was it unexpected? If yes, how long did they get? I appreciate that even if an official warning was given, not everyone may have recieved it in time. Thanks 92.15.2.23 (talk) 12:36, 18 March 2011 (UTC)

I don't know, but the quake was a warning in itself. --85.77.4.120 (talk) 14:20, 18 March 2011 (UTC)
The news reports I heard said that in the hardest hit areas, a tsunami warning was immediately and automatically issued as soon as the earthquake struck. The tsunami then first hit land only 15 minutes later. 148.177.1.210 (talk) 14:24, 18 March 2011 (UTC)
Some places had more than a half hour warning. Rmhermen (talk) 14:41, 18 March 2011 (UTC)

How much radiation protection from an Armored car (military), Armored Personnel Carrier or Tank?[edit]

I understand that modern APCs and tanks are sealed from chemical weapon attack. How much protection from radiation would a modern tank, armoured car or APC such as the Type 96 Armored Personnel Carrier give? I'm wondering why they are not being used to drag fire hoses to approriate positions in the Japanese nuclear emergency. Thanks 92.15.2.23 (talk) 12:43, 18 March 2011 (UTC)

I don't think they need that level of radiation protection to drag hoses. Around the plant, it is not "zap you dead" amounts of radiation, it is "the equivalent of lots of chest x-rays" amount (and the latter, while not good, is still only a probabilistic rise in the chance of developing cancer). The people who are probably getting way too much are the people working in the control rooms (where they are getting a year's allowed exposure in an hour, and sometimes having quite dangerous spikes of radiation), not the people dragging hoses around. I don't think the nature of the problem is not being able to drag hoses around, either. --Mr.98 (talk) 13:24, 18 March 2011 (UTC)
I wonder if there is any such thing as anti-radiation armour? It would be heavy. Radiographers in hospitals wear an apron, perhaps lead-lined. 92.15.2.23 (talk) 13:56, 18 March 2011 (UTC)
I don't think the heroic workers and fire fighters on the Japanese reactors site are at risk of being zapped by bursts of gamma rays or X-rays. Long-term health risks arising from contanimation from fallout must be a bigger concern. In this respect, the best prophylactic measures are surely a well filtered or self contained air supply and thorough decontanimation procedures. Gandalf61 (talk) 14:12, 18 March 2011 (UTC)
There is certainly radiation outside the buildings (hence the evacuation of most workers, the suspension of outside work several times during radiation spikes, etc.) There also is certainly gamma radiation - and alpha and beta. Their suits and respirators are only to keep radioactive particles off their skin and out of their lungs, altough they will stop alpha and some beta. Mainly the suits are so they don't need to be decontaminated so often. Rmhermen (talk) 14:41, 18 March 2011 (UTC)
Some tanks, such as the Abrams M1, have depleted uranium armour, but that's intended for extra resistance to armour-piercing projectiles, not gamma radiation. The primary radiological hazard isn't radiation external to the vehicle (there aren't nuclear bombs going off or big chunks of gamma emitters lying around. The danger is instead from particles of radioactive material getting onto people and contaminating surfaces: these are carried in the plume from a burning reactor (which is why the exposure in helicopters above the reactor is so great) and they fall out onto the land under the plume. Modern main battle tanks have air filtration systems and some (such as the Abrams) run an overpressure a/c system (fed through extensive filters) which are designed to keep out smoke, chemical agents, sand, dust, and other contaminants. Those fitted for urban or desert warfare have yet more filtering. But a tank is massive, and incredibly heavy, and the places to which the water needs to be delivered are inside ruined buildings. So instead they're using military fire engines - I think that means modern aviation-style ones, which have hoses on turrets that can be operated without the firefighter leaving the cabin, and have air filter systems designed to allow them to operate close to burning aircraft (which should help to keep out those radioactive particulates). -- Finlay McWalterTalk 14:27, 18 March 2011 (UTC)
I understood that the helicopter pilots problem was that they were in a direct-line from the gamma emmisions from the exposed fuel rods in the spent fuel tanks (now that the building roofs are gone while those on the ground are still somewhat protected by the large mass of the concrete pool walls. Not sure how absorbtive concrete is, though, maybe they are lead-lined as well? Rmhermen (talk) 14:41, 18 March 2011 (UTC)
Lead is only a little more effective than concrete. A 10 cm layer of either of them will be quite effective to block most gamma rays. (Equivalent to about 1 km of air). Dauto (talk) 00:48, 19 March 2011 (UTC)
It appears they are steel-lined anyway. Rmhermen (talk) 04:55, 19 March 2011 (UTC)
Levels of 400 millisieverts per hour of radiation were reported at some time during the past week. That does pose an immediate radiation hazard, with radiation sickness likely after 2.5 hours of exposure. This is aside from any danger of inhaling particles, ingesting them, or taking them home on your clothes. A tank may have thick armor on the turret and parts of the hull, but the floor is typically quite thin, and radioactive isotopes on the ground would "shine" through the bottom and harm the personnel inside if they stayed there very long, unless the tank provided shielding or unless shielding such as lead plates were added. Radiation protection says that 2.5 cm of steel would cut the radiation in half, the same as 1 cm of lead, 0.2 cm of depleted uranium, or 6 cm of concrete. Cutting the dose in half would mean radiation sickness after 5 hours rather than 2.5 at the extreme hotspot. More thickness would give proportionately more shielding. M1 Abrams#Armor says it has "chobham armor," layers of steel, ceramic, plastic and kevlar. Only the steel would provide much shielding. The article says some tanks have depleted uranium mesh only at the front of the turret and front of the hull, not particularly helpful if the radiation is coming from stuff on the ground under the tank. Armoured personnel carrier says "Armour on APCs are usually composed of simple steel or aluminium, sufficient for protection against small fire arms and most shell fragments." Thin steel or or around an inch of aluminum armor way would provide only slight reduction in the radiation the crew was exposed to. The M113 APC had 2.86 cm of aluminum on the floor and 4.45 cm on the top.I could not find a ref for the thickness of aluminum to cut the ionizing radiation dose in half. The best bet would be to operate a device remotely. Howe & Howe Tech has converted many of their tracked vehicles to full remote control, for firefighting or combat applications, and the nuclear industry would do well to similarly develop remote operated firefighting, demolition, and observation equipment. You do not need a human in the cab of a crane, or behind the wheel of a firetruck, to operate the hydraulic controls to move the vehicle around, or to move a boom or to aim a nozzle. Even the guys on Mythbusters regularly convert cars and trucks to full remote control. See also Military robot, "Thermite" fire fighting unmanned ground vehicle, which could observe and could likely be modified to deploy a hose. Edison (talk) 14:53, 18 March 2011 (UTC)

Messenger Mercury orbiter[edit]

A new "MESSENGER" orbiter has just gone into orbit around Mercury. History shows the lifetime of Mercury landers is very short because of the high heat. How does the solar heating in Mercury orbit compare to that in Earth orbit? Mercury appears to have an average distance from the Sun of about 58 million km, (avg of aphelion and perihelion) compared to about 150 million km for Earth. Inverse square law would then suggest about 6.7 times the solar radiation intensity falling on a Mercury orbiter, than if it were in orbit around the Earth. An earth orbiter gets very hot on the Sun side and very cold on the other side, if not rotated. The Mercury orbiter has solar panels, so "barbecue mode" seems to be out. It looks like they have a white heat shield on the "back" of the instrument package, and a gold covered instrument package facing Mercury. Would the "Mercury shine" be a significant infrared heating source, leading to the designers not painting the non-sun side black for better heat radiation, with lots of surface area to radiate off heat? I realize that "space has no temperature," but things in space certainly do. Edison (talk) 17:15, 18 March 2011 (UTC)

"Mercury shine" is an issue: "The spacecraft’s orbit is elliptical rather than circular because the planet’s surface radiates back heat from the Sun. At an altitude of 124 miles, the re-radiated heat from the planet alone is four times the solar intensity at Earth. “By spending only a short portion of each orbit flying this close to the planet, the temperature of the spacecraft can be better regulated,” NASA documents explained."[2] Rmhermen (talk) 17:32, 18 March 2011 (UTC)
"MESSENGER will operate at room temperature behind a sunshade made of heat-resistant ceramic cloth."[3] but "the sunshade is made of Nextel™ AF-10 fabric[7], which not only can withstand extreme temperatures, but also has excellent thermal properties that limit the spacecraft’s temperature to below 140 C."[4] 140 C is a bit warmer than I keep my room! Rmhermen (talk) 17:42, 18 March 2011 (UTC)
Is that made out of crushed Nextel cell phones ? :-) StuRat (talk) 18:39, 18 March 2011 (UTC)
Operating the spacecraft close to the hot surface of mercury it is like opening an oven and looking into it. The temperature is only 250°C but your face gets hot. The sun is more like a normal light bulb at the same distance. The tungsten filament is 1500°C or more but the heat you get is much smaller. The spacecraft orbit is designed in a way that they heat up in close approach . (They have to do it on the day side because they want to take pictures). And than they go out into space to cool down. A lander was proposed landing at the day night boarder and slowly moving away from the sunrise, but upto now nobody dared to go to mercury to land. --Stone (talk) 19:16, 19 March 2011 (UTC)
Isn't the dark side of Mercury cool enough for a landing ? (I realize that Mercury doesn't have a permanent dark side, but I believe night is very long there, allowing a mission to complete in that time). StuRat (talk) 20:37, 19 March 2011 (UTC)
The dark side isn't cool, it's freezing cold. It can get down to nearly -200C. There is no atmosphere (or at least, no atmosphere worth mentioning - there is a little outgassing, etc.) so once you are out of direct sunlight, it gets very cold very quickly. The terminator (the day/night border) is the only place with reasonable temperatures for a long-lasting lander. --Tango (talk) 02:23, 20 March 2011 (UTC)
Tango, I don't think cold weather matters much at all for these probes unless we're at cryogenic levels. The problem is that Mercury is at a 3:2 (3 rotations per 2 revolutions) tidal-lock with the sun and has an orbital period of about 100 days. This means that the dark side of Mercury will become the scorching side after only 60 days, way too short for a science mission. SamuelRiv (talk) 05:25, 20 March 2011 (UTC)
60 days doesn't seem too short, to me. For an unmanned mission, the robot could explore, pick up some good samples, and leave with them, in that time. Alternatively, they could drill a hole and climb down inside to survive the heat when day comes. Or, for a second alternative, how about rolling towards the darkness continuously to increase the length of the mission while "seeing the sights". Let's see, at the equator Mercury has a semi-circumference of about 7665 km. Divide that by 60 days to get 128 km per day. If that's too much, land closer to either pole, to shorten it, as needed. Of course, a straight path may not work, since dangerous craters may be in the way, and that would need to be taken into account, too. StuRat (talk) 08:53, 20 March 2011 (UTC)
-200C is crygenic levels, isn't it? While those kind of temperatures aren't a problem in space, since you lose heat quite slowly in a vacuum, you wouldn't be able to maintain a reasonable temperature while in physical contact with rock at -200C. --Tango (talk) 20:46, 20 March 2011 (UTC)
Driving to stay ahead of sunrise? Ha! In seven years on Mars Opportunity has driven about 26 km. Opportunity's minimum operating temperature is -40 C (with heaters to keep it that warm), so -200 C is far below current spacecraft standards. Operating on the dark side of a planet requires an energy source other than solar. So it seems landing on the dark side is not reasonable at all. anonymous6494 21:57, 20 March 2011 (UTC)
We certainly have another choice for energy: nuclear. Many spaceships have had nuclear power, so it doesn't seem impossible to put it on a rover. (The only reason it's not practical on cars is the safety concern, and, once it's on Mercury, we needn't be worried about it poisoning anyone, except maybe the little green men. :-) ). If you land it quite near a pole, say 10 km away, then it wouldn't have to move far to stay on the dark side, and it could perhaps still get solar power with a solar panel on a long vertical mast. If you can find a polar crater, it might always stay in darkness as Mercury rotates, without needing to move. So, there are plenty of options. StuRat (talk) 05:23, 21 March 2011 (UTC)

Using nuclear bombs to generate power[edit]

I remember reading about a concept to use nuclear bombs detonated in a deep underground cavity to generate power. I can't remember what it's called though, does anyone know? There's an article on wikipedia about it... ScienceApe (talk) 19:39, 18 March 2011 (UTC)

Studying that was one of the objectives of Project Gnome. In a vaguely related vein, the Soviet Union's Nuclear Explosions for the National Economy programme included explosions designed to help the harvest of fossil fuels. -- Finlay McWalterTalk 19:51, 18 March 2011 (UTC)
I think what you want is PACER. --Mr.98 (talk) 20:02, 18 March 2011 (UTC)
Thanks. Does anyone know what the advantages/disadvantages of this would be compared to conventional reactors? Would this be safer?ScienceApe (talk) 01:35, 19 March 2011 (UTC)
It's terribly uneconomical, requires you to be constantly producing nuclear weapons, and while I'm not sure the risks have been fully worked out, they are certainly not non-existant, both from an exposure point of view and a theft point of view. It's the kind of thing that weapons designers think is a really great idea, but it is most certainly not the best way to do things. Keep in mind that to produce said weapons, you still have to run reactors or enrichment facilities anyway. --Mr.98 (talk) 02:40, 19 March 2011 (UTC)
I wonder how much is known about stress fatigue in large geologic formations. Wnt (talk) 21:28, 19 March 2011 (UTC)
Not only do you need enrichment facilities, you need much greater enrichment. For a bomb, you need about 90% U-235. For a reactor, 3% will do. (The starting point is 0.7%.) See Enriched uranium for details. I can imagine that using existing nuclear bombs to generate power as a way to make use of them while getting rid of them could be worthwhile (although even that seems unlikely to me), but creating bombs specifically for power generation is just ridiculous. --Tango (talk) 02:29, 20 March 2011 (UTC)
It would be interesting to calculate how much electricity would be used to enrich uranium for a bomb versus how much energy you could produce by blowing it up in a PACER scenario. Presumably these numbers are out there, somewhere. You might end up in the black if you did this — generating more energy than it takes to enrich. I don't know. (That still wouldn't make it a good idea.) --Mr.98 (talk) 17:23, 20 March 2011 (UTC)
I'm not sure what the energy requires for uranium enrichment are. It wouldn't surprise me if you would end up in the black. However, you would be far better off with a conventional nuclear reactor. --Tango (talk) 21:30, 20 March 2011 (UTC)
Agreed. --Mr.98 (talk) 16:59, 21 March 2011 (UTC)

Evolution and sex[edit]

Male early humans were obviously more likely to use weapons and tools than female early humans. Tool usage selects for intelligence, dexterity, etc. To what extent will these traits be males, but not females? The reason I ask is that my friend tried to use this argument to "explain" why men were better than women (I think Darwin did the same)...the argument seems specious, but I don't know enough to rebuke it.

Also, more generally, when will the inheritance of some trait be sex-specific? 74.15.137.130 (talk) 20:09, 18 March 2011 (UTC)

Men only have one chromosome that women lack, the Y chromosome, so any difference between the sexes would have to lie there (or in lacking one of women's two X chromosomes). However, a "trigger" can exist there which turns on larger sections of any of the other chromosomes.
As for the argument, that would only make men better at use of weapons and tools. Women, on the other hand, seem to have developed superior communications skills and social intelligence, those being more important for living in a group while raising children. StuRat (talk) 20:23, 18 March 2011 (UTC)
I don't think it is obviously true that early human males were more likely to use tools than early human females. Assuming that the typical gender stereotypes hold (I'm not a paleosociologist, so I don't know how accurate this assumption is), men were probably more likely to use tools such as spears and axes, but women were probably more likely to use tools such as mortars and pestles and sewing needles. If anything, the smaller tools used by women might encourage more dexterity than the larger tools used by men. So it's not clear to me that the hypothesis has merit—it really requires better evidence and justification. —Bkell (talk) 20:33, 18 March 2011 (UTC)
I agree with StuRat. By default anything that changes in the human genome will affect men and women equally -- you have to add a dependence on sex hormones such as estrogen or testosterone in order to cause a feature to be sex-specific. Looie496 (talk) 20:52, 18 March 2011 (UTC)
There are many traits that are sex dependent. In fact, I remember reading somewhere that women are believed to have BETTER fine skills than men (for things such as weaving) while men are better at wider movements (For things such as throwing spears). If you ask my opinion, I don't believe it either way. Dauto (talk) 00:16, 19 March 2011 (UTC)
Yes, there are many traits that are sex dependent. But this is mainly because there are a large number of genes whose transcription is regulated, either directly or indirectly, by sex hormones. To get such regulation, though, there has to be a binding site in the DNA either for one of the hormones or for something regulated by one of the hormones; the default state is not to have such a binding site. Looie496 (talk) 00:34, 19 March 2011 (UTC)
I don't remember seeing anything like that in Darwin, and I frankly don't believe it. Darwin was one clever, perceptive writer. Wnt (talk) 23:03, 18 March 2011 (UTC)
http://en.wikipedia.org/wiki/The_descent_of_man#Part_II_and_III:_Sexual_selection
Yes, that sounds more like Social Darwinism, to me. StuRat (talk) 23:27, 18 March 2011 (UTC)
Reading [5], to me it sounds like Darwin recognizes some rather sexist assertions by his colleagues, but again and again blunts them - accepting only that men might be driven toward greater competitiveness, but not greater intelligence. For example, "It is, indeed, fortunate that the law of the equal transmission of characters to both sexes prevails with mammals; otherwise, it is probable that man would have become as superior in mental endowment to woman, as the peacock is in ornamental plumage to the peahen." Wnt (talk) 09:48, 21 March 2011 (UTC)

So, based on the comments, would it be safe to say that men using tools more frequently (if that was indeed the case) would likely benefit the species as a whole? 74.15.137.130 (talk) 01:39, 19 March 2011 (UTC)

Both men and women used tools. They may have used different tools for different jobs, but the use of tools has been a human trait for a very long time. As for the gender difference commonly cited in hunter-gatherer societies, it does not have to be genetic. Humans were smart enough to know that if a man dies, it doesn't have a large effect on the ability of the group to have children. If a woman dies, that reduces the number of children that can be born. So, men were tasked with risking their life for the betterment of the group while women were kept in relative safety. Because children must be safe also, it meant that women were kept with children while men went out to see if they would be lucky enough to return home. Of course, it could be a combination of genetic difference and social knowledge about child-birthing. Very few things are clearly one thing or another. Most things are a combination of causes. -- kainaw 04:31, 19 March 2011 (UTC)
Then why do female lions do the hunting? 74.15.137.130 (talk) 04:38, 19 March 2011 (UTC)
Compare the number of lions who die while hunting to the number of humans who died back when they were trying to take down large game with rocks and sticks. It is not comparable. You may as well argue "Then why do worms crawl underground?" as it is just as nonsensical. -- kainaw 17:08, 19 March 2011 (UTC)

Also interesting to consider perhaps: why do men have nipples? WikiDao 04:05, 19 March 2011 (UTC)

Earth going in a straight line while spacetime is curved[edit]

I read somewhere that objects under the influence of gravity (like the Earth) are actually travelling in straight lines while the massive object is bending spacetime. What is the meaning of "straight line" in this context? And how does one picture (or explain) this phenomenon? Thanks. 41.132.13.180 (talk) 20:46, 18 March 2011 (UTC)

General relativity says that objects move along geodesics; see that article for an explanation of why geodesics are considered "locally straight". Looie496 (talk) 20:49, 18 March 2011 (UTC)
I just think of that as a way to describe it, not that the objects really do travel in straight lines. StuRat (talk) 21:55, 18 March 2011 (UTC)
As Looie explained above, objects follow a geodesic which is the equivalent of a straight line for a curved space. For instance, if I say to somebody: "Follow a straight line due north and eventually you will get to the north pole", it is tacitly understood that a mean that person to stay on the earth's surface which means they are really not following a straight line because that would send them into space given that the surface of the earth is curved (curved space). I really mean that person to follow a geodesic and keep their feet on the ground. But in a sense their path is still straight. As straight as it can be without leaving the surface of the planet which is curved. Well, under the action of gravity objects follow a path that is as straight as it can be without leaving the universe which is curved by the action of gravity.
(EC) The Earth's world line is a "straight line" in the sense that it's as straight as possible, given that spacetime is curved. It's a straight line in the same sense that a line of latitude or longitude or a great circle on the Earth's surface is as straight as possible, given that the line is constrained to lie on the Earth's surface. However, it's important to note that the "straight line" that the Earth travels along is a line in a four-dimensional spacetime, in which straightness is measured using a pseudo-Riemannian metric, and is not a straight line in a three-dimensional Euclidean space. Three-dimensional Euclidean space does not physically exist, although Euclidean space can be used as a very good approximation in the common case of dealing with low speeds (compared to the speed of light) and little gravity. See also Geodesic (general relativity). Red Act (talk) 23:33, 18 March 2011 (UTC)

Thanks for your answers. I read the articles (as best I could!) and I have one follow up question. Let's say we collapse the the 3 space dimensions into 2 and then the time dimension is perpendicular to these two (ie pointing upwards) and look at this from the sun's perspective. So at t=0 the sun is at the origin and it goes straight upwards through time and the earth starts some distance away and moves in a helix so that at t= 1year it is back at the same point in space. My question is: does the mathematics say that this helix is a straight line? And what about the vertical line connecting the earths position at t=0 and t=1; is this also a straight line? 41.132.13.180 (talk) 10:39, 19 March 2011 (UTC)

Yes (the earth's orbit is a geodesic) and no (the "vertical" line is not a geodesic).
But -- and this is a rather large "but" -- your picture is not accurate. It is okay, so far as it goes, to remove one space dimension, but the result will not be the ordinary familiar 3-dimensional space you're picturing your helix in. The underlying four-dimensional space is curved, and the three-dimensional one you get by removing one dimension is still curved. If you squeeze it to make it fit into an Euclidean model, what used to be geodesics (such as the helix of the earth's orbit) will bend, but that is a product of your squeezing and doesn't say anything about the unsqueezed situation.
Also, the appropriate conversion factor between time and space is the speed of light, so the "vertical" height of one revolution in your helix would be one light year. That's huge compared to the earth-sun distance, so your helix does look extremely straight except in when you consider large times/distance (which is when the squeezing becomes relevant). –Henning Makholm (talk) 13:16, 19 March 2011 (UTC)
Here's an analogy. If you idealize the Earth as a sphere, a geodesic on the surface is a great circle. So let one person travel along the equator (which is a geodesic, i.e., "straight") and let another person set out at a slight angle to that, also in a "straight" line. Their paths will diverge at first, but they will start to converge a quarter of the way around the globe, and cross halfway around the globe, then diverge again until they start to reconverge 3/4 of the way around the globe... even though they're both traveling in straight lines. This is surprisingly close to what happens in general relativity. -- BenRG (talk) 20:27, 19 March 2011 (UTC)

transmutation of nitrogen[edit]

Since oxygen(8)has greater mass than nitrogen(7),how could Rutherford's transmutation of nitrogen into hydrogen and oxygen by iradiation have produced oxygen without a high temperature fusion event? Is fusion NOT always neccessary to produce a heavier element? Where does the extra proton come from?190.148.132.226 (talk) 21:25, 18 March 2011 (UTC)

Note that oxygen doesn't have to have a greater mass than nitrogen. It just has more protons than nitrogen. If [1]you convert a neutron to a proton, you can increase the atomic number with no gain in mass (a slight loss, actually, at least on average). Wnt (talk) 22:57, 18 March 2011 (UTC)
For your specific instance, according to Ernest Rutherford, "In Cambridge in 1919, after taking over the Cavendish laboratory, Rutherford became the first person to transmute one element into another when he converted nitrogen into oxygen through the nuclear reaction 14N + α17O + p". Note that the alpha particle is a helium nucleus, making this a case of nuclear fusion. Wnt (talk) 23:01, 18 March 2011 (UTC)
Note also that the alpha particle was produced by some radioactive decay and had a lot of kinetic energy allowing the fusion to happen. Dauto (talk) 01:39, 19 March 2011 (UTC)

Thank you Wnt. for confirming that the said transmutation did involve fusion. However, since cold fusion (as a theoretical process)has been soundly debunked over the last 20years or so by uiversities, independent reserchers, governments and the U.S. patent office, the original quandary remains. ie.that Rutherford (according to historical record)achieved this fusion by the simple expedient of leaving a piece radio active material in a sealed jar of air. No heat was applied to the sample. Therefore is seems that fusion was achieved without temperatures of millions or billions of degrees, which current scientific wisdom declares to be neccessary. Question, how is this possible.190.148.136.72 (talk) 17:07, 21 March 2011 (UTC)

Thank you Dauto. Are you saying that the kinetic energy in that radioactive decay would be sufficient to overcome the proton to proton repulsion.190.148.133.58 (talk) 23:13, 21 March 2011 (UTC)


There's a difference between cold fusion and coupling a fission and a fusion event. Cold fusion is a chain reaction involving fusion of low-mass nuclei at low temperatures. The energy released by this reaction entirely sustains future reactions. At low temperatures this is simply not possible (not enough collisions per second). You do however have alpha ray emitters -- alpha particles that are released by already unstable nuclei with well-defined half lives. In cold fusion, you start with nuclei with very very long half lives (i.e. they are stable). 137.54.20.172 (talk) 17:28, 22 March 2011 (UTC)

Safe energy?[edit]

Is any form of energy totally safe when improperly handled? Which is the safest (in the broadest sense) form of energy for human use? --78.150.226.145 (talk) 21:44, 18 March 2011 (UTC)

If you mean source of energy, there are plenty, such as solar and wind, but they have other disadvantages, like not always being available.
If you mean ways to store energy, then probably not, if they store large amounts. This implies unstable chemical or nuclear bonds (otherwise you would need to use more energy to break those bonds than you would get from them, once broken), which can then catch fire and/or explode, like wood, gasoline or hydrogen gas. StuRat (talk) 21:49, 18 March 2011 (UTC)
No, the whole point of energy is that it has energy, and if you handle it in such a way that the energy is released uncontrolled it causes damage. The safest in the broadest sense is nuclear power. By far. It releases the least amount of waste, and has killed the fewest people. (Adjusting by amount of energy produced from that type of energy of course.) Ariel. (talk) 21:51, 18 March 2011 (UTC)
In declaring nuclear power's safety, I doubt that you have factored in future deaths from exposure to poorly managed nuclear waste. HiLo48 (talk) 22:29, 18 March 2011 (UTC)
We haven't had any in 60 years (from waste, poorly managed or not), and I have no crystal ball. Have you factored in future deaths from pollution generated by other forms of energy? Ariel. (talk) 22:52, 18 March 2011 (UTC)
Haven't had any? I'd love to see proof of that. And anyway, 60 years is a tiny fraction of the period for which this stuff remains dangerous. Given world political history for the past thousand years, can you tell me with certainty who will be managing India's nuclear waste in a thousand years time? HiLo48 (talk) 22:59, 18 March 2011 (UTC)
You're asking me to prove a negative? And given the world political history for the past thousand years it will be the human descendants of the current generation who will be managing the stuff. And they will be highly technical. Sorry, but I don't believe in theories that think humans will regress. Historically it may happen at a regional level, but it has never happened globally, and as long as a reasonably sized group exists anywhere in the world, dangerous places like waste sites will be managed. Ariel. (talk) 23:16, 18 March 2011 (UTC)
You made the negative claim as a point on your side of the debate, so of course I'm asking you to prove a negative. And yes, problems will occur at a regional level. That's why my question was explicitly regional. Most of your argument seems to be based on faith that we can mange a very complex science. HiLo48 (talk) 23:22, 18 March 2011 (UTC)
For nuclear waste-related illnesses/death, see Mayak. While tracing any individual death to waste exposure is hard, the odds are very high that waste exposure there has led to quite a number of deaths amongst workers and those living in the area. It's probably the most polluted place in the world from a radiological point of view. I don't think it's indicative of how radioactive waste hazards have to be — they were exceptionally poorly managed there, by a government that cared not at all what happened to the people who lived there — but it certainly is indicative of what happens if you don't try to be responsible about it. Let's not forget that we're not just talking about the US and Europe here. --Mr.98 (talk) 00:45, 19 March 2011 (UTC)
Excluding renewable sources like solar and wind, presumably? They haven't killed anyone to my knowledge! Regards, --—Cyclonenim | Chat  21:59, 18 March 2011 (UTC)
During BBC Radio 4's prestigious political debate programme Any Questions? yesterday, panellist Toby Young stated (and was not challenged by fellow panellists environmental lobbyist Jonathon Porritt, Minister of State for Energy and Climate Change Greg Barker, or Shadow Health Secretary Diane Abbott) that over the last 10 years the Nuclear industry had averaged 7 worker deaths per year, while the Wind Energy industry had averaged 44 deaths per year. {The poster formerly known as 87.81.230.195} 90.197.66.165 (talk) 02:23, 19 March 2011 (UTC)
And presumably also excluding the people who died from political infighting over control of uranium supplies, or in mining accidents, or mining-related illnesses. --Stephan Schulz (talk) 22:05, 18 March 2011 (UTC)
Who died from infighting over control of uranium supplies? --Mr.98 (talk) 00:46, 19 March 2011 (UTC)
I would be very cautious about raw, unqualified figures like these. Over what time period? Over what geographical area UK, Europe, worldwide? How do the number of employees in the industries compare? Richard Avery (talk) 08:21, 19 March 2011 (UTC)
Solar and wind have not generated enough energy to count (over the last 50 years or so). And I did include mining accidents - on the plus side for nuclear power, since all other forms of energy require much more mining. And has there really been that much infighting for control of uranium supplies? Ariel. (talk) 22:52, 18 March 2011 (UTC)
The safest energy source is conservation. The next safest is investments in efficiency, but perhaps I'm just confused because "tax cuts" have been renamed "tax expenditures". Hcobb (talk) 22:15, 18 March 2011 (UTC)
Conservation isn't an energy source, it's an energy-saving proposition. Investing in efficiency isn't an energy source either. Regards, --—Cyclonenim | Chat  22:20, 18 March 2011 (UTC)
Nuclear energy chain is the safest form of energy production world-wide, but in the EU hydroelectricity is yet safer (ref). Comparison of "safety" is hard because standards vary accross the globe, nuclear might be safer because the West has large reserves of uranium. --85.77.98.144 (talk) 14:24, 19 March 2011 (UTC)
I found an article with numbers: http://nextbigfuture.com/2011/03/deaths-per-twh-by-energy-source.html I can't vouch for its reliability, but it does agree with my position that nuclear is the safest. Ariel. (talk) 23:58, 20 March 2011 (UTC)

Leg injury[edit]

Hey all! I'm writing a short story for a composition class and I want it to at least make sense on the surface. Are there any leg muscle injuries that one could sustain that would prohibit the riding of a bicycle but that would not be noticed or impede with other activities such as walking, jumping, or climbing stairs (etc)? Thank you. 72.128.95.0 (talk) 22:48, 18 March 2011 (UTC)

Not that I can think of. How about an injury to the buttocks that makes sitting painful ? StuRat (talk) 22:53, 18 March 2011 (UTC)
I don't think there is any leg injury that has zero effect on walking, but it's pretty easy to get a knee injury that makes riding a bike impossible but has only a minor effect on walking, because of the much greater bending of the knee required for bicycling. Injuries of that type also make climbing stairs a problem, though -- at least in my experience. A strain of the gluteus maximus can also have a larger effect on bicycling than on walking -- but it will also making rising from a chair painful. Looie496 (talk) 00:00, 19 March 2011 (UTC)
Bruising of the soft tissues covering the medial surface of the inferior ramus of the ischium often makes it difficult to ride a bike, but has little effect on walking. -- Scray (talk) 02:44, 19 March 2011 (UTC)

Would painting your pool bottom and sides black made the water any warmer?[edit]

Just a random thought. Topic says it all. I can't recall ever seeing a pool that wasn't white-bottomed, though I suspect this is more to aid in cleaning than anything else. The Masked Booby (talk) 23:02, 18 March 2011 (UTC)

Yes, it would make the water slightly warmer. I think the reason for white pools is that they are safer, since you can see when you're about to hit bottom or the side. In a black pool people would also run into each other, as there would be less reflected light for them to see each other. StuRat (talk) 23:30, 18 March 2011 (UTC)
If you do a Google Image search for "black bottom pool," you can see examples of this. Some people prefer it. I have swam in such a thing. --Mr.98 (talk) 01:34, 19 March 2011 (UTC)
I'm not sure why people would RUN in a swimming pool but certainly many HAVE SWUM in one. Cuddlyable3 (talk) 11:55, 19 March 2011 (UTC)
Athletes or anyone in rehab might run in or near the shallow end of a pool, as part of their strengthening and conditioning program. The reason is because the water offers some resistance or "pushback" which helps develop muscle strength. ←Baseball Bugs What's up, Doc? carrots→ 12:03, 21 March 2011 (UTC)
run into: (intransitive but with prepositional object) To collide with / To cause to collide with. Perfectly correct use of the term by StuRat; it does not have to mean literal running took place (as many who have ran their cars into various things know). The only thing worse than a pedant is an incorrect pedant. --Mr.98 (talk) 13:58, 19 March 2011 (UTC)
While we're on the subject of pedantry (aka knowledge of basic English), it should be "have run their cars". 86.183.1.249 (talk) 14:47, 19 March 2011 (UTC)
Indeed. See List of English irregular verbs. Cuddlyable3 (talk) 01:35, 20 March 2011 (UTC)
cudnt help laughingg out loud, you guys are indeed pedantic :)))--Fragrantforever 05:16, 20 March 2011 (UTC) — Preceding unsigned comment added by Fragrantforever (talkcontribs)
According to this black-bottom pool enthusiast, the black bottom is used more for a classy "mirror" effect. My guess is that the water looks more mirrored because there is much more contrast when looking at a reflection of a colored object on it, but it does make me wonder if the diffraction in water might cause a different effect with a black bottom? I am reminded, however, of the solar oven. SamuelRiv (talk) 05:32, 20 March 2011 (UTC)
Diffraction occurs at the surface of the water. It can't possibly be affected by the bottom. — DanielLC 05:09, 21 March 2011 (UTC)
I think I understand the argument. The surface of water has the ability to act like a "one-way mirror", if there's significantly more light on one side than the other. Therefore, if all the light below the water line is absorbed by the black bottom, then there is no light reflected back to a viewer above, so the reflections on the water surface become far more visible, and hence it seems more "mirror-like", especially while the water is calm. StuRat (talk) 02:40, 22 March 2011 (UTC)
    • ^ transmutation