Wikipedia:Reference desk/Archives/Science/2013 March 5

= March 5 =

Does drinking water right after a meal makes us fatter?
humans who drink water (let's say a glass or two) right after eating a bowl of pasta \ or a sandwich, would they theoretically gain more fat then those who won't drink ?, does this phenomenon has a literal name? 79.183.98.234 (talk) 03:51, 5 March 2013 (UTC)


 * Seems very unlikely. What's the source of that bizarre idea? Looie496 (talk) 05:23, 5 March 2013 (UTC)


 * No Primary source, it's just something i heard and in first sight made some sense (when thinking about the water interrupts the enzymes in their work).


 * Probably because they will gain weight, that being the weight of the water, at least until they pee it back out. StuRat (talk) 05:43, 5 March 2013 (UTC)


 * There are a bunch of theories running around (Google [ water with meals ]) involving diluting digestive acid and thus hindering digestion, diluting digestive acid and causing more to be produced, thus helping digestion, causing the food to empty into the intestines faster and thus hindering digestion, causing the food to empty into the intestines faster and thus making you become hungry sooner, and the fascinating theory that if your body really needs food you will have no trouble eating enough to meet that need without extra water, but once you start overeating you need to "wash the food down".


 * As far as I know, there are no scientific studies supporting any of these theories, but we do know that simply keeping a log of what you eat helps with weight loss, so anything that makes you think about what you are eating rather than finishing off a large bag of chips while watching TV is probably a Good Thing.


 * The best unproved diet theory I have heard of is a fellow who kept all his food in a guest house a mile from the main house, and any time he wanted to eat anything, from a full meal to a snack, he had to walk two miles to fetch the food, and another two miles if he had leftovers he wanted to put in the refrigerator afterwards. If anyone wants to buy me some property with a lot big enough to try this, let me know. I suppose having the food at the top of ten or twenty flights of stairs would work as well. :) --Guy Macon (talk) 09:32, 5 March 2013 (UTC)


 * There's a more modest approach which can help, too. During a meal, you can put healthy things on the table, and make people get up to get the unhealthy things.  For example, leave the salt in the cupboard, but put other spices on the table.  Laziness can accomplish changes in our diets which willpower can't. StuRat (talk) 22:18, 5 March 2013 (UTC)


 * There is such a thing as water weight gain (as explained here), but as Guy Macon said, its only temporary. Some quack diet pills are simply diuretics, which cause temporary weight loss, but neither have anything to do with "fat", only the "weight" of water. — Preceding quack comment added by 74.60.29.141 (talk) 12:25, 5 March 2013 (UTC)


 * This may come from a junk-science email that's circulating in social media right now (my g/f sent me a copy yesterday) - it lists a bunch of very specific times (before/after meals, before/after sleeping, exercise, you name it) when you should or should not drink very specific amounts of water. That's a complete load of B.S.


 * The best dieting book I've seen is "The Hacker's Diet" (Wow! We have an article about it!) (it's a free download, or read online at http://www.fourmilab.ch/hackdiet) - and it really does work. The beauty of it is that it's science-based, with actual references - and he spends much time explaining the science behind how it works.  The author doesn't make money from it - so you also know it's not some get-rich-quick idiot promoting yet another fad diet.  SteveBaker (talk) 15:26, 5 March 2013 (UTC)

Manipulating water intake -- more, less, timing or intake, even diuretics-- will make no significant long term difference to anyone's weight. Total distraction. All effective weight loss plans involve eating less over an extended and indefinite period of time. All the popular diets work to the extent that following them results in eating less for a prolonged period of time. Even following the low carb recommendations of Gary Taubes, whom many people extol as opposing the "calories in" approach, actually results in reduced calorie intake to be effective. No matter how you dress it up and what you call it and what you eat, the only thing that produces long term significant weight loss is eating less over an extended period of time. alteripse (talk) 22:50, 5 March 2013 (UTC)


 * You can be tricked by salt. Eating more salt causes increased water retention and increased weight thereby.  This is one or the reasons for the "My low calorie diet did not reduce my weight" phenomena - their low calorie diet wasn't very tasty, so they added salt.  Salt can be at the heart of the "I exercised more, and gained weight" experince, incorrectly blamed on the "muscle weighs more than fat" nonsense (volume for volume it weighs about the same, and the body can't change muscle size as readily as it can change fat quantity anyway).  This is more of a problem for older people - in middle age and younger the kidneys are pretty effective at getting rid of excess salt.  Wickwack 120.145.159.132 (talk) 01:41, 6 March 2013 (UTC)
 * Wickwack, total nonsense. Salt and water fluctuations from day to day amount to less than a kg (2 lbs). In severe kidney failure (needing dialysis) or congestive heart failure a patient can retain more than that but temporarily, but it needs to be removed by dialysis or diuretics. The fantasy that overweight people have that it is just water is just a wishful fantasy. alteripse (talk) 11:03, 6 March 2013 (UTC)
 * There appears to be zero evidence supporting the above claim.
 * BTW, I have finally found the answer! --Guy Macon (talk) 10:10, 6 March 2013 (UTC)
 * It would appear that Wickwack is correct. Muscle density is about the same as fat density.  It happens that I was diagnosed with Meniere's Disease.  The standard advice you get for this, from medical practictioners, is to reduce your salt intake as much as possible.  My doctor told me that when I do this, my weight will drop a few kg.  Initially the drop is typically because patients then find food boring, and eat less.  He told me that this is temporary as after a few weeks I will get used to how food tastes without salt and my appetite will return.  But there is typically a permanent weight loss as without so much salt I'll have less fluid retention.  I won't look much different, and won't need new clothes, as I won't have lost any fat on a long term basis, but I'll weigh less because I'll have less fluid in me.  Experience has shown that doctor was right on the money.  Floda 124.182.38.179 (talk) 12:41, 6 March 2013 (UTC)


 * Alteripse, do you have any references to support your nonsense? Perhaps you should check your facts before posting.  One should be very wary of advice on this subject given by those who work in the diet/weight loss industry.  However, take a look at this Government website: http://www.betterhealth.vic.gov.au/bhcv2/bhcarticles.nsf/pages/Fluid_retention.  It says in the first paragraph (titled "Summary") that fluid retention is caused by excessive salt intake, as well as a few other causes. Googling salt and water retention will give you quite a number of sites supporting this fact, quite well known to jockeys and other sports people whose weight determines what class or handicap they compete in.  Incidentally, day to day variation in weight can be and often is, considerably greater than the single kg you indicated.  I've worked outdoors for much of my career.  In temperatures of 45 C and greater, quite typical in many parts of Australia, the average person in outdoor work can loose 5 to 8 kg or even more per half-day shift in a full shift day.  We sure do get thirsty though.  Younger men need to take considerable extra salt or quenching their thirst after their shift will cause problems.  But just so we are clear, in regard to salt in my earlier postings, I was talking about reasonable long term changes in weight due to changes in salt intake.  Wickwack 58.167.229.59 (talk) 13:42, 6 March 2013 (UTC)
 * Wickwack, this started out as a question about water drinking and its relation to fat weight. Such questions are rarely about fluctuations of 1 kg, but from people who are distressed at degrees of overweight from 5 to 50 kg. When someone well-intentioned but relatively ignorant of physiology posts about fluid retention and salt intake without explicitly referring to end-stage kidney or heart disease, the information is usually irrelevant to the questioner's fat weight concerns or simply erroneous. The salt and water answer to "how can I weigh 0.5 kg less tomorrow" of course can be "empty your bladder before you step on the scale", or "temporarily drop a kg with a diuretic pill or a few hours of work in the hot sun". If you want to gain half a kg you can eat very salty food and drink a lot of fluid and it might take your body a day to unload it. While this might be of intense interest to a wrestler who needs to drop 0.5 kg to wrestle in his usual class in 2 days, do you really think this is at all relevant to significant long term weight loss for significantly overweight people? Of course not; it was distracting nonsense. There simply are NO "reasonable long term changes in weight due to changes in salt intake", just short term fluctuations much smaller than your claims. Healthy people do not lose 10% of their body weight ("5-8 kg") by a day of working and sweating and stay healthy. alteripse (talk) 19:35, 6 March 2013 (UTC)
 * Go look up climate stats for northern Australia. Or Saudi Arabia.  Then, ask yourself, do humans live and work there?  The answer of course is: temperatures around 45 C qite normal in summer and yes, we do live there and like everyone else we need to work and earn a living.  And no, we don't get sick or suffer health issues, we get used to it.  But we can't change physics.  Such temperatures are so far above body core temperature (37 C) that we are wet with sweat all day - and that means weight loss well above 1 kg.  Few people would find a gain or loss of only 1 kg of note - you started this particular nonsense.  The odd silly tourist from England sometimes dies, because they are not used to it.  Wickwack 120.145.63.10 (talk) 00:12, 7 March 2013 (UTC)


 * Wickwack, Alteripse is right and you are wrong. In particular, your comment above that starts with the words "You can be tricked by salt" is completely unsupported by any actual evidence. Your "This is one or the reasons for the 'My low calorie diet did not reduce my weight' phenomena - their low calorie diet wasn't very tasty, so they added salt" statement is not science. It goes against everything we know about human physiology. It is just plain wrong to say that the small amount of water retention from increased salt can overcome the much larger reduction in body fat that comes from eating less and moving more. There is no evidence supporting your claims. --Guy Macon (talk) 21:21, 6 March 2013 (UTC)


 * I'll say this one more time: Take a look at this website: http://www.betterhealth.vic.gov.au/bhcv2/bhcarticles.nsf/pages/Fluid_retention. It is a Government health website of a Western country, so is trustworthy, unlike many websites established by those in the diet/weight loss  business.  Here is a direct quote form the summary: Causes [of fluid retention] include the body's reaction to hot weather, a high salt intake, and the hormones associated with the menstrual cycle.  Neither Guy Macon nor Alteripse have provided any refrences to support their refutation of this.  Wickwack 120.145.63.10 (talk) 00:12, 7 March 2013 (UTC)


 * You can say it as many times as you like but it doesnt make it true. And your website doesnt even back up your claims of a large or long term weight effect. Dont demand we "prove" your misconceptions are misconceptions. You have no idea what you are talking about, and dont understand the websites you are citing. alteripse (talk) 00:21, 7 March 2013 (UTC)


 * Wickwack, perhaps this is a partly a language problem. No one is arguing that weight cannot fluctuate due to salt and water intake and loss. We are arguing that the magnitude is both small and not cumulative. By small, we mean usually less than a kg in people with normal hearts and kidneys. Loss of enough water and salt to reduce your weight by 10% gets you to prostration and the edge of hypovolemic shock, not just thirsty, so you need to re-adjust your understanding of sweat losses and salt gains to much smaller amounts. More importantly the weight gain or loss with salt is not cumulative. You can gain a kg or so by salt intake in a week but you dont gain another kg every week you continue the high intake-- the body readjusts. Look up atrial natriuretic hormone for one of the mechanisms. So for people interested in long term weight gain or loss of more than a kg, your information about salt and water intake is completely irrelevant. Worse, it promotes a common fantasy among overweight people that "I'm not fat, it's just fluid retention". A kg might be, the other 20 kg are not. That is why your comment was both inaccurate and unhelpful even though a nugget of it--- that small weight fluctuations can be attributable to fluid and salt gains and losses--- is true. You have not, and will not be able, to find a citation that proves that large or cumulative weight changes over time can be caused by either. Sorry to get you so riled up, but can we lay this to rest please? alteripse (talk) 02:58, 7 March 2013 (UTC)


 * This reminds me of an old Friends episode, where they were viewing a rather fat Monica on old home video:


 * Monica: "Remember, the camera adds ten pounds."


 * Chandler: "OK, so exactly how many cameras were on you ?" StuRat (talk) 03:18, 7 March 2013 (UTC)

Are there any research about compounds of francium?
I am just struggling with some information seemed to be original research in Chinese wikipedia.--Inspector (talk) 10:25, 5 March 2013 (UTC)
 * How about: Electronic properties of francium diatomic compounds and prospects for cold molecule formation
 * Have you tried Google Scholar → "francium compounds"
 * 74.60.29.141 (talk) 13:05, 5 March 2013 (UTC)
 * That might be helpful about Francium, but I still wonder if there are any about francium hydroxide and francium sulfide, because some data in those two articles in Chinese looks like OR.--Inspector (talk) 00:50, 7 March 2013 (UTC)

Unknown elements in the universe?
Is it possible that some unknown elements exist in the universe, even in the Milky Way galaxy, which are not known to humans? And if unknown elements really exist, how reliable and accurate is astronomical spectroscopy? --PlanetEditor (talk) 12:45, 5 March 2013 (UTC)
 * Have you read Chemical element, Periodic table and Island of stability? What makes an element is the number of protons in its nucleus (its atomic number), and we know all the elements with ANs between 1 and 118.  Elements with higher numbers than this would probably be very unstable, and would quickly decay into other elements.  However, see the Island of stability article for a possible exception.  Rojomoke (talk) 13:22, 5 March 2013 (UTC)
 * Thanks! --PlanetEditor (talk) 14:48, 5 March 2013 (UTC)


 * So the quick answer is "No"...as Rojomoke points out, the behavior of atoms (and therefore how we name them) depends on the number of protons they have and we've found absolutely all of them up to element number 118 or so. Above that, they get unstable and fall apart about as fast as you can make them!  The only slight possibility is that "Island of stability" idea.  It's thought that the general trend is that adding more and more neutrons and protons makes atoms increasingly unstable...which means that above element number 118, things get even more unstable - so there aren't any more elements worthy of discussion beyond that.  However, the concept behind this "Island of Stability" thing is that there might maybe be some much larger number of neutrons and protons that would again produce stable atoms...so maybe something with an atomic number like 200 would be stable enough to exist for a long time.  That's not a proven theory though - it's not clear whether it's true.  HOWEVER - even if it's true, it might well be that there are no natural processes (inside stars, basically) that would create such gigantic atoms in the first place.  So even if the island of stability is real - the odds are good that these elements would still not be found in nature.  Indeed, if they were commonplace, you'd expect to see mysterious unaccounted spectral lines in the light from distant stars caused by these gigantic atoms.  I don't think we see that - Astronomers aren't scratching their heads about a bunch of mystery spectral lines.  Hence either the island of stability isn't real - or these elements are too hard to create for them to exist naturally in large enough quantities.
 * (One of the things that upsets me about watching Star Trek is their continual reliance on mysterious new elements with crazy behaviors. That kind of thing just isn't scientifically reasonable. Their "dilithium" crystals - that power their engines - are a weird exception.  There really is a substance called that - it's a compound formed from two lithium atoms - but you can't have crystals of it because it's a gas...still, those guys are smart...maybe they can fix that!)
 * SteveBaker (talk) 15:09, 5 March 2013 (UTC)
 * If they can have transparent aluminum, they could have crystalline lithium. ←Baseball Bugs What's up, Doc? carrots→ 15:18, 5 March 2013 (UTC)
 * Ah - be careful: Scotty was talking about Aluminium oxynitride - aka "transparent aluminium". It's a completely clear, bullet-proof material - and it would be great for making large water tanks for transporting whales. SteveBaker (talk) 15:40, 5 March 2013 (UTC)

How about unknown molecules that exist in the universe? ScienceApe (talk) 17:35, 5 March 2013 (UTC)
 * Well, for sure, yes. We find new molecules in plants, animals - even within our own bodies all the time.  There are (potentially) an infinite number of different molecules. SteveBaker (talk) 20:55, 5 March 2013 (UTC)
 * There still is not (imo) a satisfactory explanation for dark matter. — Preceding opinionated comment added by 74.60.29.141 (talk) 19:33, 5 March 2013 (UTC)
 * True - but it's not going to be anything like normal atoms - so we wouldn't call that material "an element". SteveBaker (talk) 20:55, 5 March 2013 (UTC)


 * By the way: it looks like the OP had their question answered with the 1st response.
 * I agree - but a little clarification sometimes helps.
 * Neutron stars may contain unusual elements, as they can add neutrons as fast as they drip out of the atoms. Graeme Bartlett (talk) 09:41, 6 March 2013 (UTC)

Thank you everyone. --PlanetEditor (talk) 16:22, 6 March 2013 (UTC)
 * One great example (of recently discovered crystal, not a new element) is crystalline deuterium, a macguffin I used in fiction to build a new generation of hydrogen bombs. It's real, and the super-strength nukes would be quite compact, too, as the 15,000 tons per bomb would occupy no more than 105m3 in real life, a density that exceeded my expectations. (They were weapons of sterilization, not merely of mass destruction. However, some tons of mass do get lost in the process...) - ¡Ouch! (hurt me / more pain) 08:07, 8 March 2013 (UTC)
 * I think this all comes back to the question of "how reliable and accurate is astronomical spectroscopy?" - and I have no idea. What are the odds that the spectral lines of Element 200 have been written off as some combination of hydrocarbons? Wnt (talk) 00:09, 10 March 2013 (UTC)

Top and Tail trains - how can both locomotives be helping?
This discusses push-pull trains, saying ".... a push–pull train, especially a long one, may have a locomotive on both ends so that there is always one locomotive pushing and one locomotive pulling. In this case, caution must be used to make sure that the two locomotives do not put too much stress on the cars from uneven locomotives....  It is usual to arrange things so that the trailing locomotive supplies less power and that the locomotive at the front does more pulling than the locomotive at the rear does pushing."

I think I can grasp how both locomotives could help to move a rigid train. (If I'm pushing a car and you're pulling it we both help it to move.) But exactly what would happen if every coupler had a centimetre of slack? If the rear locomotive is of any use it will be compressing at least some fraction of the car (wagon) couplings and pushing some fraction of the train. Assuming the front locomotive is doing anything useful at all it will be stretching the couplings of the front cars (wagons) and pulling that part of the train. But how can both locomotives be useful unless they have exactly the same power (with the middle coupling slack)? If all the couplings are compressed or stretched, won't one locomotive have to do all the work?

I am sure I've misunderstood something or that I have not explained my puzzlement very well. Obviously the actual answer, which I cannot visualize, arrives at an equilibrium. I'm sure one of you clever clogs will help me resolve this. So that I can sleep. Your Username 19:18, 5 March 2013 (UTC) — Preceding unsigned comment added by Hayttom (talk • contribs)


 * Gee thanks, now I won't get to sleep either! — Preceding sarcastic comment added by 74.60.29.141 (talk) 19:41, 5 March 2013 (UTC)
 * It could be that the front engine is pulling the front half, and the rear engine is pushing the rear half. CS Miller (talk) 20:03, 5 March 2013 (UTC)


 * Ok, try this for a thought experiment (I make no claims for real trains). You've worked out the rigid case, but wonder about small amounts of slack. Consider instead very large amounts of slack. In this instance, the back pusher is really only pushing a few cars that have compressed linkages, and the puller is only pulling cars with taut linkages. If pusher and puller and all cars are equal, then only the central linkage is neither taut nor compressed. So far, I think you are right in that regard. However, if the back pusher is weaker, that will just push the loose linkage toward the back of the train, until at some point, as you say, the puller does all the work. Of course in the real world, linkages don't have that much play, because of backlash_(engineering) issues. But I thought thinking about very loose links would make the situation more clear. Also modern linkages appear to have some range of elastic deformation, meaning that they have states between totally taut and totally slack. See the many options at Railway_coupling SemanticMantis (talk) 20:07, 5 March 2013 (UTC)


 * Good question and I think we really need a loco driver to answer it definitively. I would imaging that the leading cab informs (continuously) the rear cab of both the speed and horse power (load)  the rear cab is required produce to to achieve a bit bit less than  50% of  traction  required to push buffer-to-buffer all of the wagons based on its own speed (ie. Push less than half  of all the wagons).  The rear loco delivering some what less than 50% could then leave a safety margin of several wagons in the middle,  that are either pushed or pulled but ovoids all wagons being pushed which would lead to wagons  being buckled  off the line. The lead cab would be able to tell from its own speed changes if this balance was being kept. Long heavy trains such as these would have a lot of inertia, so I would imagine that there is ample time for the crew to make fine adjustments.  --Aspro (talk) 20:14, 5 March 2013 (UTC)


 * It seems that Railway coupling buffers act as hydraulic dampers, which dissipate kinetic energy, (and make the connection semi-rigid) so it is essentially the same as your rigid example. See photo→


 * ~:74.60.29.141 (talk) 20:50, 5 March 2013 (UTC):~


 * I don't actually know the answer either - but I used to work with railroad locomotive simulators and a few non-obvious data points are worth mentioning:
 * On very long trains, of the sort that have rear locomotives, the length of the train typically exceeds the length of the longest hill on the trip. This can lead to some very counter-intuitive situations, such as the locomotive needing the most power as it heads down over the crest of a hill because all of the weight is still climbing up it - and in reverse, the brakes often need to be on during the uphill parts because most of the cars are still heading downhill.
 * When the train stops on level ground, rookie drivers tend to come to a very gentle stop rather than stopping more abruptly as they should. This leaves you with tight couplings and the train may find it impossible to start moving again because it has to overcome the static friction of all of the wheels at the same time rather than accelerating one car at a time.
 * The issue of acceleration, starting and stopping is by far the hardest part. Long trains have very little air resistance (their cross-section is tiny compared to their length) - and steel wheels on steel rails produces very little rolling resistance.  The amount of power required to keep the train moving is much less than you might expect.  I suspect then that the rear locomotive is there for those kinds of situation where acceleration or braking (or perhaps both - but at opposite ends of the train) is required.
 * There are other complications when going around long, tight turns where the train is like a string that wants to be pulled straight - the forces that pull the train cars sideways are enormous - so derailment is alarmingly easy. Having the train being pushed from behind as well as pulled from the front dramatically reduces those forces.
 * Train dynamics are a deeper subject than you might think! SteveBaker (talk) 20:51, 5 March 2013 (UTC)


 * (ec) One problem I see with pushing only, is that, when the train stops, all the couplings will be fully compressed, and starting a train like that would be difficult, as you must overcome the static coefficient of friction for every car, simultaneously. This could be handled by either having the rear locomotive brake when it stops, to stretch out the couplings, or it could back up a bit, for the same purpose, before starting.


 * Another possible problem with pushing is that it will tend to push cars off the tracks, while pulling will tend to pull them back onto the tracks. Under normal conditions, this wouldn't be a problem, but there may be situations where pushing will cause it to derail, but pulling won't.  With this in mind, only using the rear locomotive to pull it's own weight, and not the train, might make sense.  When the train heads the other way, that locomotive can do all the work, while the now rear locomotive just pulls it's own weight. StuRat (talk) 20:57, 5 March 2013 (UTC)


 * I think people are overthinking this -- it's actually pretty simple. Just imagine the case of a single weight that is being supported from above by a stretched string.  Suppose somebody pushes upward on the weight from below, but not strongly enough to lift the entire weight.  Still, the force the string needs to exert will be reduced, even though it remains stretched. Looie496 (talk) 22:59, 5 March 2013 (UTC)


 * No. The mass is distributed along the whole length of the train. Your analogy is not applicable. Think of a heavy chain. Lift the bottom link and the total wait can only reduce by the mass of that link. Now your out of bed, have a strong cup of coffee.--Aspro (talk) 00:00, 6 March 2013 (UTC)


 * Funny – the above two posts are a bit like two locomotives themselves: Each carrying part of the weight; both of the two models are valid in a certain range of applicability. The chain model ceases to be valid when all buffers are pushed in, and the front locomotive will be directly pushed. That is of course rather rare (such as when it decelerates faster than the rear locomotive, or maybe in certain terrain situations). In that event, the solid model applies. &mdash; Sebastian 00:57, 6 March 2013 (UTC)


 * Oh Really? What about when  all buffers are pushed in  and the forward cab brakes or comes to a curve and buckles or pulls  the wagons off the track. As I said above, a balance of force needs to be maintained to prevent this (ie. less than 50%) to prevent what you call the solid model. See SteveBaker's post above. He has got the handle on this too. Looie496's analogy was a was a piece of string. Have you ever tried to 'push' a piece of string? What's funny about that? P.S. Maybe you’ve drunken  way too much coffee this morning. --Aspro (talk) 02:11, 6 March 2013 (UTC)

Lots of good food for thought. Thanks, everybody! [more inputs welcome...] Your Username 05:55, 6 March 2013 (UTC) — Preceding unsigned comment added by Hayttom (talk • contribs)


 * Not a proper answer, just deduction: Logically, the front locomotive should tug on all the cars with (up to 100% of) maximum power. The train should be designed not to rip apart under maximum power, regardless of whether it is stopped or moving.  So the locomotive should pull the front car, second car, third car etc. all with the maximum length of the connector, until either the train is moving slowly or else it is totally stopped because the coefficient of static friction on the last car, plus the coefficient of rolling friction on the cars already moving, is enough to totally counter the front locomotive's pull.  Either way, the back locomotive, once started, should start pushing cars ahead of it, all with the minimum slack between them in a pile - until the total friction totally stops it, or until the front in the series of stacked cars is moving at full desired speed, for example if it is being pulled by the front locomotive.  So I predict that you'll have longest lengths at the front of the train, short connector lengths at the back, maybe something in between at a point between.  If the locomotives vary in power relative to one another, the number of cars at each length should vary accordingly to take up the load.  (i.e. moving a middle car from "all the way forward" to "all the way back" reflecting whether it is being pulled from in front of it or pushed from behind it) Wnt (talk) 00:19, 10 March 2013 (UTC)

Technical side of feeding electricity into the grid
Can you feed in electricity at any point of an electric grid? Even if these point of access was conceived for households to consume energy? OsmanRF34 (talk) 19:41, 5 March 2013 (UTC)


 * Not sure on the technical aspects of it, but this power company has a special two-way meter that calculates the difference between power generated at home versus power consumed from the grid, and has an option that pays you for excess power generates. See .  This website here also implies that a simple two-way meter is all that is needed; there does not appear to be any other special equipment needed for you if you are producing excess power that is getting added back to the grid (i.e. production exceeds consumption).  -- Jayron  32  19:53, 5 March 2013 (UTC)


 * Regulations would require a safety systems which ensure that in the event of a local distribution circuit going down, that one doesn’t continue to pump power in to a dead circuit which could endangers the utility workers trying to fix it. Other than that, there is is not technical reason that stops one and many households in Europe already do so.--Aspro (talk) 20:26, 5 March 2013 (UTC)


 * That's true so long as most people don't do it. However, if everybody had a windmill and their property was covered with solar panels, you might find that too much power was fed into the grid on windy, sunny, cool days, when neither electric heaters nor A/C use that electricity up. StuRat (talk) 20:46, 5 March 2013 (UTC)


 * Too much power! Oh, that will be the day. We humans will find lots of ways to use it up as fast as its produced. On sunny days, why bother with charcoal? Just have and electric barbecue, or an under-floor cooled garden patio, etc.--Aspro (talk) 21:01, 5 March 2013 (UTC)


 * "Too much power" is a very real issue in load balancing of an electrical grid. If the power generated exceeds the power consumed, the grid voltage will rise, with all the problems involved in running things above their design voltage. --Carnildo (talk) 03:11, 6 March 2013 (UTC)


 * A commercial power-station's output is in the range of gigawatts.    I don't know of an grid where total domestic input exceeds  the main generating force. The large utilities already have to adjust between base load and peak demand, so  micro-generation is not likely (in the foreseeable future) lead to such a scenario that you suggest. --Aspro (talk) 15:56, 6 March 2013 (UTC)


 * The total domestic production doesn't need to exceed the main generator production, to cause problems. Kiet has an example, right below, where this happens in Australia.  As electricity prices go up, and prices for solar panels and windmills come down (especially with government subsidies), we might see more of this. StuRat (talk) 16:36, 6 March 2013 (UTC)


 * As others have said, electricity can be injected into the grid at any point. Essentially, this is what power companies do anyway - most grids have evolved over time with various large and small power stations at various locations.  Technically, no special meter is required for customers who generate their own power (by sola panels) and feed it back into the grid.  The standard electrodynamic meter inherently reads correctly in both directions - it will run backwards if power flows from house to grid.  However, in most cases, what the power company pays you for power is different to what they charge you for power.  For example, the power company in my area charges approx 12 cents/kW.hr for what you draw, and pays you 22 cents/kW.hr for what you give back.  There's no sense in this - they do it because the Government passed a law to compell them to, to encourage take up of solar power and keep the greenies happy.  Where there is no Govt law or Govt subsidy to the power company, the power company will typically pay less than what they charge for power drawn.  They need to install special 2-way meters in order to tell which is which.
 * A minor reason (can be a strong reason for some power companies) is that their existing accounting systems cannot sensibly handle a meter reading lower than the previous reading - it will be treated as the meter having gone round the clock. The extra reading for power feed back by consumers with solar is entering into a special add-on computer system for accounting.
 * A technical issue does exist with two much power feed by consumers back into the grid. The isssue has arisen with Australian power companies due to the rediculous and stupidly high subsidies on photo-voltaic power.  A power company must supply to consumers a voltage within specified limits, otherwise appliances will not work correctly and/or be damaged.  A typical specification in 230 V countries is +,- 7%.  (Note that in many British Commonwealth countries the spec is more complex due to political reasons).  A power company will traditionally install and configure its infrastructure so that the voltage at any one consumer at 4 AM (when draw is mimimal) is under the upper limit. The voltage at houses during business hours can go to the upper limit during business hours, while everyone is at work and school.  The voltage will drop in proportion to draw, so that at about 6:00 PM when people com home from work, turn on aircon and start cooking, the voltage will drop the most.  The power company installs transformers and cables etc large enough so that the 6:00 PM voltage reamins greater than the low limit.  If too many consumers install solar power, it can cause the street voltage to rise above the upper limit, as the peak solar output will always be during business hours.  This may force the power company to alter transformers for a lower zero load voltage, which of course will cause the evening droop in voltage to go below the lower limit.  To counter that, the power company will have to install heavier cables to reduce the variation.
 * Generally though, for solar power installations, the invertor used is required by law to shut down if the voltage on the line is outside (too low or too high) limits. This has led to the Australian experience that the first people in the street to install solar power get quite a large reduction in the power bill.  They tell their neighbours, who then install their own systems.  Sooner or later, there is too much capcity, the voltage rises, and the invertors automatically shut down.  Then everyone (including the early adopters) finds their power bills drop a lot less than they expected.
 * Kiet 120.145.55.126 (talk) 01:24, 6 March 2013 (UTC)


 * That's relly interesting Kiet, do you have a reference where I can read up more about that? Searching "Solar Power Oversupply" only gets articles about too many panels being on the market. 124.191.176.111 (talk) 06:34, 6 March 2013 (UTC)


 * I don't know any online references. It has been reported in Australian trade journals.  When I get a spare moment, I'll check back issues and post some references.  Keit 124.182.29.31 (talk) 07:55, 6 March 2013 (UTC)


 * Something nobody has mentioned yet is that the waveform of the power being added to the line must be in phase with the waveform existing in the line - phase differences can cause all sorts of problems. Roger (talk) 16:03, 6 March 2013 (UTC)
 * This last issue is resolved by using an inverter approved for grid connection. Itsmejudith (talk) 17:34, 6 March 2013 (UTC)