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Since an avalanche can be triggered by a loud noise, it seems almost certain that they could be caused by strong winds -- but I can't see how it could ever be proven that any specific avalanche was caused by wind. Foehn winds tend to occur in specific weather conditions, so I don't see how you could disentangle the wind from the weather. Looie496 (talk) 01:22, 7 June 2010 (UTC)[reply]
Well, triggers section of avalanches says they can be caused by warming, and the Foehn wind article says "Winds of this type are called "snow-eaters" for their ability to make snow melt or sublimate rapidly." so i think it's quite reasonable to suggest that they could cause avalanches. Vespine (talk) 06:36, 7 June 2010 (UTC)[reply]
At some european ski resorts, they use explosives to trigger controlled avalanches to make the snow safer for skiiers - so a sufficiently powerful shockwave will do it. Whether you call that "sound" or "explosive pressure" or "wind" is a matter of degree and nomenclature. A sufficiently powerful movement of air is enough. After all, in the few seconds before a 'natural' avalanche, almost zero disturbance is needed...and at the precise moment when the avalanche starts, zero disturbance was needed to trigger it. So there must be some balance point where a strong gust of wind could set it off...but that might be just minutes before the thing would have started all by itself. SteveBaker (talk) 13:13, 7 June 2010 (UTC)[reply]
I actually was going to say something very similar, but then i had a quick read of the avalanche article and it specifically says that they are not random or spontaneous events. So melting or pressure or shock can trigger it, so it might be as simple as the sun shining or a gust of warm wind. Of course it depends on what you mean by "all by it self" but I would avoid using that phrase, we're not talking about schrodinger's cat after all;). Vespine (talk) 03:45, 8 June 2010 (UTC)[reply]
Good, my intention was to make the Chinook wind trigger the avalanche and also hamper rescue efforts. Thank y'all, and clear skies to you! 67.170.215.166 (talk) 06:05, 10 June 2010 (UTC)[reply]
The lithium/lithium phosphide cell looks interesting (two different patents ):
Questions:
[1] would this cell produce phosphine if it got wet? and would that prevent it ever being used (eg any comparative examples of potentially dangerous substances contained in a product)?
[2] a conducting allotrope of phosphorus is used - this suggests black or purple phosphorus - if the cell is discharged would this make polymeric polyphosphide ions , and if so would these ions react with water to produce phosphine or not? or would it be intercalation instead?
also an Intercalation (chemistry) reaction known anywhere between either Li or Li+ and conducting phosphorus?
also in the cell described above - I'd like to know whether the conducting phosphorus would be expected to reduced when in operation - are any polyphosphide anions known? In fact is anything much at all known about the surface chemistry or modification of black phosphorus?
While I'm partly guessing (although did glance thorough that section which seems to support my guess), I suspect the article only lists real world examples, in other words, chemistries that have actually been used to produce batteries used (at some time) in the real world (even if only in obscure/specialist areas or a few cases), rather then more theoretical examples, even ones that have been demostrated in the lab. However further discussion on this point would belong in the article talk page. BTW, one of those patents is from 1989. If there's no significant ongoing research, I would suspect that that proposal can be considered dead at the current time. Perhaps advances will change that but I would guess you'd see some published research when that happens. Nil Einne (talk) 17:11, 7 June 2010 (UTC)[reply]
There are dozens of different goal-directed and goal-oriented pedagogy programs. You're going to have to be a bit more specific. If you are interested in neural substrates in general, see the article Cognitive neuroscience, which discusses them. --Jayron3204:25, 7 June 2010 (UTC)[reply]
I deal with a lot of computational and theoretical models of learning. The granddaddy of mathematical memory is Hebbian learning. One of the most promising real biological explanations is BCM Theory which is a calcium-based physics model for long-term potentiation. Hopfield nets are more abstract, and their biological relevance is extremely doubtful, but it does provide a neat simulation of learning, extrapolating, confusion, false memories, and extinction. In terms of goal-directed learning, there are self-organizing maps that work without a teacher and the standard adaptive algorithms that require training (like backpropagation). For the cells themselves, there is again LTP and STP (long-term and short-term) for two ways memory is encoded in the neurons (that is, how the neurons learn to rearrange themselves), accompanied by various models of how the synaptic "weights" change. I'm not sure where you're headed with your question, but I can give more details on at least some of these if you want. SamuelRiv (talk) 05:48, 7 June 2010 (UTC)[reply]
"Goal directed learning" is not a very meaningful term, but if you mean reward-driven learning, there has been a lot of work recently pointing to a central role for the basal ganglia, especially the projection from the ventral tegmental area to the nucleus accumbens that uses dopamine as neurotransmitter. Those articles should be enough to get you started. Many other brain areas come into play, but those are thought to be especially central to the neurobiology of action. Looie496 (talk) 05:56, 7 June 2010 (UTC)[reply]
Could you please tell me the difference between engines of vehicles using CNG(Compressed Natural Gas) as fuel and those using petrol/diesel as fuel. If there is no difference then why not such bikes are made that can use CNG as fuel? And even if there is difference does this difference only made it costly or undesirable to use CNG as fuel in bikes?--Myownid420 (talk) 04:18, 7 June 2010 (UTC)[reply]
See Compressed natural gas which discusses advantages and disadvantages of using CNG as a fuel, and Natural gas vehicle which discusses some about the vehicles themselves. They operate on a variety of different designs, so you'd want to see List of natural gas vehicles and select various models to see how they operate, and how they differ from "standard" gasoline or diesel vehicles. --Jayron3204:23, 7 June 2010 (UTC)[reply]
Well i don't think there is THAT much of a difference since it's not difficult to retrofit convert most petrol cars to LPG. Also, googling LPG motorbike it didn't take me long to find motorbikes converted to LPG. The obvious hurdle there is for LPG you typically don't remove the petrol tank and replace it, you just ADD an LPG tank, on a motoribke, there is obviously less space or convenience to add on a bulky LPG tank. As to maybe why there aren't bikes factory fitted with LPG to begin with, i suppose it's still just a matter of economics.. eventually petrol will get expensive enough that maybe bikes will get factory fitted LPG too.. Vespine (talk) 06:11, 7 June 2010 (UTC)[reply]
My father used to run a business converting gasoline cars to run "dual-fuel" on either gasoline or propane/LPG - so I have quite a bit of first-hand knowledge about this. The modifications to a car were as follows:
A honking great pressurized gas cylinder - which would typically fill up most of the trunk of the car - but could be easily accommodated in the bed of a pickup truck or inside a larger commercial vehicle.
A liquid propane line running from the tank into the engine's fuel intake - with a solenoid that switched between propane and gasoline, operated by a switch that was tucked away under the dashboard.
A mechanism to route heat from the engine to the fuel inlet in order to prevent freezing as the propane expanded from a liquid into a gas. I think this took the form of a thin water pipe that ran from the radiator top-hose, around the fuel inlet and back into the radiator bottom-hose.
There are no changes whatever to the engine - you could certainly convert a motorbike to run on the stuff - and as a kind of sales pitch, my father converted a lawnmower to run on LPG. It's a very simple process.
The result was a car that had to be started on gasoline (because when the engine is cold, the carburettor would ice up if run on propane) - and could be switched over to propane within a couple of minutes of driving. If you were getting low on propane and there was no refilling station handy - you could simply switch back to gasoline at the touch of a button. You also needed to run about one tank of gasoline for every 10 tanks of LPG because gasoline contains engine cleaning agents and such that LPG doesn't have.
Liquid propane has a similar energy density to gasoline - so you got about the same MPG on propane or gasoline - but the propane was much cheaper, and (this was in the UK) didn't attract the killer taxes that are levied on gasoline there. Result: It cost you about 25% of the price per mile driven! A Taxi company that my father refitted with LPG systems paid for it in just 2 months of fuel savings. I don't know whether that's still the case though...and in places with lower gasoline taxes, the benefits are very much smaller.
The car produced only CO2 and water out of the exhaust - which was good for vehicles like busses that have to drive down city streets where people are walking. Of course this does NOTHING to help global warming - CO2 was still produced and in identical quantities to gasoline.
The problem with converting motorbikes is that of where to put the large, heavy gas cylinder. To get the same range, it would be the same volume as the bike's regular fuel tanks - but made of thick, heavy steel. You can't replace the existing gas tank because you have to run on gasoline until the engine gets hot...so you need both. So it's not a simple conversion. You could possibly design a bike from scratch to run on LPG with a really small gasoline tank with just enough fuel to get the engine started...but the inconvenience factor of having to fill a tiny little tank every few dozen engine starts would be a pain...and because LPG filling stations are not everywhere - you really need that ability to run on gasoline when the propane runs out. Also, unless the bike is water-cooled, you'd need to run exhaust gasses to the propane fuel inlet to prevent it from freezing.
LPG is widely used throughout the world for running fork-lift trucks. These machines need to be driven inside buildings where the build-up of carbon monoxide (not to mention the smell) from a gasoline engine would be unacceptable. Producing only CO2 and water means that these engines can be run indoors with zero problems for people in the building.
"The car produced only CO2 and water out of the exhaust" only? c'mon Steve, surely the combustion fixed some nitrogen as well so NOx in exhaust, unless it really ran at low compression ratio? --BozMotalk12:59, 7 June 2010 (UTC)[reply]
There was nitrogen of course - but no NOx. I guess that the combustion temperatures are lower or something. However, it's true - you could breathe what came out of the tailpipe. My father had trouble from the local planning authorities who complained that he operated motor vehicles indoors without exhaust capture systems - but he ran LPG cars in his garage all day with the doors shut. Again, note that fork-lift trucks are routinely run indoors on propane. SteveBaker (talk) 13:06, 7 June 2010 (UTC)[reply]
All vertebrates have the same basic eye structure. Could you be a bit more specific about what you're trying to find out? Looie496 (talk) 06:04, 7 June 2010 (UTC)[reply]
They do not have binocular vision like humans, which enables humans to have finer depth perception and detail. Instead, they have bulging eyes which cover a lot of area; each eye looks at a separate area. --Chemicalinterest (talk) 10:50, 7 June 2010 (UTC)[reply]
There is a ton of literature on frog/toad and salamander eyes and retinas. What kind of difference from the human eye you are asking about specifically? Color vision? Anurans see colors, but not in the same L, M, S cone fundamentals as humans. Eye movement? They can lower the eyeball towards their throat and lift it back into the orbit; they have a pair of muscles for that. They can't make saccades like we do, though. Frogs don't really see non-moving objects unless the frog itself moves. Is that what you are asking? Ir is it something else? --Dr Dima (talk) 11:31, 7 June 2010 (UTC)[reply]
why the planets revolve around sun?
and why they revolve around sun in an Elliptical path?
is it due to the change in gravitation of sun? —Preceding unsigned comment added by 120.60.141.39 (talk) 05:42, 7 June 2010 (UTC)[reply]
Such a question was answered mathematically, with a final stamp, by Kepler's Laws. You're familiar with gravity - a big heavy object like the Sun has a big gravitational force that it pulls on the planets (as well as comets, asteroids, and anything else in our solar system). This gravity doesn't change like the mass of the Sun doesn't change, so why ellipses?
Let's say you have a big rock going really fast toward the sun (but just to the side) - as the rock gets closer, the Sun drags on it more and more, eventually swinging that rock around. But the rock is still going so fast that the Sun can't hold on and so that rock goes flying back out into space. BUT the Sun is still pulling, just much weaker, and the rock got slowed down when it went around the sun such that eventually the rock gets pulled to a stop and starts speeding up toward the Sun again, everything starting over.
That's a kind of intuitive description of an elliptical orbit. A perfect circle orbit can form, but it has to form kind of precisely, otherwise the planet will always have a moment where it will be moving faster than before and slip away a tiny bit before being pulled back. Note that these orbits are stable - that rock always comes right back where it started, speeding up toward the sun and flinging back away in the same paths every time. This is because there is no friction in space, so without the Sun changing or the rock changing, we get this great situation of Conservation of Energy, and you can do all sorts of nice math to prove all of Kepler's Laws from there. So the Sun doesn't change and gravity doesn't change, and that's precisely why orbits are elliptical. SamuelRiv (talk) 05:59, 7 June 2010 (UTC)[reply]
The strength of gravity follows the inverse-square law, as per Newton's law of universal gravitation. Thus if the planet were to double its distance from the sun, then the gravity between them would now be 1/4 of the original value. Strictly speaking, its the distance between the centres of the planet and the sun. Also the orbit is only stable if there are only the sun and the planet - other planets' gravity can affect the first planet's orbit. Even with one planet, the orbit's major axis will slowly rotate around the sun, see orbital precession for the details. CS Miller (talk) 12:54, 7 June 2010 (UTC)[reply]
I think that at such extreme acidities, definitions become important, and I'm not even sure that everyone agrees on the definition. pH is supposed to be the negative base-10 log of the hydrogen ion activity, whatever that is, in moles per liter. The easiest thing to explain is the hydrogen ion concentration, and at low concentrations, that's (almost) the same thing as the activity. But when you get down to pH of zero or below, they may be rather different. I have never quite followed what "activity" means exactly, but I gather that it's some statistical-mechanics concept, and sometimes there are fuzzy aspects to some of those. --Trovatore (talk) 18:28, 7 June 2010 (UTC)[reply]
The explanation that the difference of aging between the two twins is caused by acceleration or reversal of direction is not convincing if we compare two trips:
The voyager travels to a star distant 10 light years, starting with acceleration of 10 m/s till he attains 99% of the speed of light (what should take a little less than one year). He then proceeds with this speed until the same distance of about 1 light-year from the star and decelerates at 10 m/s2 until reaching a star at speed zero. Finally he returs to Earth in the reverse manner.
He then makes a second trip in the same way but to a star 20 light years from earth. The period and speed profile of the accelerations and decelerations will be the same in both cases, but in the second case the distance and time traveled at 99% of the speed of light would be about double.
If the age difference (and the respective clock markings) of the two twins were due to acceleration and / or direction reversal, the difference would be the same in the two cases.
Note: During the constant-speed step it is assumed that the ship rotates in order to generate a gravity of 10 m/s2 for the twin in order that he stays under normal living conditions.
Question: if the twin´s pulse rate were regular 60 per minute on Earth, how many beats he had accumulated at the main points of the travel? (start = 0, end of acceleration, start of deceleration, arrival at the star and the equivalent points at return.)
If the number of pulses of the traveler is much less than of his twin, then actually he has lived less during his trip. Also all chemical and biological reactions would follow this pattern. It is broadly similar to what happens to bull sperm kept in bottles immersed in liquid nitrogen: although its normal life is only a few days, it can be kept “alive” (actually in suspended animation) for more than an year because at this low temperature all chemical and biological reactions practically cease until the sperm is returned to normal temperature to revitalize and fertilize cows.
Question: If, starting with the ship´s departure, Earth sends a light signal every second, how many signals the ship has received at the main points indicated above? Also, if the ship sends a sequentially numbered signal every second (measured by the ship´s clock), at what time (earth clock) do these numbers arrive?
Question: IF the light signals from earth and from the ship are sent with a wave length of 600 nm, with which wave they will arrive at destination during the several steps of the voyage?
Question: in which reference frame (earth´s, ship´s, star´s) is the twin with his clock during acceleration and during constant speed, going towards star and going towards Earth?
In my opinion the problem of “understanding” the paradox as such does not exist because there is nothing in the universe what we really understand. We accept gravity, magnetism, electricity, matter, universe and life as facts and can treat many of them mathematically, but we do not really understand any of them. In the same way, what we need in the case of the twins is to know what happens, and this should be calculable by the formulae of restricted relativity and the Minkowski diagram.
Your questions seem a bit loaded, and I get further every day from my physics degree, but I'll bite.
First, it is important to understand that the twin paradox is not a paradox at all; the problem and the system contained within are completely solvable with the known laws of physics. From your wording, I'm not sure if you understand this point.
Second, your assertion that the twin paradox is close in principle to Cryopreservation does not make any sense: one is physical time dilation due to relativity, and the other is merely the slowing/cessation of aging and decomposition processes due to extremely cold temperatures. These aren't even the same branch of physics.
Third, your calculation of taking around a year to accelerate to 99% of light speed is misleading. You are assuming that acceleration for a given force is linear—that the same force which accelerated you from 0 m/s to 10 m/s will get you from c - 20 m/s to c - 10 m/s.I'll leave the math to someone with more time on their hands, but if you wanted to have a truly constant acceleration, by the time you reached 99% of c, you would need inconceivable amounts of energy to get that last 10 m/s. But I digress...
Fourthly, you seem to misunderstand the very point of the whole paradox. Say all humans live exactly 1000 years (and, for the sake of your argument, let's make this a "triplet paradox", and assume that these are super-triplets whose bodies have all the same chemical reactions going on in exactly the same sequence). The first triplet is born, lives a fulfilling life, and dies exactly 1000 years later. The second triplet is born, then at age 20 goes on your 10-light-year trip. He comes back to earth, lives a fulfilling life, and dies. If he kept a clock, which is 100% accurate according to both his perception and the laws of physics in his reference frame, it will read that he died exactly 1000 years after he was born. Same goes for the triplet who went on the 20-light-year trip. Now, according to the triplet on earth, his siblings who went on the journeys will be much younger than him when they return, but this is only in his inertial reference frame. The cells in all three triplets bodies will undergo the same reactions and live the same length of time; this time will just seem different to someone who strayed from that person's inertial frame.
I hope that someone smarter and less busy will come along and answer your math questions, but they seem a bit trivial to understanding the problem as a whole.
And finally, well, your last paragraph is straying from the realm of science into the realm of philosophy. Science is not reality, it is a way of mathematically and quantitatively describing reality. "Accepting" science is a matter of personal choice I suppose, but the great thing about science is that anyone can do it—you can test and verify any law of science for yourself—and I assert that you will find the same values of the universal constants as everyone else (within experimental error of course). In my mind, I "understand" the science of magnetism, gravity, and relativity, but understanding science is easy, because science is the way we break down reality to make it easy to understand. -RunningOnBrains(talk)14:08, 7 June 2010 (UTC)[reply]
Without doing the math, here's how to get out of the paradox. You state "If the age difference... of the two twins were due to acceleration and / or direction reversal", and that's the problem.
The observed age difference is because of travel at relativistic speeds. Period. Each twin observes time moving more slowly for the other while they are departing (because it's perfectly valid for each to claim that he's the one at rest). However, this creates a paradox -- if the twins are returned to each other, they can't each be older than the other one. The acceleration needed to stop and return resolves this. However, that's not the same as saying that the difference in time experienced is because of the acceleration. Note also, while philosophizing about guessing and the like, that this behavior has been experimentally confirmed. It's not guesswork. — Lomn13:58, 7 June 2010 (UTC)[reply]
Careful! The observed age difference is due to the acceleration, since it is due to them traveling at relativistic speeds with respect to each other, when they begin in the same inertial frame. If they both go out at the same relativistic speed in the same direction and return, they will still be the same age. -RunningOnBrains(talk)14:15, 7 June 2010 (UTC)[reply]
Or, as brilliantly put in one of the great underrated movies, "Do the math, motherf***er!". One interesting thing is that you can re-fold space (change its topology) such that the two ships actually can meet and synchronize without proper acceleration, but this violates global conservation of energy, which can occur when one changes topologies. Incidentally, this also occurs in wormholes, where the twin paradox is specifically exploited for time travel. SamuelRiv (talk) 15:19, 7 June 2010 (UTC)[reply]
It's always amusing to see what people think "causes" the twin paradox, and how passionate they might be about their convictions. For the analogous Euclidean situation, what "causes" two distinct paths between Kathmandu and Istanbul to have different (or possibly the same) lengths? Is it the angles of deviation from a straight-line course? Maybe so, depends on how you want to look at it. Either way, no one seems to regard the fact that two paths between these cities don't generally have equal lengths as paradoxical. Go figure.
By the way, there's no need to invoke exotic topologies or energy conservation issues to have two ships with zero proper acceleration meet at different times. Two freely orbiting clocks in the influence of a common central force, one in a circular and one in an elliptical orbit, can repeatedly meet and show different elapsed times between meetings. Tim Shuba (talk) 21:05, 7 June 2010 (UTC)[reply]
Well, here's the disanalogy from your proposed Central Asian sojourn: A path from Kathmandu to Istanbul can be marked out along the ground, the physical structure, that is, of the Earth. You could even dig it into the soil and then measure the length later. Hard to argue with that.
The Twins, though, are making a journey across notionally empty space, and one of the basic driving ideas behind relativity is that there is no aether (or at least that, if there is, it's experimentally unobservable). So, one thinks, shouldn't we basing everything off of concrete objects? Well, what if the Twins and their spacecraft are the only objects there are? Where then does the asymmetry come from?
Now, it certainly could be "just the way things are" that there is no absolute frame of reference for position, or for its first derivative, but that there is one for the second derivative. But that definitely reduces the sense of a-priori canonicity with which the story of special relativity is usually told.
The other possibility is that you can just as well describe the events from the point of view of the Twin that we see as the one who went off and came back — but that to do so correctly, you have to include the acceleration of distant galaxies in the computation, and that when you do so, you wind up with the same observable outcomes. Here by "possibility" I just mean that I don't know this isn't possible — I'm no expert on GR and I don't know whether the equations really come out that way.
Once again, the relevant article is Mach's principle (whether that principle is true or false, I don't know — I'm not advocating it per se, just saying this is what we're talking about). --Trovatore (talk) 04:05, 8 June 2010 (UTC)[reply]
There is no need for keeping the path on the Earth's surface in the analogy, except for familiarity of visualization. Take a path in the air or space, go through the Earth, no problem. In fact, let's take the path straight through the Earth to Istanbul. That is the shortest distance, of course. So what is the cause of all other paths being longer? To me, it is simply a fact of the geometry, no more in need of a cause than one side of a triangle needs a cause to be shorter than the sum of the other sides. It is precisely analogous to the twin paradox for the standard set-up, in which case the stay-at-home (assumed inertial) twin has the longest proper time; all other spacetime paths correspond to less proper time, a simple fact of the Minkowski geometry.
Asserting the geometry is a result of Mach's principle is even more problematic than asserting that acceleration is a cause. Currently, the Mach's principle article states, "Einstein's formulation [...] is not a fundamental assumption of general relativity." This is rather understated, e.g. top-notch relativity historian Michel Janssen gives a detailed account of the history in an essay entitled "Of pots and holes: Einstein's bumpy road to general relativity", especially from page 68 second paragraph through page 74, where Janssen writes, "Einstein had to concede that [Mach's principle] does not hold in general relativity."
I've yet to see any convincing invocation of Mach's principle to show anything concrete, and certainly not for the twin paradox. What general relativity does say is in the absence of nontrivial masses and energies, spacetime is flat, that is Minkowskian, and special relativity applies. Tim Shuba (talk) 13:24, 8 June 2010 (UTC)[reply]
Although one twin may be "younger" than the other due to hyperrelativistic speed travel, cosmic radiation may cause that twin to age by damaging cell chromosomes enough to reverse the "de-aging" effect. ~AH1(TCU)00:15, 8 June 2010 (UTC)[reply]
What determines the frame of referance for spinning?
The Solar sail article describes some designs of solar sail which would be kept stiff by the forces from spinning them. But why should the frame of refeance be the rest of the universe? If you took as your frame of referance the solar sail, then it would be static and the rest of the universe would be spinning around it. The solar sail should then go floppy. How can this be explained? How does "God" decide what the frame of reference should be in those circumstances? 92.24.182.231 (talk) 10:49, 7 June 2010 (UTC)[reply]
You are confusing rotating frames of reference from those in linear motion relative to each other. Rotating frames of reference can be distinguished precisely because of effects like this. Frame of reference explains this. SteveBaker (talk) 11:48, 7 June 2010 (UTC)[reply]
The relevant article here is I think Frame-dragging. The rest of the universe spinning around would drag the local space round so overall you see no difference -the sails would still go out stiff. Dmcq (talk) 12:23, 7 June 2010 (UTC)[reply]
Yes to above, people are getting confused. A rotating object creates a non-inertial reference frame, which is totally distinguishable from any inertial reference frames around it. The difference is that rotating objects have non-zero acceleration, so, as with relativity questions above, this makes all the difference and observers always know who is rotating and who is not. The sail is rotating because it feels centrifugal force. The universe is not rotating because, for example, humans on Earth do not (except from the Earth's own rotation). SamuelRiv (talk) 15:33, 7 June 2010 (UTC)[reply]
So ... if the sail were the only object in the universe - could you still tell whether it was spinning or not ? And if you fire rockets tangentially around the rim of the sail, will that make any difference ? What if you fire the rockets in the opoosite direction ? Now look at the sail from the other side - clockwise becomes anticlockwise and vice versa. So have you introduced a parity violation in an isotropic universe ? Gandalf61 (talk) 16:06, 7 June 2010 (UTC)[reply]
In order for there to be an observer - the solar sail can't be the only object in the universe. If you and it were the only two things in the universe, you could certainly tell whether either it, or you, or both were rotating. However, this hasn't made the universe isotropic because everything depends on where the observer is relative to the sail. SteveBaker (talk) 18:56, 7 June 2010 (UTC)[reply]
But if the only two things in the universe were the sail and the observer, and they were rotating together, could you tell? Mach's principle says no. Is Mach's principle correct? That's the question. --Trovatore (talk) 19:08, 7 June 2010 (UTC)[reply]
I don't know about Mach's principle, but yes you could tell. In order to rotate the edges of the object need to be "pulled" from a straight line, and that force can be detected. Ariel. (talk) 07:05, 9 June 2010 (UTC)[reply]
Another thing I've wondered about - I walk from A to B on the surface of the globe. I am stationary and the earth is moving. How much energy do I need to rotate the earth? 92.15.24.29 (talk) 20:08, 7 June 2010 (UTC)[reply]
The net energy is zero, since the Earth's rotation is the same at the beginning as the end (stationary, with respect to you). --Tango (talk) 20:20, 7 June 2010 (UTC)[reply]
No, it doesn't. You and the car might need to use energy for other reasons different than the one you're thinking, such as replenishing energy lost due to air friction. Dauto (talk) 01:21, 8 June 2010 (UTC)[reply]
Well, energy is required to get it in motion but there is no minimum requirement and whatever anergy is used can be recooped once it stops. Dauto (talk) 05:26, 9 June 2010 (UTC)[reply]
Do the above explainations also explain why "God" forces the earth to be the frame of reference rather than me? If I was the frame of reference, then I would have to deal with a huge amount of energy to start and stop the earth rotating under me when I went for a walk. 92.24.182.110 (talk) 11:31, 9 June 2010 (UTC)[reply]
Production of phosphine and glow from white phosphorus
Can't they build themselves a simple fume cupboard at least rather than trying to depend on a mask? I'm agin all this nanny codding stopping any experimentation but it is a good idea not to kill oneself. Dmcq (talk) 12:51, 7 June 2010 (UTC)[reply]
So if its disproportionation, why the glow? Does phosphorus glow during disproportionation? Or does it have nothing to do with the disproportionation? --Chemicalinterest (talk) 14:33, 7 June 2010 (UTC)[reply]
Sorry, here I am referring to a response is to use White Phosphorus [P] reacts with Potassium Hydroxide solution [KOH] (concentration) and heating this mixture at high temperature, then you will get a Hydrogen gas like sulfur [H2S] Phosphorus is Hydrogen [H3P]. itself highly flammable substance and characteristic fish smell, its own fire in the air with a characteristic blue --I love chemistry (talk) 15:03, 7 June 2010 (UTC)[reply]
I sometimes do this ^ while going for a jog, pantomiming being dragged and making a worried face, just to confuse people. SamuelRiv (talk) 15:52, 7 June 2010 (UTC)[reply]
(I have a rule: Everytime someone makes this error, they have to donate a dollar to Wikipedia! My plan is to make a million dollars that way...and I think I'm getting close!)
The problem is that you are confusing "force" with "energy".
A magnet exerts a force - it does not "produce energy". Your fridge magnet will stay stuck to your fridge until hell freezes over without "running down" because it doesn't need to expend energy to do it. Energy is produced or consumed when something that's exerting a force moves. So when you peel the magnet off of your fridge, it costs your muscles energy to do it. When you release the magnet from a half inch away from the fridge, it releases that energy by moving back onto the fridge - and the energy you added is turned into heat and sound as it snaps back onto the metal. The magnet itself doesn't HAVE energy to give away - it's not like a battery or a wound-up clockwork toy. The situation is exactly the same as with another common force...gravity. If you place a book onto a table, the book and the table are exerting forces on each other - the book is being pulled downwards by gravity - and the table has interatomic forces that resist motion and press upwards onto the book to prevent it from moving. Again - there are lots of forces being applied here - but no energy is being produced or consumed in the process because nothing is moving (technically: because nothing is accelerating). The book will stay there, resting on the table forever without the table or the book "running out of energy" because they are doing that. But if you exert chemical energy from your muscles to pick up the book - you are giving it energy because you are moving it against the force of gravity...and when you drop it onto the floor, this energy will be released as it falls, turning into kinetic energy (motion) and then into heat and sound as it hits the floor. So just as we can't get "free" energy from gravity - so we also can't get it from magnets.
Oops. I had better explain that the word "energy" here comes from me -- the OP had titled this section "Question", which I thought needed to be changed. The title as it is now seemed to capture the spirit of the question, but it didn't occur to me that the OP might be blamed for using a word that the OP didn't use. Sorry, Looie496 (talk) 20:26, 7 June 2010 (UTC)[reply]
I see this frequently in food ingredients, but the entry on Corn Syrup doesn't seem to mention solids specifically. What are they and why are they used as opposed to liquid corn syrup? --70.167.58.6 (talk) 16:21, 7 June 2010 (UTC)[reply]
As a side question: "Does anyone know of an online source that defines what ingredients on food products in the USA are?" It seems like the FDA should have a website giving precise definitions on some of the vaguer ingredients. Almost like a MSDS for food, if you will. --Rajah (talk) 16:50, 7 June 2010 (UTC)[reply]
I have never heard any scientific view that nature is perfect. And for straight lines, have you ever looked really closely at a grain of salt? Googlemeister (talk) 16:45, 7 June 2010 (UTC)[reply]
There are straight lines in crystals, I put some images in that show crystals with straight or very nearly straight edges
This question may be based on a very old (and easily debunked) theory that coastal cities had more advances in art and science than inland cities because the only straight line in nature is the horizon and the only place you can see it is looking out across the ocean. Of course, that isn't the only straight line in nature. It isn't the only place to see an unobstructed horizon. And, there is no correlation between being able to see a straight line and being more advanced in art and science. -- kainaw™16:53, 7 June 2010 (UTC)[reply]
The horizon is curved - not straight (and even at sea: waves, tides and gravitational anomalies disturb that "perfect" arc - as does the fact that the earth isn't perfectly spherical). Crepuscular (and in fact all rays) that pass through air (refraction) or a gravitational field are bent by that passage - and there is nowhere in the universe where there are no gravitational fields. Those crystals aren't perfectly straight if you zoom into them enough (see third picture to the right here). So all of those examples are busted.
Zoom into the highest res version of this photo of a salt crystal...the edges are not straight!
The truth is that if you are picky enough, then outside of pure mathematics, there is no such thing as a "perfect" anything. No material substance can be used to make a perfect straight line because of quantum effects - I suspect that the uncertainty principle guarantees that you can't be sure that even an idealized light beam in a gravitation-free universe would travel in a straight line because you can't nail down the positions of the photons that accurately...but we don't live in a gravitation-free universe - so light rays are bent all the time (albeit not by very much). The standard of "perfection" is just too high a bar for anything outside of theory.
However, if you apply a more realistic view of "perfect", there are things like crystals, light rays and many other things that result in straight lines that are close enough to perfect to satisfy any reasonable person.
What is intensely annoying about this statement that "nature" doesn't produce straight lines is that humans can't do any better...even by "unnatural" processes (whatever that means!). There is simply no such thing as perfection in anything that you can measure. The universe simply doesn't work that way. But "nature" (whatever that is) is as capable of approximating a straight line as anything else that's real. So the statement is really a tautology.
Not exactly. Shorter lines typically vary from perfection to a lesser degree than longer ones - but the probability of exact perfection is still zero...so "more likely" isn't a very good description! SteveBaker (talk) 19:13, 7 June 2010 (UTC)[reply]
I may be wrong about this, but since antineutrinos rarely interact with anything (I know they do interact occasionally), wouldn't they travel in straight lines? Or would they still be curved by gravity? Regards, --—Cyclonenim | Chat 23:16, 7 June 2010 (UTC)[reply]
Anti-neutrinos travel in straight lines in spacetime, which would mean they would necessarily be "warped" by gravity (like photons). Whether this constitutes "straight" or not depends on whether you are trying to force Euclidean geometry on inherently non-Euclidean space. --Mr.98 (talk) 00:32, 8 June 2010 (UTC)[reply]
I think Steve is being way over pedantic, of course we aren't asking for some sort of platonic version of a straight line, we're looking for a pretty good approximation of two points joined by a line, not obviously bent or crooked.. Those salt crystals look pretty straight to me even at highest magnification. And if they're not, it could be due to lens aberration. Another example of straight lines in nature is spiders can form straight lines with their webs, straight lines a plenty.. Vespine (talk) 03:35, 8 June 2010 (UTC)[reply]
And another somewhat arbitrary example: when a body like a meteorite strikes the planet, the blast and fragmenting ejecta can be deposited in "straight lines" or rays. Don't know if that qualifies as "nature," as in living organisms type nature, but since gravity is natural and so are force vectors, I leave my example. DavidH (talk) 00:57, 11 June 2010 (UTC)[reply]
The first thing to consider is no one is sure why moths are attracted to lights. See Moth#Attraction to light. The second thing is that whatever the reasons, there's a fair chance it wouldn't work if they are active during the day since the light is so bright then that there's nothing to be attracted to. Nil Einne (talk) 17:03, 7 June 2010 (UTC)[reply]
I think I may have misunderstood the question since I didn't pay much attention to the header. I thought the OP was asking why moths sleep during the day if they are attracted to light not how come they can sleep during the day Nil Einne (talk) 21:41, 7 June 2010 (UTC)[reply]
AFAIK, nocturnal moths are not attracted to light; on the contrary, if you wake one during the day it will fly straight to the nearest shade and go back to "sleep". Moths do use the moonlight to navigate, so the streetlights simply throw their navigation equipment off. Circadian rhythms and insect navigation are both fascinating subjects, and, as you can see by the color of the links, the prior is better understood than the latter :) --Dr Dima (talk) 20:40, 7 June 2010 (UTC)[reply]
Thanks Nil u really understood the question correctly. but not at first. simply the question is
why not moths go flying towards sun in the morning? as per the first article suggested it could be becoz they want to hide from predators in light, then why not they should hide from them in night. going near light will make them a vulnerable prey.--Myownid420 (talk) 05:03, 8 June 2010 (UTC)[reply]
It's interesting about using the moon (which I had never heard and have to read about), Dr Dima, because I thought we always use random-walk Levy flights to model, for example, flies. Interestingly, on a very macro scale, human movement is also modelled by a Levy flight (though along specific paths), which is philosophically nuts if you think about it. Then again, maybe it's a failure of trivialness at some scale of the Levy flight model? Ugh, last thing I need is another research topic. SamuelRiv (talk) 16:21, 8 June 2010 (UTC)[reply]
Hasn't that already been answered? Moths evolved to sleep during the day so that's what they do, probably a combination of their circadian rythms and other factors. As for Hsaio theory, I don't really know how to explain more then what's in article. You may want to listen to the source. Bear in mind in flying to the light you are discussing human artificial light sources, which moths wouldn't have evolved to properly respond to, part of the complexity is trying to understand what response to natural light sources is causing moths to behave in such a way to artificial light sources. Nil Einne (talk) 18:51, 9 June 2010 (UTC)[reply]
In nuclear fission, "It was clear to a number of scientists at Columbia that they should try to detect the energy released in the nuclear fission of uranium from neutron bombardment. On 25 January 1939, a Columbia University team conducted the first nuclear fission experiment in the United States,[18] which was done in the basement of Pupin Hall; the members of the team were Herbert L. Anderson, Eugene T. Booth, John R. Dunning, Enrico Fermi, G. Norris Glasoe, and Francis G. Slack. The next day, the Fifth Washington Conference on Theoretical Physics began in Washington, D.C. under the joint auspices of the George Washington University and the Carnegie Institution of Washington. There, the news on nuclear fission was spread even further, which fostered many more experimental demonstrations.[19]"
In Chicago Pile-1, "Chicago Pile-1 (CP-1) was the world's first artificial nuclear reactor.[4] CP-1 was built on a rackets court, under the abandoned west stands of the original Alonzo Stagg Field stadium, at the University of Chicago. The first artificial, self-sustaining, nuclear chain reaction was initiated within CP-1, on December 2, 1942."
So, my question is Was the Columbia experiment not artificial, not self-sustaining, or not a chain reaction? or some combination of those? I ask because it seems like to fission uranium, there would have to be a chain reaction, but I"m not clear on the self-sustaining part.
Columbia was not self-sustaining. It was just a small experiment to detect energy released from fission. CP-1 was a real reactor, where the fission reaction is critical (self-replenishing). --Mr.98 (talk) 17:59, 7 June 2010 (UTC)[reply]
Just to elaborate a bit more: the Columbia experiment involved coating an ionization chamber with uranium oxide, and then putting a neutron source of known intensity inside of it, and then measured the energy set off by the fission of U-235 (in the form of recoil of the fission fragments). This provided direct experimental evidence confirming both the Hahn-Meitner observations and explanation of fission, as well as the Bohr elaborations (in particular, the U-235 was responsible for fission, not U-238). This is basically a desk-top experiment, just showing that the theory of fission seemed to be true. It was not self-sustaining at all—fission happened only when the neutron source was applied, and stopped immediately as it was removed. There was no critical mass. CP-1, by contrast, was a room-sized apparatus of uranium oxide embedded in blocks of graphite, arranged in a critical mass, with the graphite serving as a moderator to slow down the resultant neutrons so that they could more easily be used for future reactions. It went critical—meaning it could have kept exponentially reacting indefinitely (until it melted itself into the floor) had they not stopped the reaction after a few minutes. Each fission reaction produced more than one secondary fission reactions on average. Does that clarify the differences and the significance of each? The Columbia thing was important but not world-changing—it just confirmed what was already expected to happen, and gave some more precise understanding of it. The Chicago experiment was a bigger deal—it proved that reactors could be built. --Mr.98 (talk) 21:20, 7 June 2010 (UTC)[reply]
yes it's sleeve - I'd imagine if you install the light outside the supply will come through a wall - the cable is probably plastic or rubber so a sleeve would prevent any abrasion on the wire when it passes through rough brick or stone.
Also is it possible that the sleeves are heat resistance (woven glass?) and are intended for use on the inside of the light fitting were it may get warm - I'm clutching at straws.
Also where the cable exits the bulkhead light though a conduit -if it's bent or curved- may help the cable pass through (reduced friction)77.86.124.76 (talk) 19:48, 7 June 2010 (UTC)[reply]
(ec)As you say, sleeving, but I would (also) suggest it is some sort of 'high' temperature sleeving. I have seen (wp:OR) similar come with halogen downlights. It may be to protect wiring from the heat of the lamp? --220.101(talk)Contribs19:56, 7 June 2010 (UTC)[reply]
Yes, heat-resistant sleeving. If the plastic insulation on normal wiring gets hot, it can melt and short either to earth or to the other wire. This sleeving protects against this eventuality by maintaining insulation even at high temperatures. Dbfirs20:52, 7 June 2010 (UTC)[reply]
I expect that this only happens in certain situations, but when I took this photo, the autofocus only operated up to a certain zoom, after which it could not focus anymore. Switching to manual focus, I seemed to reach the maximum in either direction without focusing the image. The camera is a Panasonic HDC SD60 but I'm not sure this is relevant. Should I have opened or closed the iris? --78.148.138.14 (talk) 18:49, 7 June 2010 (UTC)[reply]
Aye, it has a macro setting. There was a picture of a flower than flashed when I zoomed in close enough. Unfortunately, I don't believe that my camera was designed to accept alternative lenses. The camera was about 30-40 cm from the article and then zoomed as far as it could maintain focus. I would like to understand what was limiting its capacity to focus, but if it involves 30 different lenses and prisms jostling around, I should probably just forget about it :( --78.148.138.14 (talk) 19:14, 7 June 2010 (UTC)[reply]
That's normal. Just like your eyes (and for the same reason, roughly speaking) a camera cannot focus closer than a certain distance because there is a limit to the maximum power of the compound lens (if you have two lenses, the power increases as they move closer together, and obviously you can't get closer together than touching, so that is the maximum). Changing the zoom changes that distance. I don't think there is anything you can do about it other than change the lens (if the camera has changeable lenses - I haven't looked up that model). See Lens_(optics)#Compound_lenses for some more information. --Tango (talk) 19:17, 7 June 2010 (UTC)[reply]
I should clarify - your camera may well have more than two lenses, but the principle is the same (the maths is just more complicated). --Tango (talk) 19:19, 7 June 2010 (UTC)[reply]
According to the manual [5] (p.39) macro mode can focus to distances down to 1-4cm (wide angle) or 70cm (telephoto) (it has a zoom lens) - did you try adjusting the zoom?87.102.17.246 (talk) —Preceding undated comment added 20:19, 7 June 2010 (UTC).[reply]
and select "intelligent macro mode" whatever that is.
I was looking for info and found this youtube video of your video camera/camera in macro mode [6] really nice video - the worms are fantastic - nice camera - I recommend you all click on it if you're ok with creepy-crawlies.87.102.17.246 (talk) 20:25, 7 June 2010 (UTC)[reply]
Et3N seems a worse leaving group than Cl-. After all Et3N is a nucleophilic catalyst but Cl- is not. Or is that R4N+ has repulsion with the partial positive charge on the carbonyl, allowing the Cl- to reattack and displace the tertiary amine? So actually, shouldn't the tertiary amine react, but not stay on for very long? John Riemann Soong (talk) 18:52, 7 June 2010 (UTC)[reply]
They do react - eg
MeCoCl + NMe3 >>> MeC(O)N+Me3 Cl-
It's rare to (try to) isolated these, but simple tertiary amines are used to 'activate' acyl chlorides , and as an intermediate step when the acylation of an acid sensitive (or HCl sensitive) substance is attempted.
Okay because some resources appear to state that tertiary amines "don't react with acyl chlorides" and use them as a means of distinguishing them from secondary and primary amines. John Riemann Soong (talk) 19:46, 7 June 2010 (UTC)[reply]
Yes, they do say that - of course any compound formed won't survive an aqueous work-up. - you'll get the tertiary amine back (as a salt) - so in that sense it is true.
Okay, for a pendulum to have a period independent of its amplitude, the curve of the pendulum's bob must follow a cycloid. How was Huygens able to change the path that pendulum took? I would imagine that he had curves near the pivot so that as the pendulum swung, it would wrap around it. But what shape would these curves have to be? Would they be cycloids too? 173.179.59.66 (talk) 19:37, 7 June 2010 (UTC)[reply]
The period of a circular-arc pendulum is approximately independent of amplitude for small amplitudes. Huygens calculated that the cycloid was the ideal curve, and he had pendulum clocks built, but I don't know whether he bothered with the tiny curve corrections because, in practice, pendulum clocks usually have a fairly constant and small amplitude. Dbfirs20:22, 7 June 2010 (UTC)[reply]
Does it make any difference to accuracy? I would think that stability of the suspension and temperature correction would be much more significant in real clocks. Huygens seems to have been an exceptionally talented scientist so he may well have tested out the design (or got his clockmaker to try it out - he wasn't very practical). If he did, then I suspect that he found that it wasn't worth the trouble. Has anyone access to any of his research papers? Dbfirs21:02, 7 June 2010 (UTC)[reply]
A spring is used to support the pendulum on some clocks to compensate pretty accurately for this problem. Some early clocks used cycloidal jaws but they were more trouble than they were worth. Dmcq (talk) 22:47, 7 June 2010 (UTC)[reply]
Thanks, though I don't see what "problem" needs compensating for if the amplitude is kept small and almost constant. I would have thought that a perfectly rigid support would be the way to improve accuracy. Dbfirs00:17, 8 June 2010 (UTC)[reply]
OP: Yeah I know that he ended up abandoning the cycloid thing. My question was more of a mathematical one. Is there an easy way to see that cycloidal jaws maked the bob move in a cycloid? 173.179.59.66 (talk) 03:04, 8 June 2010 (UTC)[reply]
Perhaps I'm imagining this the wrong way, but placing any curve under the pivot would, the way I imagine it, increase the "problem" by causing the bob to move in a tighter curve, not the flatter curve of a cycloid. Allowing the pivot to move sideways would introduce additional inaccuracies greater than those eliminated wouldn't it? Is my intuition wrong, or am I missing something that is obvious to others? Can anyone draw or link a picture? Dbfirs08:36, 8 June 2010 (UTC)[reply]
Thanks for the link which confirms my intuition on accuracy, but yes, I had wrongly assumed the same radius. Dbfirs15:23, 8 June 2010 (UTC)[reply]
1) @Dbfirs: I'm not 100% sure, but I think that a cycloid is tigher in this case, because you're comparing it to a semi-cirle of twice the radius of the generating circle in the cycloid...I think. In any case, the equation for the motion of the pendulum is d2x/dt + ω2sinθ = 0. If sinθ becomes θ, then the pendulum moves with simple harmonic motion. But sinθ < θ, so the time it takes for the bob to return to the equilibrium point would increases with increasing amplitude. But by letting ω increase with amplitude, this can "offset" the deviation of sinθ from θ. Because ω2 = g/L, L being the distance of the bob from the pivot, we require that L decrease with increasing amplitude, and thus the curve must be tighter.
2) @Dmcq: Yeah, I understand why the bob has to move in a cycloid, but why would cycloidal jaws cause the bob to move in a cycloid? After-all, semi-circular curves would not make the bob move in a semi-circle, right? 173.179.59.66 (talk) 14:14, 8 June 2010 (UTC)[reply]
I thought that the melting point of an ionic solid would be determined by the difference in electronegativity between the two elements composing the salt. But even though each alkali metal and alkaline earth metal halide follows the rule, some other ones don't. For example, why isn't beryllium oxide low melting because it is more covalent than caesium oxide, which is very ionic? --Chemicalinterest (talk) 20:21, 7 June 2010 (UTC)[reply]
The main factors are:
Size (mp increases with decreasing size due to inverse relationship between force and electric charge at distance)
Charge - higher charge means higher mp.
[9] -see section "The physical properties of sodium chloride" which repeats what I said above.
Fusion is clearly a process that requires phenomenally high temperatures to work. How the hell have scientists managed to, or at least planned to, contain the plasma? I would have thought any conceivable metal unable to withstand such temperatures, but clearly I'm mistaken. Is the plasma prevented from touching the walls of the chamber, and conducted in a vacuum to prevent direct transmission of heat, or are there other components at work? Regards, --—Cyclonenim | Chat 23:20, 7 June 2010 (UTC)[reply]
Yes and yes. Hope you don't mind but I'm just going to paste a template in:
In theory you use a "magnetic bottle"—you ionize the plasma and then contain it with an electromagnetic field, so it doesn't touch anything. In practice, you try very hard to make a stable magnetic bottle, and it inevitably leaks out heat and is very hard to keep stable and the plasma generally leaks out. A plasma physicist friend once described it as trying to push all of the water from one side of your tub to the other by just using your bare hands—the plasma generally finds a way through. This is basically why controlled fusion (that is, outside of H-bombs) has proven so difficult and elusive. The current front-runner of this type is the ITER program, where they are trying to overcome the instabilities of the bottle by just making it huge. Whether it will work or not (to the point of achieving more energy from the fusion reactions than it takes to start the reaction) is not clear (scientists disagree).
The other main approach to fusion, inertial confinement fusion, doesn't use any kind of confinement at all. You basically just try to start up a fusion reaction inside of a big chamber, and then catch the energy that shoots out from it. The trick here is that starting controlled fusion reactions in small spaces in a short amount of time is not easy. The current front-runner of this type, the NIF, uses a set of gignormous lasers to try and ignite fusion reactions in a capsule the size of a pinhead. Whether it will achieve the "break-even" point is also not known, but it seems reasonably likely that it will happen. (Whether either of these will ever prove to be an economical way to generate energy is a separate, and far more problematic, question.) --Mr.98 (talk) 00:16, 8 June 2010 (UTC)[reply]
Many things get darker when they get wet. Many types of cloth, wood, concrete just to name a few. I don't know what the phenomenon is called. --Chemicalinterest (talk) 00:09, 8 June 2010 (UTC)[reply]
You are in a spaceship without any windows or external instruments. a) You are travelling at say 50% of the speed of light, and are captured into orbit by a massive object such as a star or black hole. Your spaceship traces a sharp curve around it, almost doubling back, and begins to orbit it. Does anything feel different from normal inside the spacecraft? Could you detect that your direction has changed or feel any force from the change in direction? b) You fall into a black hole. Does anything feel different from normal inside the spacecraft, or are you unaware of this happening? 92.28.247.252 (talk) 23:35, 7 June 2010 (UTC)[reply]
You wouldn't feel the gravity directly, but you might feel the tidal forces, particularly from a black hole. Incidentally, you won't usually go into orbit about a massive object just by passing it - you would have to decelerate in order to be captured. In order to end up in a periodic orbit without thrusting, you have to have started out in a periodic orbit. Oh, and you couldn't orbit a star at 50% the speed of light - the orbit an object the mass of the sun at 0.5c you would need to have an orbital radius of about 6km, which is much much smaller than the sun, so is impossible. If you want to orbit something at those kind of speeds, it needs to be a black hole (or possibly a neutron star). --Tango (talk) 23:49, 7 June 2010 (UTC)[reply]
"Incidentally, you won't usually go into orbit about a massive object just by passing it - you would have to decelerate in order to be captured." Are you sure that is always true? If it is true, then how do planets capture moons? There would be no reason for the would-be moon to deaccelerate. And how does the planet or gravity know that the object has deaccelerated, which would imply it has a memory of some kind? Surely only position mass and instantaneous speed are relevant? 92.28.243.59 (talk) 10:57, 8 June 2010 (UTC)[reply]
Asteroid capture is possible through transfer of energy to an existing moon through gravitational interaction, or loss of energy through impact with an existing moon or planetary ring. In either case a third body is involved. There is an explanation of this somewhere in one of our astronomy articles, but I can't find it right now. Gandalf61 (talk) 11:13, 8 June 2010 (UTC)[reply]
Is it being implied that if you fire something towards a neaby star, either it falls into the star or it just goes past without going into orbit? 92.24.187.9 (talk) 13:45, 8 June 2010 (UTC)[reply]
Yes - unless it interacts with a third body or reduces its momentum in some way e.g. with rockets or a light sail or outgassing. This is discussed in our article about Neptune's moon Triton - see Triton (moon)#Capture.Gandalf61 (talk) 15:21, 8 June 2010 (UTC)[reply]
Yes, only position, mass and instantaneous speed are relevant, but you can work out those values for any other time given them at one time (ignoring quantum effects, which we can safely do). If the current position, mass and current speed are such that the object is in a periodic orbit then we can calculate that it's position, mass and speed at all other times will also be such that it is in a period orbit. The easiest way to do those calculations is to consider energy. If nothing happens to change the energy of the object then all that can happen is energy transfers between kinetic and potential. An object with escape velocity will have higher energy than one in a periodic orbit, so you need to add energy to escape. Exactly the same is true in reverse - an object that has approached from a great distance (ie. along the reverse of an escape velocity) will have more energy than an object in a periodic orbit, so you need to remove energy to be captured. See Specific orbital energy for some details. --Tango (talk) 16:34, 8 June 2010 (UTC)[reply]
Are you really saying that there is no combination of speed (from almost zero to C), mass, or direction, that will put something in orbit without it slowing down? If that is the case, then why should the speed, mass, or direction of an object after its been slowed by a third body somehow magically put it into orbit, when the identical speed mass and direction but without the interaction does not do so? 92.28.249.23 (talk) 19:29, 8 June 2010 (UTC)[reply]
Without the additional interaction, a closed orbital speed is not attainable. Remember that in addition to the velocity the approaching object has by virtue of its approach, it also experiences acceleration due to the gravitational attraction between it and the 'target', as if it had fallen from infinity, giving it an additional velocity equal to that of the escape velocity of the 'target'. No matter how small its initial approach velocity was, this additional free-fall acceleration will always raise its total velocity to more than closed orbital speed, so if it doesn't actually hit the 'target' it will only be diverted into a hyperbolic path (an 'open orbit'), and cannot achieve that "identical speed mass and direction" you mention. Some additional decelerating force is necessary to lower its velocity to that of a possible closed orbit. This could come from the gravitational force of a third object, friction from the local medium (such as dust in local space or the 'target's' atmosphere) external force from local sunlight, or a reaction force from the object itself, such as an engine exhaust or, possibly, a combination of spin and breakup that would fling a small portion of the object away and leave the remainder in orbit - this latter would also apply if it was a mutually orbiting binary object, and indeed some satellites in the Solar System and some multiple stars and 'runaway stars' in the Galaxy are suspected to have attained their orbits or trajectories in this fashion. 87.81.230.195 (talk) 20:49, 8 June 2010 (UTC)[reply]
It couldn't achieve that speed, mass and direction without the interaction. If it has that speed, mass and direction now then it must already be in a periodic orbit and have been in one since the last time it interacted with a third body (or had a rocket burn or similar). --Tango (talk) 00:29, 9 June 2010 (UTC)[reply]
In case you don't know, tidal forces are because the gravity on the side of the spaceship closer to the star is slightly greater than the gravity on the side farther from the star - and you might be able to measure the difference. But otherwise no, you won't be able to tell. And tidal forces are not always very small, see Spaghettification. Ariel. (talk) 06:01, 8 June 2010 (UTC)[reply]
In fact, not only would the tidal forces in that scenario instantly rip you to shreds, they would rip your spacecraft into its constituent subatomic particles. Looie496 (talk) 20:38, 8 June 2010 (UTC)[reply]
![[3]](http://i.imgur.com/ycmvH.jpg){kind=link}
