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In my childhood, I remembered reading a lot of Calvin & Hobbes humor comics. Calvin waited 6 weeks for a Beanie to arrive in his mail, after ordering it through a magazine catalog (IIRC.)
While he waited, he dreamed of flying all through the sky with the beanie, while holding it by the handles.
Then one day, it arrived. He put it together and got it to run. However, the motor wasn't giving it enough of a spin to lift him off the ground. Calvin got enraged, smashed it up, and vented to his tiger pal Hobbes about it.
I had wondered what it would take to make a beanie that actually flies the wearer, but back in the mid '90s, there was no Wikipedia (and its Reference Desk), no Answerbag, no AOL Answers, nor any other Q&A sites I was aware of back in the days of crawling through the "Information Superhighway" on 14.4k modems.
(On what online venue could I have asked this question back in '95-'96 anyway?)
For one, bigger blades; I am not sure the standard propeller blades on a beanie could generate enough Lift to actually pick you up off the ground. You'd also need a motor to drive the blades. Furthermore, helocopter blades will tend to twist you out of alignment; so you'd need a counteracting set of blades; either a second set which rotated in the opposite direction, like the this one or a second blade at right angles to the main ones, like the this one. --Jayron3202:25, 31 December 2010 (UTC)[reply]
But a smaller one. My guess based on some googling is that you need approximately 8 foot blades to lift a human. There are some website that give formulas to calculate the lift of a helicopter. You can't increase the lift forever - once you hit the speed of sound you can't do any better. At lower altitudes you can use smaller blades because the air is thicker, so you need to take into account how high you plan to go. Ariel. (talk) 09:18, 31 December 2010 (UTC)[reply]
I never knew that was actually possible in the laws of physics. Of course, it's got support and a seat for the rider. Looking at the actual C&H strips, you can see it's just a hat with a propeller.
My local news just showed a video of a suspected DUI traffic stop in which the driver failed in his attempt to walk a straight line — however, after stumbling and giving up, he proceeded to turn a cartwheel. How is it that alcohol can prevent someone from walking straight but not from maintaining the balance and agility necessary to avoid falling over like I do when I try to turn a cartwheel while sober? Nyttend (talk) 04:09, 31 December 2010 (UTC)[reply]
The issue is that DUI tests are standardized; they need to be to be admissable as evidence in a court of law. Officers perform the same set of standardized tests, and videotape the evidence. Turning cartwheels isn't a part of the standardized set of DUI tests in any state I know of. So the fact that one could do so doesn't make any difference in establishing reasonable suspicion to arrest someone for DUI. If litigated in court, I suppose that the defendent could perhaps use such cartwheel as evidence that he was sober enough to drive; but that wouldn't mean that the officer at the scene would be wrong for arresting the person for DUI if they failed the standardized tests. They may argue to get their license back based on that evidence, but that has no bearing on how the arresting officer should act at the scene. The officer should always do it "by the book": administer the standard tests, and if the standard tests are failed, place them under arrest. Let the courts deal with anything that comes later. --Jayron3204:24, 31 December 2010 (UTC)[reply]
I wasn't trying to question the claim that he was drunk; the rest of his behavior made it obvious that he was very much under the influence. My question asked how he could perform a cartwheel despite being unable to do much simpler things. Nyttend (talk) 05:04, 31 December 2010 (UTC)[reply]
Balance depends heavily on feedback-driven control, which is particularly impaired by alcohol. A cartwheel is a complex motor sequence, but it doesn't depend very heavily on feedback. Looie496 (talk) 04:59, 31 December 2010 (UTC)[reply]
Can you explain what you mean by "feedback"? I've read the "Biology" section of our feedback article; its final paragraph somewhat resembles what I think you mean, but it seems somewhat irrelevant, so I suspect that you mean something different. Nyttend (talk) 05:08, 31 December 2010 (UTC)[reply]
In order to balance, your body needs to get information about where it is now and use that information to maintain that position. So your brain analyzes a whole shitload of information; some from your inner ear, some from the tension on your muscles, some from your vision, to determine where you are in relation to, say, the ground, and then based on that information, it makes minute adjustments to your muscles to keep you standing straight and balanced. Alcohol short-circuits that feedback, so while you may be fully in control of your muscles while drunk, your brain is receiving bad information about your "location" in the world, so it cannot easily autocorrect if you are falling. That's why drunks fall over; not because they don't have muscular control, but rather because their automatic balancing system is receiving fuzzy information. --Jayron3205:15, 31 December 2010 (UTC)[reply]
... and safe driving relies on this "feedback", so the walk test is better than the cartwheel test. Even drunk drivers can make a car cartwheel! Dbfirs10:16, 31 December 2010 (UTC)[reply]
... and turning a cartwheel cannot be performed by anyone able to drive, so it has no value as a test, since a DUI test should be positive for all (or most) drunk drivers and negative for all (or most) non-drunk drivers. However, why put people to walk? In Europe, as far as a I know, they always use a breathalyzer. If you want, you can request an additional, more precise blood testing. Quest09 (talk) 20:51, 31 December 2010 (UTC)[reply]
Not to say any of the above comments are wrong, but to answer the question a bit more directly, the "biology" part of the feedback article doesn't really cover its role in motor control -- the "control theory" section is more relevant. Basically feedback control of a movement means noting errors in the execution and adjusting the movement to reduce them; feedforward control means generating the correct muscle activity before any useful feedback comes in. In a cartwheel, by the time you have noticed that you screwed up, it is too late to do anything about it. (The feedback is however useful for learning -- if you get something wrong, you can try to change it on the next attempt.) Looie496 (talk) 23:47, 31 December 2010 (UTC)[reply]
The article Universe says that "The universe is very large and possibly infinite in volume.". However, this seems at odds with logical thinking.
The universe is expanding at a very high speed, but not at an infinite speed. Let's call this speed x
The universe has been expanding for a very long period of time, but not an infinite period of time. Let's call this time t.
Therefore, the current size of the universe should be a direct function of t and x. If neither x nor t are infinite, how can the size of the universe be infinite at any given moment? Thanks. Leptictidium (mt) 07:43, 31 December 2010 (UTC)[reply]
I'm not a cosmologist, but my general understanding is that if the universe is infinite in extent now, then it always has been. It's often claimed that the big bang means that the universe sprang from a single point, but that is not in fact a necessary part of the theory. If you run the equations backwards you get to a moment when the universe was infinitely dense, but it did not necessarily have zero volume. --Trovatore (talk) 07:55, 31 December 2010 (UTC)[reply]
If the infant universe had infinite volume and infinite density, then it must have had infinite mass (since density is a function of mass and volume). However, our present universe does not have infinite density, so where did all the mass go? Leptictidium (mt) 08:17, 31 December 2010 (UTC)[reply]
If it has infinite volume now, and density finite but bounded away from zero (that is, there is some nonzero density such that every part of the universe is at least that dense), then it also has infinite mass now. So why did it have to go anywhere?
For one thing, the big bang wasn't necessarily the whole universe, it could have just been a portion, and there may have been many other big bangs, far away. We wouldn't know about these other big bangs unless one of them expands into ours. StuRat (talk) 08:45, 31 December 2010 (UTC)[reply]
I recently completed a second year cosmology paper, and what I was told is that the universe is not infinite, neither does it have an edge. The best definition of the edge of the universe you can get is halfway between the centre of the universe and itself. In other words, if you had a ridiculously long pole, growing longer at the same rate as which the universe is expanding, you should be able to tap yourself on the shoulder. --Plasmic Physics (talk) 09:10, 31 December 2010 (UTC)[reply]
Well, you can write down models where space is finite in total area without "ending" anywhere. The surface of the Earth is also like that. There's no central point on the surface of the Earth, and any point on the surface could be described as "halfway between the antipodal point and itself", though it's probably better not to describe it that way. There's no evidence favoring those models over other models, but they are at least consistent with the available evidence. -- BenRG (talk) 10:32, 31 December 2010 (UTC)[reply]
What cosmologists mean by the "total size of the universe" and the "expansion of the universe" doesn't exactly match the everyday meaning of the words, in part because in general relativity the distinction between space and time is a bit fuzzy. Shown to the right is the Milne universe, which is basically a simplified (numerically inaccurate) version of standard cosmology that's easier to visualize in a 2D diagram. This model is spatially infinite at all times, in the sense meant by cosmologists. See the previous Ref Desk threads Light from just after the Big Bang and Infinite amount of mass in the universe? where I mention the Milne model, and Speed in space which has a more accurate picture. And also see Ned Wright's cosmology tutorial. -- BenRG (talk) 10:32, 31 December 2010 (UTC)[reply]
@Leptictidium: You asked why we have finite density now if we had infinite density in the past. It is important to realise that this question is actually the wrong way round (no blame - I make that kind of mistake too often myself). We do not know that the density was infinite in the past; what we do know is that now it is finite. The "correct" question would then be why we had infinite density in the past when we have finite density now, and we can answer that one. We also know that the universe is expanding (i.e. mean density is decreasing) and we have a theory for that expansion. Using that theory and our knowledge about the present state of the universe as initial condition, we can determine how the mean density of the universe evolved in the past. Doing that we find that the density increased (going backwards in time) without bound. The point where the density formally diverges (becomes infinite) is called the "Big Bang". The Big Bang is just that: an extrapolation backwards in time. However, we also know that our theory breaks down at some point when the mean density becomes larger than what we can handle with our knowledge of quantum theory and general relativity. Hence, the big bang is a simple extrapolation beyond our secure knowledge of physics. Unfortunately, the big bang is, especially in popular science books and documentaries, often represented as a secure fact and it is way overstressed. The initial question, whether the universe is finite or infinite, is not answered yet. We do have some constraints on the minimum possible size (if it is finite), nothing more. There is a nice book about this topic (the topology of the universe) by Jean-Pierre Luminet, which is available in English, I think. --Wrongfilter (talk) 11:45, 31 December 2010 (UTC)[reply]
The way I see it, if the universe were expanding at the speed of light in all directions, and we are correct that it is impossible to exceed the speed of light, then for all intents and purposes, the universe would be infinite since you can never get to any edge. Googlemeister (talk) 15:07, 31 December 2010 (UTC)[reply]
The universe is not expanding at the speed of light. The rate of increase of distances (which is not a velocity!) is proportional to the distance (suitably defined) and is not limited by the speed of light. --Wrongfilter (talk) 15:35, 31 December 2010 (UTC)[reply]
The connection with the visible universe is not correct. The rate of increase of the distance between two objects (they are not "travelling" - where would they travel to?) is equal to 300000 km/s (the speed of light) when they are at a distance of 4.28 Gpc (for H0=70 km/s/Mpc), that corresponds to a redshift of 1.45 (per Ned Wright's cosmology calculator), nowhere near the horizon. The speed of light has nothing to do with the metric expansion of space. --Wrongfilter (talk) 18:03, 1 January 2011 (UTC)[reply]
Expanding on Wrongfilter's reply: in the Milne universe, shown above, there's no upper limit to the cosmological recession speeds, even though the special relativistic speeds of the very same objects are all less than c. The cosmological recession speed in this case is equivalent to rapidity (times c, since rapidity is usually given as a unitless number). A rapidity of 1 (or c) has no special significance. It's like an angle of 1 radian. -- BenRG (talk) 01:31, 2 January 2011 (UTC)[reply]
If I evaporated a solution of antimony trichloride in a solution of concentrated hydrochloric acid, would the antimony trichloride precipitate as a solid? --Chemicalinterest (talk) 12:47, 31 December 2010 (UTC)[reply]
Most of the chlorides if produced by this way would give the oxide and hydrochloric acid. I have to have a look for the example of antimony trichloride, but I doubt that the compound is stable in water at all.--Stone (talk) 13:34, 31 December 2010 (UTC)[reply]
The group 15 elements form a huge range of oxohalides as well. SbOCl is the simplest example in this system, but there are many others. Also, SbCl3 is described as deliquescent, so you are not going to be able to form a solid by evaporation at ambient temperature and pressure. Raising the temperature or lowering the pressure will drive off HCl before it drives off water, and so promote hydrolysis of the chloride to the oxochloride. In short, no. Physchim62(talk)16:05, 31 December 2010 (UTC)[reply]
Sorry! I have to correct myself. Butter of Antimony [1] is the very old name of the compound. Best produced by dissolving antimonsulfid in conc. hydrochloric acid. When always an excess of hydrochloric acid is present the liquid can be evaporated. You get a white substance of waxy consistence ( therefore called Butter of Antimony). If the solution of Antimony trichloride in water is diluted in water a complicated mixture of oxides is formed which at the end will be antimony oxide.--Stone (talk) 17:16, 31 December 2010 (UTC)[reply]
I have a small mains-powered transistor radio, which had an aerial wire sticking out of the end. The aerial wire has now broken off and I will need to re-attach it, or bin it. The wire was 970mm long. I mostly use it to listen to a radio channel on about 94Mhz, FM. Where I am, reception is not very good. What would be the best length for the new wire aerial? If 970mm was the best length, would there be any advantage in having the wire a multiple of this length, such as twice, thrice, four times, etc? A particular problem I have is that another more intense signal is right next on the dial to what I want to listen to. I still get some reception without the wire attached, but it is not as good. Please do not suggest buying a DAB radio as the signal is not good enough here, regretably. Thanks 92.24.176.169 (talk) 19:45, 31 December 2010 (UTC)[reply]
Under ideal conditions the best length, a quarter wavelength with some adjustment for end effects, would be about 760 mm. It sounds like your biggest problem is the stronger station, and a longer wire would just make the strong station stronger. If the weak station and the strong station are in different directions, try putting some kind of metal screen in the direction of the strong station. Jc3s5h (talk) 20:00, 31 December 2010 (UTC)[reply]
Odd multiples will match the impedance of the input stage better, an even multiple will have a high impedance. However boosting the input will overload the radio more and increaase the problem due to the interfering station. If you have the space a dipole antenna connected via a balun to the antenna input will get you more control. Match the polarization and rotate to minimise the interfering station. Graeme Bartlett (talk) 22:08, 31 December 2010 (UTC)[reply]
The aerial has only one connection, I would not know where to connect the other half of a dipole. From what I remember of other radios, dipole antennas for VHF are not very big. If the interfereing channel is ignored, would for example a 3/4 wavelength monopole be better than a 1/4 wavelengtn monopole? 92.24.185.151 (talk) 23:23, 31 December 2010 (UTC)[reply]
Please explain why a 1/4 wavelength aeriel will be better than an aerial three times as long as this, I'm sceptical. The longer aeriel will collect three times the signal energy. (And three times the signal energy is nothing compared to the higher signal intensity of someone living nearer the transmitter, so the radio should be easily able to cope with it). The Wikipedia antenna article also says: "The random wire antenna is simply a very long (at least one quarter wavelength) wire with one end connected to the radio and the other in free space, arranged in any way most convenient for the space available. Folding will reduce effectiveness and make theoretical analysis extremely difficult. (The added length helps more than the folding typically hurts.)". That says nothing about a quarter walevlength being best, and the links I've read also say nothing about a quarter wavelength being best, although warning against using a half-wavelength or multiples of that. 92.15.14.57 (talk) 14:46, 1 January 2011 (UTC)[reply]
The thing about a random wire antenna is for cases where a 1/4 wavelength is impossible to string - it's just too long. So instead a wire is placed randomly. If you actually can run the wire to the correct length then do that. Ariel. (talk) 23:50, 1 January 2011 (UTC)[reply]
I'm getting a little weary of your sarcasm and argumetativeness. Volunteers have answered your questions with detailed and accurate answers. Please show some politeness to them, even if you don't like the answers. If you would rather follow your own advice, that is ok with us too. SpinningSpark14:21, 2 January 2011 (UTC)[reply]
I'm also wondering where the impoliteness is? Would you like to give excerpts of the text that you consider sarcastic, argumentative, or impolite? 92.15.31.128 (talk) 12:17, 3 January 2011 (UTC)[reply]
Where's the sarcasm? Where's the argumentativeness? Perhaps you mistake my asking why a 1/4 wavelength aerial would be better than the larger 3/4 wavelength, to which I've had zero answers. Maybe you also mistake my pleasant surprise at discovering fractal aerials, as they may offer a compact solution that's better than a length of wire because it gets a more intense signal, can cover a wide range of wavelenths, and does not need any extra aerial circuitry. If Maplins sold one like the ones they have in mobile phones, I'd buy one and try it out. 92.28.255.126 (talk) 19:49, 2 January 2011 (UTC)[reply]
A ideal 1/4 wave antenna need to be straight, in line with the polarisation of the wave, have a infinite conducting plane perpendicular to its base, a purely resistive load of 36.5 ohm and no other conducting or dielectric material in its vicinity.
Since this is not likely to be the case 1/4, 1/2, 3/4 wavelength and so on are irrelevant. 1/4 of a wavelength can give you an idea of a suitable length, much shorter will not be as efficient and the inductance of a much longer antenna will normally prevent it from being much better. Other than that it is trial and error to find a good length and placement. It can of curse be calculated but the complexity of a typical home environment, will require an advanced computer model.--Gr8xoz (talk) 11:40, 3 January 2011 (UTC)[reply]
I did not say that it is worse, I did say it is not much better. You could think of it as a elastic rubber band floating on the water surface, wave energy picked up at the far end will mostly be re-emitted in to the water before it reaches you. The inductance is similar to the elasticity of the rubber. This is not an exact analogy but it shows the principle. A longer antenna has a more complex sensitivity pattern and can be more directional but since it is hard to predict the direction of sensitivity that is not an advantage. --Gr8xoz (talk) 12:55, 3 January 2011 (UTC)[reply]
In any superheterodyne receiver you have a trade off between sensitivity and selectivity. The laws of physics prevents you from having the best of both. Asking why a 1/4 wavelength antenna would be better than the larger 3/4 wavelength antenna when the receiver gets influenced differently by each of them is like asking which is better Bush or Obama? Both have their own properties and a specific scenario in which each of them will be the best choice. There is no one-size fits all solution to your question. For your small mains-powered consumer transistor radio a 1/4 wavelength antenna would be the best choice to maintain the designed quality factor of the receiving LC circuit and to keep the sensitivity and selectivity within the range that the designer intended. That is why the designer/manufacturer of the radio used a 1/4 wavelength antenna with some extra length to compensate for the lack of a ground plane and the parasitic capacitance introduced by the operator. --Sage.electcon (talk) 07:17, 5 January 2011 (UTC)[reply]
