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

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January 4

Bike mechanics - removal of rear sprocket (single speed - no cassette)

My bike has internal hub gears. The time has come to replace the rear sprocket but I don't know how. I need to remove something. Here is a picture to illustrate the situation. You can see the sprocket, a tube coming from the center (the gear changing pin goes in there) and a big, thick (1cm), round metal thing keeping my sprocket in place against my will. Can anyone identify that metal thing and advise on its extraction? Thx. in advance ----Seans Potato Business 00:58, 4 January 2008 (UTC)

This may be what you need [1] Spinningspark (talk) 10:56, 4 January 2008 (UTC)

What I have is a 'snap ring' and it should be removed with a screw driver (I needed two and a pair of pliers...). There's a picture available here. --Seans Potato Business 21:47, 4 January 2008 (UTC)

Strategic Defense Initiative ("Star Wars")

I would like to know if the theory of "laser beams" from orbiting satellites with the capability of shooting down missiles is even slightly plausible, and if yes, how so? No, I don't plan to build one, I'm just curious as to how it is done, and what exactly the "laser beam" consists of.SDI

EWHS (talk) 13:37, 4 January 2008 (UTC)

You should read the article you linked to: Strategic Defense Initiative. Short opinion/speculation: yes, it's certainly plausible that a space-based laser could shoot down a missile; no, it's not plausible that this could ever win in an arms race against anti-SDI countermeasures. Also check out Boeing YAL-1. --Sean 13:48, 4 January 2008 (UTC)

Thank you. EWHS (talk) 14:19, 4 January 2008 (UTC)

There were a few "laser" plans. Most were just big, conventional lasers. One which got a lot of attention (because it was later discovered to have been vastly oversold) was the x-ray laser, where the x-rays released by a nuclear bomb would be channeled into specific directions. Anyway, the problem with SDI—and all missile defense—is that the economics of it are not very good. It is always going to be cheaper to send up counter-measures than it is to shoot them down; it is always going to be much easier to attack with missiles than it is to defend against them. And even if one did get a reasonably good missile shield in place, the existence of it alone would change the threat landscape quite a bit (e.g. instead of shooting a missile, an enemy would try to smuggle the weapon in, or just do something else). At the moment the US spends a HUGE amount of money on a technology which may or may not ever work as advertised, but even if it does work it only is one small thing in an overall landscape of threats. Personally I think the money could be better spent elsewhere. --24.147.86.187 (talk) 16:24, 4 January 2008 (UTC)

Here's one: not satellites from space but American Airlines is participating in tests of anti-missile lasers installed in commercial airliners link. These are used to confuse shoulder-fired missiles, not destroy ICBMs but show the targeting systems be workable. Rmhermen (talk) 14:37, 7 January 2008 (UTC)

What is the difference between Nernst equation and Nerst's equation?

--78.145.168.125 (talk) 14:18, 4 January 2008 (UTC)

I think the latter was a mistake (misspelled, not a great article). I've changed it to a redirect to the former. -- Coneslayer (talk) 14:35, 4 January 2008 (UTC)

Amazonian insects and fruit for identification

Greetings,

I went to the Amazon Rainforest and took plenty of pictures for Wikipedia. Unfortunately I couldn't identify some of the insects I saw, so I ask for your help. In addition, you're asked to identify or at least classify the yellow fruit locally named Pitabao (or Pitabau). Its flesh has a smooth texture, tasting a bit like Eugenia uniflora, and its seed is expectedly bitter. Thanks, Lior (talk) 14:41, 4 January 2008 (UTC)

• 
  Who am I?
• 
  A particularly annoying big moth
• 
  Same moth
• 
  A butterfly also found in southern Brazil
• 
  A huge tree ant
• 
  Unidentified wasps emerging from an unidentified tree
• 
  The same wasps with a better look on the tree
• 
  Pitabao tree
• 
  Pitabao fruit - whole, peeled and consumed

The "pitabao" look like a palm fruit, possibly Bactris gasipaes. However, your fruits look too small for that species.--Eriastrum (talk) 17:34, 5 January 2008 (UTC) Oops! My bad. I failed to look at your photo of the tree, which is not a palm! It looks like it belongs to the Melastomataceae, because it has three prominent veins on the leaves.--Eriastrum (talk) 17:41, 5 January 2008 (UTC)

How much gas can a candle burn?

Imagine I had a gas cooker with four burners inside a kitchen with the standard gas ventilation measures. If I lit a candle in the kitchen before turning the gas on, and then opened the gas to all four burners at maximum rate;

• would the gas eventually build up in great quantities and explode

• or would the candle burn the gas quicker than it can build up?

I'm not going to commit suicide by blowing myself up or anything, I'm just wondering if the gas explosion in the film Hot Fuzz is possible or just artistic license. Thanks for your answers. -- Leptictidium (mammal talk!) 14:58, 4 January 2008 (UTC)

There would be an explosion once the gas-air mixture reached the candle flame in the critical proportions. The further away from the source, the greater the quantity of gas and the bigger the bang. The candle burns its wax, not the gas.--Shantavira|^{feed me} 15:27, 4 January 2008 (UTC)

Yup. And if the gas burns at all without blowing out the flame, then the fire will travel back from the candle through the gas-filled air and then just burn at the source of the gas. In other words, if there isn't a big enough explosion from the gas reaching the candle to blow out the fire, then you just end up with flame burning at the source(s) of the gas. -- HiEv 23:54, 4 January 2008 (UTC)

I would expect that even if the candle was extinguished... --Seans Potato Business 19:12, 6 January 2008 (UTC)

But a small explosion could put out the flame entirely, so then the gas would just leak without burning. -- HiEv 01:47, 10 January 2008 (UTC)

Gluing plastics together

What's a good glue for gluing plastics together? Super glue doesn't work. 64.236.121.129 (talk) 15:15, 4 January 2008 (UTC)

Not all plastics can be glued. You need to know exactly what you are trying to glue. Then have a look at this chart.--Shantavira|^{feed me} 15:33, 4 January 2008 (UTC)

Depending on the plastic and application you might consider welding it together too. --BozMo talk 12:55, 5 January 2008 (UTC)

Yes indeed, with plastic welding rods.--Shantavira|^{feed me} 17:08, 5 January 2008 (UTC)

Speed of Light

I think that Einstein held that nothing can travel faster than the speeed of light, yet his famous equation states that mass is the equivalent of energy times the speed of light squared. How can one logically square the speed of light in the "real" world? —Preceding unsigned comment added by Lasummrs (talk • contribs) 17:31, 4 January 2008 (UTC)

By multiplying it by itself:

$c^{2}=(299792458~{\rm {m/s}})(299792458~{\rm {m/s}})=8.98755179\times 10^{16}~{\rm {m}}^{2}/{\rm {s}}^{2}$

Seriously, I don't understand the question. It sounds like you're assigning some tangible meaning to "squaring the speed of light" that it doesn't have. It's a mathematical operation. It absolutely does not imply that anything is moving faster than the speed of light, if that's what you're getting at—after you square c, it doesn't even have units of speed anymore. -- Coneslayer (talk) 17:44, 4 January 2008 (UTC)

Note that you square the speed of light in a very equation that tells you you cannot go faster than the speed of light! (see Lorentz factor, which makes it pretty clear that v can never exceed c). As for why squaring the speed of light is so important—the speed of light as a fundamental constant in the universe, and so its no surprise that there is a mathematical relationship between it and many other things related to mass and energy. Doing mathematical manipulations to the speed of light does not itself violate anything—the only problems come in when you try to make a velocity larger than the speed of light. It's a specific constraint. --24.147.86.187 (talk) 19:04, 4 January 2008 (UTC)

1 kg m²/s² is the amount of energy it takes to move an object one meter against a force capable of accelerating one kilogram by one meter per second in one second, thus e=mc² converts between units of mass and units of energy. It has nothing to do with speed or any physical process. — Daniel 19:31, 4 January 2008 (UTC)

Einstein wasn't the first to say that the speed of light is constant. According to Speed_of_light#Special_relativity: 'From the work of James Clerk Maxwell, it was known that the speed of electromagnetic radiation was a constant defined by the electromagnetic properties of the vacuum (permittivity and permeability)'. AndrewWTaylor (talk) 20:14, 4 January 2008 (UTC)

It's worth pointing out what Einstein and earlier researcher's wording actually was. It was not accepted or postulated that the speed of light was constant; rather, the common expression was something along the lines of "the speed of light has the same measure no matter how you measure it," which is what was literally apparent from Maxwell's work. It's a minor distinction, but it's important in that you don't need to assume the speed of light is constant to derive special relativity. You merely need to assume the validity of the laws of physics in all inertial reference frames (and some other random stuff, depending on how nitpicky you are). Someguy1221 (talk) 20:24, 4 January 2008 (UTC)

Thanks for the clarification: I suspected I was on shaky ground with my comment... AndrewWTaylor (talk) 20:57, 4 January 2008 (UTC)

Even more specific, you can say "the speed of light is independent of the speed of that which emitted it", which is obvious if you think of light as being a wave in a medium—the speed of any wave in any medium is independent of the speed of its emitter (it only feels conceptually awkward when you start thinking of it as a particle or as a wave without a medium). --24.147.86.187 (talk) 04:08, 5 January 2008 (UTC)

The idea that light is a wave in a medium is the "luminiferous aether" theory. However, empirical evidence such as the Michelson-Morley experiment shows that the speed of light is also independent of the motion of the observer, which rules out any theory that involves an aether-like medium. Gandalf61 (talk) 08:32, 5 January 2008 (UTC)

"Rules out" might be a bit strong. It's evidence, to be sure, but not entirely dispositive -- you can argue that the Lorentz contraction, by a convenient coincidence, distorts our measurements in a way that just happens to exactly cancel the effects you'd see from the aether. Philosophically less satisfying, maybe, but it predicts the same observations. --Trovatore (talk) 08:51, 5 January 2008 (UTC)

And unfortunately for Lorentz, his hypothesis didn't account for time dilation. And then we get the Lorentz transformation, which just works a whole lot better. Someguy1221 (talk) 17:20, 5 January 2008 (UTC)

Sure, but the point is still the same. There could be an aether (read: preferred frame of reference), and it just so happens that the behavior of yardsticks and clocks conspire to prevent us from detecting its influence. It's not ruled out in principle that the question could someday come up again in a way that is experimentally distinguishible--perhaps there's some sort of interaction, as yet undiscovered, that does not behave in an (apparently) frame-independent way, and that could identify for us the preferred frame. --Trovatore (talk) 21:50, 5 January 2008 (UTC)

One question, though: Could you ever replicate general relativity with an aether theory? I've never been able to do it. Someguy1221 (talk) 08:00, 6 January 2008 (UTC)

Hey, I didn't claim there was a medium, I just said it makes things conceptually easier when thinking of light as a wave (so much so that physicists were kicking and screaming about getting rid of it because it made things conceptually bizarre—where else does one have a wave without a medium?), and in understanding why people accepted without really worrying about it that light acted in a certain way in the pre-Einstein years, that's the way you've got to think about it. And I'm sure Trovatore knows this but that's exactly where the Lorentz contraction originally came from: Lorentz's ingenious fix to preserve the aether in the face of Michelson-Morley. We now, like Einstein, find the idea of an indetectable aether philosophically unnecessary (if it is indetectable, why postulate it?), but we still use the ingenious fix. Funny how history works, no? --24.147.86.187 (talk) 17:22, 5 January 2008 (UTC)

Temperature data archives

Is there a website or database somewhere where I could find daily maximum and minimum temperatures for different locations in the united states, going back at least twenty years? Ideally something where the data could be easy sifted through (i.e. in an excel file or access database)? Thanks —Preceding unsigned comment added by 66.162.50.98 (talk) 20:29, 4 January 2008 (UTC)

Try http://www.ncdc.noaa.gov/oa/mpp/freedata.html —Nricardo (talk) 02:15, 5 January 2008 (UTC)

Photography: closing aperture

Here is a diagram. In the first part, we see what might go on inside a camera aimed at a guy with a hat. When the aperture is closed (second part) in order to limit the amount of light that enters, doesn't that mean that the outer regions are not captured by the camera? --Seans Potato Business 20:43, 4 January 2008 (UTC)

That diagram is erroneous. For a simple lens, which is what's shown, the aperture stop would have to be located at the lens, not behind it. It does not have the effect of reducing the field of view. Rather, for a simple lens, it would have the same effect as making the lens diameter smaller. It would collect less light, and produce a fainter image. (Remember that the diagram is not showing all rays. There's a ray from the subject's head through every point on the lens. Likewise, there's a ray from the subject's feet through every point on the lens. Placing an aperture stop, or making the lens smaller, just means that fewer rays get through—not that part of the scene gets cut off. In the extreme limit, the lens becomes a pinhole camera.) -- Coneslayer (talk) 20:54, 4 January 2008 (UTC)

You might also compare the aperture opening of a camera lens with the iris of a human eye. Notice that when you go from a dimly-lit space to a brightly-lit one the field of view of your eye doesn't change, even though the pupil diameter changes by a factor of five or more. TenOfAllTrades(talk) 21:15, 4 January 2008 (UTC)

Oh, that's great - thank you both! I blame whoever is responsible for AQA A level physics specification... I always said to my teacher that our diagrams didn't make sense (since I figured that light in all directions from any point to all points on the lens just as you explain... in the end I just had to memorise the diagrams without really understanding them :( ----Seans Potato Business 21:52, 4 January 2008 (UTC)

Actually, it is OK for the aperture to be positioned away from the lens, as long as it is the only aperture in the system. Specifically, as long as the lens itself is large enough that it does not act as an aperture (i.e. the edge of the lens does not cut off any ray (from any point within the intended field of view) that wasn't going to be cut off by the other aperture anyway). If you have more than one aperture in a system, you get vignetting: a darkening and eventually cutoff of the edges of the field of view. --mglg_(talk) 22:53, 4 January 2008 (UTC)