Wikipedia:Reference desk/Archives/Science/2012 November 26

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November 26

stereo amplifier output impedance

How do I best go about calculating my amp's output impedance? Should put a given ohm resistor and than just differentiate the voltage drop there? Will this do it?

I need this to figure out its damping factor. — Preceding unsigned comment added by 77.35.20.143 (talk) 03:14, 26 November 2012 (UTC)

Firstly, be advised that damping factor is a figure often quoted, but in solid state amplifiers, and most valve/tube amplifiers with any pretence of quality, it is quite unimportant and just about meaningless. It was important in valve amplifiers with low or zero negative feedback, especially with loudspeakers designed before the late 1960's as such speakers had low internal damping. Modern speakers are pretty heavily damped internally.

Damping factor is most easily measured this way:-

1. Short one input channel of the amplifier, or if an integrated (ie combines the power and preamp) turn the balance control control to zero that channel.

2. Connect an 8 ohm power resistor (or whatever value is equal to the nominal speaker impedance) between the two "hot" outputs of the two channels (assuming a stereo amplifier - if an mono amplifier, borrow another one)

3. Connect a signal source (preferably an oscillator but an AM radio program will do to get an approximate value) to the unshorted channel, and adjust the level for several volts across the resistor, without overloading the signal channel.

+++ This means you have one channel driving the other channel backwards via the 8 ohm resistor. No harm will come from this provided the signal channel does not overload the driven channel. This will not occur if the two channels of one amp are used or two mono amplifiers are used of the same power output. +++

Measure the voltage with respect to ground of the output terminal of the back-driven amplifier channel. An oscilloscope is best but a multimeter will do. If the signal is not an oscillator, use an analog multimeter - do not attempt to get sensible readings with a digital multimeter.

The damping factor is the ratio of the resistor voltage to the back-driven amplifier output voltage. With modern amplifiers the ratio will be very high - you may have trouble measuring the back-driven voltage with a multiometer due to it being very low. The output impedance is the resistor value divided by the ratio.

IMPORTANT - DO NOT overdrive the amplifiers, as if you do, the readings will be meaningless.

Keit 120.145.149.167 (talk) 03:43, 26 November 2012 (UTC)

Which caulking should I use?

I have to caulk around a ceiling light fixture, I decide to use silicone caulk for the job, since it is high heat. I took a laser thermometer in the area I need to caulk and its 102° with the light on. I have previously used a particular type and brand of silicone caulk on many jobs and was happy with the results, it is the GE 2 silicone kitchen and bathroom caulk, it has a mold inhibitor and it but when I like most about it is that it has very little smell to it. When drying, some of the caulks have a horrible acetic acid smell to them which I want to avoid. My concern is that if I use this caulking, the mold inhibitor may get released into the air when it gets heated up. I looked into some of the "food safe" silicone caulk but it seems to be the type that smells like acetic acid, I'm concerned with the acidic acid type is that when it gets heated up, it may release that acetic acid smell again. I am going to link the MSDS sheets of both types of caulk I'm considering to use and I would like you to tell me which one I should use.

The Ge 2 kitchen and bathroom 3 hour dry http://www.homedepot.com/catalog/pdfImages/80/803b3b37-83d5-40b0-a182-9e5eba6d304c.pdf

The food safe one DAP Household/Aquarium Adhesive Sealant http://www.homedepot.com/catalog/pdfImages/a1/a12dd342-15df-4ca3-92fc-8204bdf74281.pdf?cm_mmc=seo%7Caltruik%7C100128841 --Wrk678 (talk) 03:37, 26 November 2012 (UTC)

Let's back up a bit. Why exactly do you want to caulk around a ceiling light fixture ? To keep bugs out ? This isn't normally done. For one thing, won't this be a problem when you need to change the bulbs ? Moisture might also be trapped inside, if it evaporates from the caulk as it cures. And you need to allow the fixture to expand and contract as it heats up and cools down. Another concern is that this may allow heat to build up more than it would if left uncaulked. StuRat (talk) 05:18, 26 November 2012 (UTC)

The acetic acid is only produced as the caulk cures, and not later. It also becomes soft when heated up a lot. Polypipe Wrangler (talk) 10:53, 28 November 2012 (UTC)

Power spectral density of speech

What is the energy distribution of human speech like on the frequency spectrum? Is there freely available data of the form

On average, X% of the energy in speech lies below frequency Y Hz.

Thanks. — Preceding unsigned comment added by 98.114.41.201 (talk) 04:41, 26 November 2012 (UTC)

Unfortunately it's far too variable for any simple statements like that to be useful -- it depends on the voice of the speaker, the language being spoken, and even the specific content of speech. Looie496 (talk) 05:26, 26 November 2012 (UTC)

If you google power spectral density of speech, you'll get quite a range of papers describing it. As this paper [1] and others show a lot of the information is in the frequency range 1000 to 2500 Hz band, but this band contains very little of the total energy. Most of the energy lies in three distinct bands in the range 100 to 500 Hz (which the actual band centres depending on the speaker's sex and size parameters). While the lower two of these bands sets the voice quality, only the upper of these three bands contains significant information. As is weel known in the telecoms industry, you need a system frequency response essentially flat from about 70 Hz to about 5 kHz to get a natural sounding reproduction of almost all speakers, but the telephone system only transmitts 300 Hz to 3.4 kHz with negligible loss of intelligibility.

You should note that the short term (~ 100 mSec) power spectral density in te upper bands varies quite a bit depending on what is spoken, but the longer term (several seconds) averaged density is quite stable. Keit 120.145.149.167 (talk) 05:53, 26 November 2012 (UTC)

relativistic kinematics: the solar sail

Most treatments of relativistic kinematics deal with two massive bodies. How about the collision of a photon with a perfectly reflective (at rest) nanosurface of mass m? I'm supposed to calculate the frequency of the reflected photon, since the mirror will acquire momentum. I've tried solving it myself. Currently I've set the K.E. conservation equations and the momentum conservation equations equal to each other and have assumed an elastic collision, where L is the Lorentz factor:

momentum of incoming photon = E1/c = E2/c + L*m*v = momentum of reflected photon + momentum of mirror

then E1 = E2 + L*mvc

but from K.E. conservation

E1 = E2 + L*mc^2 - mc^2 = E2 + (L-1) * mc^2

then

(L-1) * mc^2 = L * mvc

m on both sides drops out. Since L is dependent on v, after distributing the v and c terms in the Lorentz factors on both sides and doing a ton of algebra, I get the cubic equation in x (where x = v^2):

x^3 + 2c^2(x^2) + (c^2 - 3c^4)(x) + c^2(x) = 0

I haven't had access to MATLAB to try to compute the roots in a sane manner yet (c is hard to run through online calculators, but substituting 3 for c says that v only has one real solution), but already the final velocity of the mirror is not dependent on E1 or the mass of the mirror. This can't be right. 207.114.92.89 (talk) 04:55, 26 November 2012 (UTC)

In your initial equation, are you sure you have the signs right ? It seems to me that the momentum of the mirror and reflected photon should have opposite signs. StuRat (talk) 05:41, 26 November 2012 (UTC)

v is negative, and the only real root I get for the cubic equation is negative, when I plug in 3 for c. However, this isn't dependent on mirror mass or initial energy. That can't be right. 207.114.92.129 (talk) 06:31, 26 November 2012 (UTC)

I think you are over-complicating the whole thing. You said the sail is at rest, or, if I am misinterpreting you, then you can put the sail at rest by a change in coordinates. The acceleration from one photon will be very small, so it will always be moving very slowly. That means you don't need to worry about relativistic effects on the sail (so L won't appear). You just need the energy and momentum of a photon in terms of frequency (hint, the formulae involve Planck's constant, h), and then you can apply your conservation laws. --Tango (talk) 09:25, 26 November 2012 (UTC)

Your problem is with the sign of the momentum. If you take the initial photon to have momentum E1/c in the positive direction, then the reflected photon should have momentum in the negative direction, specifically, -E2/c. Further, it follows that the solar sail moves in the same direction as the initial photon, hence v is positive and not negative as you assert above. If you want to do the relativistic case, then the correct equations are:

${E_{1} \over c}=-{E_{2} \over c}+Lmv$

$E_{1}=E_{2}+(L-1)mc^{2}$

It follows therefore that:

$Lmvc+(L-1)mc^{2}=2E_{1}$

Which shows that both the mass of the craft and the energy of the initial photon will enter into the solution. Dragons flight (talk) 11:16, 26 November 2012 (UTC)

What is the change in wavelength when light of wavelength k hits a perfectly reflective mirror in space? It can't be zero, because the mirror must move to compensate for the light being reflected, but it has an upper bound in order to conserve kinetic energy. I managed to reach this equation too, but v and E1 are both unknown. Yet I started out with two equations with two unknowns. Can I find a specific v or E1? Or is it a spectrum of energies, related to Black body radiation? -- 64.134.100.218 (talk) 16:10, 26 November 2012 (UTC)

Okay, using the constraint ${L=1+(E_{1}-E_{2}) \over mc^{2}}$, I managed to solve for v:

$v={(2E_{2}-E_{1}) \over (E_{1}-E_{2})+mc^{2}}$

However, it seems that both E2 and v are free to vary within a certain range, if E1 is known. Does this mean I have to use laws of Black body radiation? And why don't I notice the huge range of frequency changes and huge light pressures that this equation seems to permit? I have to use relativistic equations because my homework specifically said do not assume v << c. I understand this possibly means the photon might have to be a gazillion times more energetic than a gamma ray for this to matter, but it matters theoretically for my professor. (Plus, the light sail might be moving at relativistic speeds eventually, after enough photons.) 64.134.100.218 (talk) 16:36, 26 November 2012 (UTC)

You have to eliminate v and L which is a function of v from the pair of equations finding one equation which relates m, E1, and E2. If you don't do it right it will get messy. Hint: square both equations and subtract them finding an expression for the product E1*E2 and right that in terms of (E1 - E2) after using one of the original equations a second time. You will need an expression for L^2*v^2 in terms of L^2 -1 which can be easily obtained by inverting the defining equation for L. Dauto (talk) 16:48, 26 November 2012 (UTC)

This is why you need to learn about four-vectors and the Lorentz inner product. You can then eliminate the energy and momentum of the particle you are not interested in, in one fell swoop without having to bother about it's velocity and gamma factor.

In natural units, the four component vector (E, p), where E is the energy and p the momentum of a particle, transforms like the space time coordinates under Lorentz transforms, and is called a four-vector for that reason. The Lorentz inner product between two four-vectors defined as:

(E1,p1).(E2,p2) = E1 E2 - p1.p2

where p1.p2 is the ordinary inner product, is an invariant under Lorentz transforms. If p is the four-momentum of a particle of mass m, then we have:

p^2 = m^2 (1)

where the square means Lorentz inner product of the vector with itself. This is because it is Lorentz invariant, so you can evaluate it in the rest frame where p = (m,0).

We can then solve the problem as follows. p1 is the four-momentum of the incoming photon, p2 that of the reflected photon, q1 of the mass at rest and q2 after it has collided with the photon, then we have:

p1 + q1 = p2 + q2

We're not interested in q2, so we move this to one side:

q2 = p1 - p2 + q1

Square both sides:

q2^2 = p1^2 + p2^2 + q1^2 - 2 p1 .p2 + 2 p1.q1 - 2 p2.q1

By (1), we have q2^2 = q1^2 = m^2, and p1^2 = p2^2 = 0, so the equation becomes:

(p1 - p2). q1 = p1. p2

Taking the initial photon to move in the positive x direction, we can substitute here:

p1 - p2 = (E1-E2,E1 + E2),

q1 = (m,0)

p1 = (E1,E1)

p2 = (E2,-E2)

So, the equation becomes:

m (E1 - E2) = 2 E1 E2 ------->

E2 = m E1/(2 E1 + m)

Putting c back, this is:

E2 = m E1/(2 E1/c^2 + m)

Count Iblis (talk) 17:42, 26 November 2012 (UTC)

Can you walk me through one of the steps? After rearranging q2^2 = p1^2 + p2^2 + q1^2 - 2 p1 p2 + 2 p1 q1 - 2 p2 q1 ,

I get 2 * p2 * q1 = 2 * p1 * q1. After substituting in c and mv for the 3-momentum I get E2 = E1* (1 - mv^2/c^2). 64.134.100.218 (talk) 20:55, 26 November 2012 (UTC)

So, you have

q2^2 = p1^2 + p2^2 + q1^2 - 2 p1.p2 + 2 p1.q1 - 2 p2.q1

The reason why we brought q2 on one side and squared it, is because q2^2 = m^2, so you get rid of the unknown energy and momentum of the mirror after the collision that we are not interested in. The right hand side does not contain these quantities, only the unkown E2 that we want to solve for. Then q2^2 = q1^2 = m^2, so the m^2 from both sides cancel. And because photons are massless, we have p1^2 = p2^2 = 0. We are thus left with:

0 = - 2 p1 p2 + 2 p1 q1 - 2 p2 q1 --------->

p2.(p1 + q1) = p1.q1

Insert in here: p1 = (E1,E1), p2 = (E2,-E2), q1 = (m,0):

(E2,-E2). (E1+m,E1) = (E1,E1).(m,0)

The Lorentz inner products here are are given by (A,B).(C,D) = AC - B D, so we have:

(2 E1 + m) E2 = m E1 ---------->

E2 = m E1/(2 E1 + m)

We can write this as:

E2/E1 = 1/[1 + 2E1/(m c^2)]

where I've put c back.

Count Iblis (talk) 23:27, 26 November 2012 (UTC)

What's the momentum of the mirror after it begins moving? 128.143.1.41 (talk) 19:59, 29 November 2012 (UTC)

By conservation of momentum, this is minus the change of the momentum of the photon. The initial momentum of the photon was E1 in the x-direction, the final momentum is -E2 in the x-direction, so the change is -E2 - E1, the momentum of the mirror will thus have increased from zero to E1+E2. Putting back c, this is (E1 + E2)/c. Count Iblis (talk) 17:27, 30 November 2012 (UTC)

HIGGS BOSON

I HAVE A QUESTION IN MY MIND IT GOES LIKE THIS WHAT IS THE MASS HIGGS BOSON CONVERT ENERGY INTO ? [LIKE METAL, NON-METAL ETC] — Preceding unsigned comment added by 115.240.26.191 (talk) 06:22, 26 November 2012 (UTC)

I'm afraid your question doesn't make sense... The Higgs Boson doesn't convert energy into anything - it is involved in creating the effects of mass. --Tango (talk) 09:29, 26 November 2012 (UTC)

That's not entirely true either - it doesn't create the effects of mass, the effects already exist. Plasmic Physics (talk) 11:54, 26 November 2012 (UTC)

The Higgs (within the standard model framework) gives mass to (some) fundamental particles including the W-boson, the Z-boson, and all of the fundamental fermions - that is quarks and leptons, though it is not clear how exactly neutrinos get their masses. Dauto (talk) 15:51, 26 November 2012 (UTC)

I'm not sure I understand the question properly, but maybe you are looking for a simple unit conversion. The previously linked article on the Higgs Boson says its mass is ~126 GeV/c², which is equivalent to ~2.25×10⁻²⁵ kg. Annihilation of the particle would yield ~126 GeV of energy, which is equivalent to ~20×10⁻⁹ joules. Astronaut (talk) 20:46, 26 November 2012 (UTC)

the found or posthypnotic Higgs boson has not be as thought one which give mass to all particles .it is only boson such as others , the nature dose not obey from which we think it works really as it is.--Akbarmohammadzade (talk) 16:05, 27 November 2012 (UTC)

Sorry, I don't understand. Please write in your native language and we will translate it. --Tango (talk) 23:43, 27 November 2012 (UTC)

you never heard of the post hypnotic Higgs boson with the nature dose not obeying which we think..?GeeBIGS (talk) 00:20, 28 November 2012 (UTC)

برای بوزون هیگز قبل از آشکارسازی آن نقش های خاصی قائل بئدند و جرم پذیری ذرات رابه آن نسبت می دادند. این نقش تا حد جایگاه فراطبیعی پیش رفته بود اشکار سازی و به عبارتی حدس نزدیک به کشف بوزون هیگز که کاملا هم به یقین تبدیل نشده ،در سرن نشان می دهد که این بوزون نیز مانند سایر بوزونهاست ،منظور اینکه طبیعت هر آنچه در واقعیت ذاتی خود داراست نشان می دهد و تابع تصورات یادتئوریهای ما نیست.--Akbarmohammadzade (talk) 06:39, 28 November 2012 (UTC) با عرض پوزش مکان جغرافیایی ما با اروپا و آمریکا اختلاف ساعت دارد بنابراین اوقاتی که شما در اینجا حضور دارید و بحث می کنید احتمالا بنده در اینترنت نیستم ووقفه حاصل می گردد. با این حال بنده بسیار خرسندم که ارتباط با دوستان فرهیخته و پزوهشگر دارم.این برای من باعث افتخار است و بهره می برم از این مباحث علمی.--Akbarmohammadzade (talk) 06:44, 28 November 2012 (UTC)

V-1 flying bomb

What kinds of metal-processing/assembly machinery (other than a lot of slave labor) were used in the manufacture of the V-1 flying bomb? Also, where else were V-1s being made besides the main factory at Nordhausen-Mittelwerk? Thanks in advance! 24.23.196.85 (talk) 07:20, 26 November 2012 (UTC)

Read V-1 flying bomb facilities. Page 31 of the manual describes the Geräte und Sonderwerkzeuge (special tools and equipment) they used. Trio The Punch (talk) 13:10, 26 November 2012 (UTC)

Danke! The info on the location of the production plants and storage depots was quite valuable. The location of the storage depots, in particular, is just right for the thriller novel I'm working on. As far as the Gerate und Sonderwerkzeuge, though, these appear to describe mainly the equipment needed for the V-1s transportation and final assembly before launch -- I was hoping for info about the equipment used to actually manufacture the major assemblies prior to shipping. 24.23.196.85 (talk) 03:24, 27 November 2012 (UTC)

Germans like making lists, but I haven't found one that lists all the machinery involved in manufacturing the V1's, and I am not sure if such a list exists because they used a lot of pre-existing steel/car/airplane factories. For example, 336,600 people paid in advance for the Kdf-Wagen but when Germany invaded Poland on 1 September 1939 the automotive production was replaced by war production and none of buyers received their car. I would recommend contacting something like the Stiftung Gedenkstätten Buchenwald und Mittelbau-Dora, they know a lot about Mittelwerk. Page 37 of this PDF mentions straigthening rolls, spot welders, guillotines, press brakes and forming presses and may give some inspiration, but the Combined Intelligence Objectives Subcommittee was more interested in destroying the bunkers than documenting all the equipment used, because they had the same stuff back home. Trio The Punch (talk) 04:44, 27 November 2012 (UTC) p.s. Category:V-weapons

There must be exact copies of all procedures in some US archives. The US took everything from Mittelwerk. They tested the V-1 extensively, they even launched one from a aircraft carrier. Wernher had for sure all the plans with him. --Stone (talk) 20:31, 27 November 2012 (UTC)

They guesstimated that the Mittelwerk and Nordwerk tunnel system (which was used to produce and assemble the components for both the V1 and the V2, the Taifun, Schildrote and the engines for the Junkers) contained about 5000 machine tools. I copypasted some of the more interesting stuff. Trio The Punch (talk) 00:48, 28 November 2012 (UTC)

Think you'll find the smaller US V1 clones where developed for Landing Ship, Tank's and it was the V2 clone that was launch from the carrier.--Aspro (talk) 20:51, 27 November 2012 (UTC)

See Republic-Ford JB-2. Trio The Punch (talk) 21:13, 27 November 2012 (UTC)

This second PDF has EXACTLY the kind of info I was looking for! (I was trying to get a feel for the production process and in particular for any critical bottlenecks in it, so as to know which machines the Maquis should smash.  :-) ) Looks to me that blowing up the main transformers would have been very effective, or else destroying the overhead cranes. Danke schoen! 24.23.196.85 (talk) 05:47, 28 November 2012 (UTC)

Page 7: "In no case was any standby or local [electricity] generating plant provided". According to page 18 there was only one main high tension intake station with a number of transformer sub-stations. If you destroy it it would be difficult to fix, and the electric hoists mentioned on page 20 would be useless. The lights would turn off too, which may aid or hinder your escape. Trio The Punch (talk) 06:35, 28 November 2012 (UTC)

Clarification: the Maquis attack takes place not at Nordhausen-Mittelwerk, because it's too deep within Germany proper; rather, it takes place at another (fictional) combined V-1 factory and V-weapons storage depot in the Lille-Calais area (in real life, that area had several storage depots but no V-weapons factories). In fact, this is the reason why I asked specifically about the V-1 and not the V-2 -- the latter were all made at Mittelwerk. 24.23.196.85 (talk) 05:09, 29 November 2012 (UTC)

Insect

Unable to place this insect

What is this insect? I have based the name on a guess. How can I identify the species? The photo was taken at Idukki Didtrict, Kerala, India. — Preceding unsigned comment added by Drajay1976 (talk • contribs) 08:33, 26 November 2012 (UTC)

I found a website that helps you identify 39 different species of grasshoppers found in the region of the Western Ghats and a local expert. Trio The Punch (talk) 13:15, 26 November 2012 (UTC)

Do you have any other pictures of it from another angle? It definitely looks like a tettigoniid though from the antennae length, or at least ensiferan. Male as well from the lack of an ovipositor. Also, are the wings damaged? They look chewed off.-- OBSIDIAN†SOUL 21:42, 29 November 2012 (UTC)

Thanks. I sadly couldn't get pictures from other angles. I could not get closer to it because of the terrain. Couldnt get a better angle as well. The insect was wet from the rain and was just sitting there. I dont think that the wing was chewed off. --Drajay1976 (talk) 09:41, 1 December 2012 (UTC)

Reason for type of emotional response to music and poetry

It came out on this thread on the Miscellaneous reference desk that "The thing with Limericks is their metrical structure alone is funny, regardless of meaning", though that was also disputed. But what is it about that metrical structure that makes a limerick sound funny to most people? And why do certain tunes sound funny, or light-hearted, or sad, or pensive, or foreboding to most people? I'm not looking for a description of what strikes people in this way or that way; I'm wondering if it is known why the brain reacts the way it does. Is it that that sound pattern somehow gets processed at a spot in the brain that is physically near the spot that processes that emotion? Duoduoduo (talk) 15:54, 26 November 2012 (UTC)

I'd say (without references) that it's just a matter of expectation and normalization (psychologists have a better word for that). Say you've heard 100 limericks, and 50 of these (the figures are made up) were funny, 1 was an odd one, and 49 were just bad. Then you'd have the expectation during the next limerick that "it's gonna be funny (or maybe it's just gonna suck)" and that makes you think limericks are funny.

And if you read it, the voice in your head will probably be a funny one. Which i why it works so well with limericks - the rhythm and rhyme are easy to remember.

Once limericks were expected to be funny, more funny limericks were made, since after all, the audience was already expecting something funny.

I think that's all, no nerve tickling its neighbors or stuff like that... - ¡Ouch! (^{hurt me} / _{more pain}) 16:24, 26 November 2012 (UTC)

p.s. about the voices in my head. I'm not mad. I asked the voices, and four of them said I'm not. - ¡Ouch! (^{hurt me} / _{more pain}) 09:10, 28 November 2012 (UTC)

See "Music and emotion" and "Psychoacoustics".

—Wavelength (talk) 17:23, 26 November 2012 (UTC)

I can't find a physiological answer in those cites. Also, as for Ouch's comment that "if you read it, the voice in your head will probably be a funny one", this extends the question to include this: for what physiological reason do certain "voices in your head", or more generally certain sentence intonations, come across as funny? Or is that too known to be purely learned? Duoduoduo (talk) 18:45, 26 November 2012 (UTC)

One reason for a song's personal emotional appeal can have to do with some past association. For example, if you hear an "oldie" on the radio, it might remind you of where you were and what you were doing, when you first heard it. ←Baseball Bugs ^{What's up, Doc?} carrots→ 12:05, 27 November 2012 (UTC)

But I think I see what the OP is getting at. Why does a minor scale sound "sad" compared to a major scale? I've wondered about that myself; a couple of sound waves of different frequences; change the frequency of one of them a little bit, and for some reason you whole emotional reaction changes. What's up with that? Where in evolution does that confer a survival advantage? Amazing. Gzuckier (talk) 05:10, 30 November 2012 (UTC)

Common emotional response to specific colors

Similar question to the above. Red is commonly interpreted as an aggressive color, a certain shade of pink as a soothing color (so for example I've read that some prisons have pink walls to lower inmate aggression), pastel colors as gentle, hot pink as attention-getting, highly saturated colors as attention-getting, etc. Do we know why the human brain interprets certain colors in certain ways? Is it that, for instance, red is processed in a location in the brain that is near where aggression is processed?

Related question: What is the physical reason that it is widely perceived that certain colors "clash" -- that is, viewing them together leads to an unpleasant feeling? Duoduoduo (talk) 16:04, 26 November 2012 (UTC)

I think the difference between pink and red is just an example that stronger stimuli provoke a stronger response in our brains. Some colors and emotions are linked through (sub)culture, for example certain medieval European queens used white to mourn instead of black. In certain parts of Africa red is a color of mourning, representing death. In both Thailand and Brazil they prefer purple. Many people who see bullfighting on TV believe that the color red causes the bull to be aggressive. In reality they torture the bull beforehand (offcamera). Cattle is red-green color-blind. There have been some attempts to make lists, but they are incomplete and unreliable. The brain area that deals with face recognition sits right next to the color area, which is problematic for achromats. More interesting stuff. Trio The Punch (talk) 16:39, 26 November 2012 (UTC) p.s. Color_theory#Color_harmony_and_color_meaning

Color in Chinese culture tells us that red is a lucky colour in China. HiLo48 (talk) 18:33, 26 November 2012 (UTC)

Right, maybe I overstated the premise. So let me back up: (1) Is there some commonality across cultures, such as more saturated colors being viewed as "stronger" and eliciting a stronger response? And (2) if so, is there any known physiological reason for this? E.g., why are unsaturated colors not viewed as "strong"? (I don't see that item (2) is addressed in any of the references given.) Duoduoduo (talk) 18:40, 26 November 2012 (UTC)

The article Color psychology makes some interesting claims, but doesn't really answer most of your questions. There might be a more germane article somewhere? Dbfirs 19:32, 26 November 2012 (UTC)

Isnt' that like asking why are dense clouds of odor molecules considered "strong" and diffuse clouds of odor considered "weak"?165.212.189.187 (talk) 21:12, 26 November 2012 (UTC)

Nope. Saturated colors don't have more photons; they simply have a lower variance of wavelengths. Duoduoduo (talk) 22:00, 26 November 2012 (UTC)

Point? The purest (most saturated) color is achieved by using just one wavelength at a high intensity (dense cloud), such as in laser light. To desaturate a color of given intensity in a subtractive system (such as watercolor), one can add white (diffuse cloud), black, gray, or the hue's complement. Density across the spectrum. Maybe I should not have used the term molecules but just gas.165.212.189.187 (talk) 13:54, 27 November 2012 (UTC)

• This is an much-studied question, and our article on color psychology deals with it pretty comprehensively. Looie496 (talk) 19:40, 26 November 2012 (UTC)

Aposematism extends this topic to other species --Digrpat (talk) 22:06, 26 November 2012 (UTC)

100% electric spacecraft engines

I've looked around a bit, and do want to know if there is any real, practical, lightweight spacecraft engine that uses no fuel but electricity? I've looked at ion engines and others, but they just use heated materials as propellant. 3 bit (talk) 17:51, 26 November 2012 (UTC)

What you're really after here are engines without reaction mass. Examples include solar sails (have been demonstrated in space) or theoretical things such as magsails and Bussard scoops. Note however, that in all of these there is still reaction mass in a way, just not carried on board. Solar sails and magsails use the momentum carried by photons and charced particles from the sun. The Bussard scoop uses light atoms encountered along the way. Full reactionless drives are thermodynamically impossible, as there still needs to be a transfer of momentum onto the spacecraft in some form. Fgf10 (talk) 18:22, 26 November 2012 (UTC)

(edit conflict)Do your mean batteries? (Otherwise there must be a source of fuel to generate the electricity.) The problem with using an electric "engine" is that there is nothing to "push against" so an electric motor cannot be used to propel the spacecraft. Electricity can, of course, be used to accelerate particles to eject in an ion engine, but I cannot think of any other usage for electricity in a spacecraft, other than the obvious application of powering computers and equipment. Perhaps someone else can think up a novel way to drive a spacecraft by onboard electricity (assuming that there is a way to generate it)? Dbfirs 18:26, 26 November 2012 (UTC)

The Bussard ramjet mentioned by User:Fgf10 above would be one such application. Dbfirs 18:32, 26 November 2012 (UTC)

Hmm, generating electricity without fuel or batteries? If only there was a nearly limitless supply of energy we could use.... ;) Fgf10 (talk) 19:06, 26 November 2012 (UTC)

Well, obviously, except in deep-space, but I class the sun as a source of fuel. The OP asked about "no fuel but electricity". Dbfirs 19:24, 26 November 2012 (UTC)

Newton neatly sums up why it's not possible to have an engine with no reaction mass: "A body will preserve its velocity and direction so long as no force in its motion's direction acts on it." In a (very nearly) frictionless vacuum (like outer space) you need to have some sort of mass to carry that force. A light sail and laser engines use photons to carry that mass and bounce it off of an ablation surface or a sail. A conventional rocket engine uses molecules of hot gas to carry the momentum. An ion engine (surprise!) uses ions. It could be anything thanks to the fact for every action there is an equal and opposite reaction. Hence the name "reaction engine" and "reaction mass". You could be throwing bowling balls out the back or letting off compressed gas (as in a positioning thruster) but you have to have SOME kind of reaction mass in order to carry away the momentum in a controlled direction. HominidMachinae (talk) 22:06, 26 November 2012 (UTC)

EmDrive--Stone (talk) 20:20, 27 November 2012 (UTC)

That reminds me of Eric Laithwaite and his magic gyroscope. Dbfirs 09:22, 28 November 2012 (UTC)

Length contraction

If I have a long, thin, dense rod with radius less than its Schwarzschild radius and make it go at a relativistic speed in the direction of its axis, will it collapse into a black hole if it's going fast enough? --168.7.228.96 (talk) 21:09, 26 November 2012 (UTC)

Fast enough relative to what? AndyTheGrump (talk) 21:11, 26 November 2012 (UTC)

That doesn't matter, the answer is no. In its own reference frame the object is motionless and its length is unchanged. Motion can generate an apparent reduction in length from the viewpoint of an outside observer, but it doesn't generate a gravitational field. Looie496 (talk) 21:18, 26 November 2012 (UTC)

I'm not certain the answer is that simple. Doesn't velocity imply energy, and doesn't energy inherently effect a gravitational field? And doesn't the generalization of gravity tell us that neither reference frame is canonical? Both the "stationary" observer and the "moving rod" view a relativistic motion, as measured in the respective reference frame. The question presents a legitimate conundrum. It seems the resolution is, of course, that an event horizon must be viewed from some reference frame. Not all reference frames see the same horizon; nor do they see a singularity in the same location. By symmetry, whichever reference frame the observer happens to be in, they will see (or rather, not see) the other reference frame as on the opposite side of an event horizon. To define that apparent horizon in any kind of useful way, we need to solve a very ugly and difficult set of "elegant equations." With luck, one of our regulars who are better-versed in relativistic geometry will be along shortly to provide a more thorough explanation, and perhaps even a good book. (Unfortunately, I know of no text or web-page that makes this topic any more accessible). Nimur (talk) 22:35, 26 November 2012 (UTC)

It's worth noting that the equations that point out which bodies will become black holes (basically the various forms of the Schwarzschild metric) are solutions to Einstein's field equations in rather trivial situtations, ones which require no linear momentum. Once you change the system to one in which your potential black hole is moving at extreme velocity, you will need new equations to describe its gravitational field; you can't simply plug in the relativistic mass to the Schwarzschild metric. Someguy1221 (talk) 03:42, 27 November 2012 (UTC)

No. Whether you consider something to be moving or not is merely a change of coordinates, namely, a Lorentz boost. The geometry of spacetime isn't affected by what coordinate system(s) you choose to describe it in. I.e., what events, four-vectors, tensors and geodesics exist, and what the geometric relationships are among those geometric objects, is independent of any coordinate system. In particular, what absolute horizons exist, and what set of events they consist of, is invariant with respect to any change in coordinates. If a rod doesn't produce an absolute horizon as expressed in one coordinate system (atlas, actually), then it doesn't produce an absolute horizon as expressed in any other coordinate system. Red Act (talk) 08:04, 27 November 2012 (UTC)

Looie496 and Red Act are correct. It is a common misconception to think that space contraction and relativistic mass makes it possible for a fast moving object to turn into a black hole. That's not correct. Dauto (talk) 17:03, 27 November 2012 (UTC)

... otherwise the Earth would turn into a black hole every time an observer passed by at the speed of light! Dbfirs 22:21, 28 November 2012 (UTC)

The Earth just happens to inexplicably turn into a wicked time-warped pancake every time an observer passes by near the speed of light... oh wait. ;) -Modocc (talk) 04:13, 30 November 2012 (UTC)

average intensity of three vibrations on a string of constant frequency and individual amplitude, but random phase of pi or 0

This is a homework problem I'm trying to make sense of. The amplitude A and frequency f of each of the vibrations is constant, but phase varies "randomly and independently" in values of pi or 0. It's the "randomly and independently" part that confuses me. I think it's related to quantum wavefunctions but I am not sure. Anyway, I deduced there are 6 probability states for the string, 2 with a peak amplitude of 3A where all of the vibrations are in phase with each other, and 4 where one of the vibrations is out of phase with the other two.

I used the formula for power of a wave on a string I got from physicsforums. Then P_avg = A^2 * 2pi^2 * f * v * mu, where mu is linear mass density, v is wave velocity and A is peak amplitude. This is for an individual wave though.

If the vibrations' phases vary "independently and randomly" does that mean that mean that I_avg = (2 * 3^2 + 4*1^2)/6 *(A^2)*(2pi^2)*f*v*mu, that is, 6(A^2)*(2pi^2)*f*v*mu? 64.134.100.218 (talk) 21:38, 26 November 2012 (UTC)

The reasoning is basically correct, I think, but you are miscounting the number of states. If you have three random variables, each with two possible values, the number of global states is 2*2*2 = 8. Looie496 (talk) 21:57, 26 November 2012 (UTC)

Also, the words "individual amplitude" makes me think that the waves amplitudes are not all equal to each other. Dauto (talk) 17:00, 27 November 2012 (UTC)

The problem specifically says they are equal. 128.143.1.41 (talk) 21:39, 29 November 2012 (UTC)