Wikipedia:Reference desk/Archives/Science/2007 June 21

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June 21

Relative Motion - how I loathe thee

This is a question that I got wrong on an exam and I, nor my teacher who wrote it, know why it is wrong. The question involves a collision of 2 charged particles of the same charge and mass, say 2 protons initially EACH going at a constant speed of let's say 1*10^6 m/s. (I don't know the exact numbers but the exact answer is not what I'm interested in.) The question asks to find the distance between the 2 particles when they slow to a stop due to their charges. In order to solve the problem one must equate kinetic energies to the electric potential energy. I, being the genius that I am (sarcasm alert), used the relative speeds of the 2 particles (1 particle is stationary, the other moving at twice the original speed). I got an answer for distance at half the distance in the answer. I and my teacher can see mathematically why it's wrong 2(.5mv^2)is not equal to (.5m(2v)^2) but I don't understand it physically. Shouldn't the distance from the perpective of one of the particles be the same as the distance from any other perpective?

• This would require a special relativity correction to the lengths observed. The short answer is how we measure length depends on our relative motion - a stationary observer and an observer moving with the charge will measure different lengths. WilyD 00:59, 21 June 2007 (UTC)

(Wily: Read it again: this is a non-relativistic question.)

The idea in the original approach is that, at the moment when both particles have zero velocity, all the energy must be in the form of potential energy, so we can equate this to the initial amount of energy (which was (essentially) all kinetic). The problem with your approach is that, in the reference frame where one of the particles is initially at rest, there will never be a time when both particles have zero velocity. At the same time as one particle is being slowed by their mutual repulsion, the other one is being accelerated away. Eventually the first particle will come to complete stop, and the other one take over the entire kinetic energy. Thus there is no time when all the energy is in the form of potential energy, so equating potential energy to initial kinetic energy will not give you the right answer.

You are correct that the minimum distance between the particles should be the same regardless of which reference frame we measure it in (as long as the relative velocities of the frames are small compared to the speed of light, so that the effects of relativity can be ignored). But your approach does not calculate that minimum distance in the specified experiment, but instead in another experiment in which the velocity of one of the particles is somehow maintained constant during the collision (or when its mass is very large compared to that of the other particle – it would for example be a decent approximation of what happens when a positron collides with a proton). --mglg_(talk) 01:06, 21 June 2007 (UTC)

Nice catch, and now I feel really dumb. mglg is right, and I'm wrong. Thanks, WilyD 01:20, 21 June 2007 (UTC)

Resonances in cylindrical tube.

Whats the best way to stop overtone resonances in a cylindrical tube closed at one end and exicted at the other? --Tugjob 02:32, 21 June 2007 (UTC)

Stuff something soft in the open end, like a rag ? StuRat 04:35, 21 June 2007 (UTC)

Did any dinosaurs have a syrinx or even a larynx?

Embouchure? ..assuming you are referring to musical cylindrical tubes such as flutes. Pfly 09:00, 21 June 2007 (UTC)

No Sturat Im exciting the other end with a loudspeaker, so I cant stuff things there.--Tugjob 23:58, 21 June 2007 (UTC)

You will need a perfect, totally homogeneous tube. You will need to carefully low pass filter your acoustic input. It will be much more difficult to actually implement this in practice, because perfect filters and perfect tubes rarely exist in real world environments. Nimur 05:35, 22 June 2007 (UTC)

Yeah that sounds reasonable. So I will need an acoustic low pass filter inside the cylinder? But the energy needs to be absorbed somehow so, could I just use damping material to damp all resonances? If so what would be the best disposition of such material inside the tube?--Tugjob 15:13, 22 June 2007 (UTC)

Global Warming

How is global warming related to natural disaster?

the first will cause changes in the patterns of the latter? Can you be more specific? Pfly 09:02, 21 June 2007 (UTC)

Dear 219.75.59.207, the page extreme weather, which you just vandalized, contains some information relevant to your question. --169.230.94.28 09:10, 21 June 2007 (UTC)

That's ironic, eh? Gzuckier 19:50, 21 June 2007 (UTC)

Food test

Today I went to have a test to see if I was suitable to handle food. They took swabs from my arm pits, groin region and my nose. They also requested a sample of my faeces. Does anybody know what procedures are done on the material? What do they look for and how do they identify it's presence?Bastard Soap 10:02, 21 June 2007 (UTC)

• I don't understand the question, and forgive me but I don't think I want to understand it. YechielMan 15:17, 21 June 2007 (UTC)

looking for bacteria and virii, no doubt. just guessing, but maybe salmonella, hepatitis? Gzuckier 17:16, 21 June 2007 (UTC)

Faeces suggests cholera or poliomyelitis to this layman. Hepatitis is spread by blood and saliva, i think. It's odd that BS doesn't mention a blood test. Groin and armpits are a mystery, but maybe they're an index of general cleanliness? —Tamfang 04:56, 24 June 2007 (UTC)

Help me to ans.what is the ex.of sympathetic and parasympathetic

thers any one could help me to ans. these question.

See sympathetic nervous system, parasympathetic nervous system, and also autonomic nervous system (the division of the nervous system that includes both sympathetics and parasympathetics). This is an overgeneralization, but the sympathetic nervous system is responsible for the flight-or-fight response, while the parasympathetic nervous system is often considered the "rest and digest" system. Cheers, David Iberri (talk) 12:08, 21 June 2007 (UTC)

Another way to look at it, the sympathetic is more tuned to the outside, the parasympathetic is more tuned inwards. There tends to be a switchover to some degree at night, so at night all your aches and pains are more noticable than in the day. Gzuckier 17:17, 21 June 2007 (UTC)

thermodynamics

A proffesor of religious studies after parking his car for 8 hours in a shed,checked the pressure and found it to be normal for the car.He then drove to another town and discovered that the pressure had slightly increased.In his wisdom,he decided to let some air out in order to reduce the pressure to its original level.comment on this action.

Is this supposed to be about religious studies, car maintenance, or thermodynamics? Anyway, most cars (but perhaps not all) have recommended tire pressures suggested for when the tires are cold. Similarly, most cars have a recommended oil level when the engine is cold. However, some manufacturers have at times used hot-engine oil calibration. I would not be surprised to find that, likewise, some manufacturers have used hot-tire pressure calibration. — Lomn 14:35, 21 June 2007 (UTC)

What is your question? —Bromskloss 14:49, 21 June 2007 (UTC)

There are a variety of ways this could have happened. You could change the variables in the Ideal gas law. He could also have driven from Los Angeles, CA to Denver, CO, where the altitude changed significantly so the atmospheric pressure changed. The temperature of the tire or of the pavement could have something to do with it. Of course, you could also say that gremlins added air to his tires en route, since this is such an open-ended question. -- JSBillings 14:53, 21 June 2007 (UTC)

(*response*)He could have changed altitude or the outside temp could have changed, both of these things would change the volume/pressure of the air in the tires yet would not change the amount of air in them, so "commenting on the action" it was stupid if he has to drive back, because as soon as he goes back to the earlier conditions/ place where the pressure was normal the tires will be flat because he let air out. - Matt June 21 2007

Intelligent design of tire pressure? Gzuckier 17:18, 21 June 2007 (UTC)

This sounds like a homework question. But, you should read about the ideal gas law, and realize that driving causes friction which heats the air in the tires. Hotter air in an enclosed volume will have a higher pressure. Nimur 05:37, 22 June 2007 (UTC)

You should tell you physics professor that it's not cool to make fun of religious studies professors Nil Einne 22:18, 23 June 2007 (UTC)

I suspect that this is a riddle whose answer is a pun. —Tamfang 04:58, 24 June 2007 (UTC)

Geostationary Orbit

Why is it that there is apparently only space for 400 satellites in geostationary orbit?

Thanks, --Fadders 14:54, 21 June 2007 (UTC)

• I don't know. Obviously there has to be an upper limit, and geostationary orbit doesn't say how many satellites are allowed. One factor to consider is that satellites drift from their equilibrium positions above earth, so nobody wants a midair collision. YechielMan 15:15, 21 June 2007 (UTC)

Couldn't they stick them all together and create a huge ring around the earth. Then they wouldn't crash.

The Ringworld is unstable!Gzuckier 17:20, 21 June 2007 (UTC)

When two satellites are using the same frequencies and polarization, then they need to be 2 degrees of longitude away from each other. Otherwise, the receive antennas on earth will see too much interfrence from adjacent satellites, and transmit antennas on earth will cause adjacent channel interference. Therefore, there are only 180 "slots." But we can have more than one satellite per slot if the saltellites are on different channels. A modern satellite occupies all teh channels. -Arch dude 19:05, 21 June 2007 (UTC)

It's not about collisions - the radius of a geocentric orbit is about 42,000 km - so the circumpherence of the orbit is something over a quarter of a million kilometers. If there were 400 satellites up there and they were equally spaced, around the equator - they would be 660 kilometers (410 miles) apart! Because the entire point of a geostationary orbit is to be STATIONARY, these satellites are not moving at any kind of speed relative to one another. If there were 250,000 satellites - spaced a kilometer apart - the odds of a collision would still be pretty small. However, it's not as simple as that. There are a few 'sweet spots' which are better places to put a satellite - and hence in high demand. For example, if you are broadcasting cable TV, you want to be on the side of the planet where your audience is. If this is a communications satellite for transmitting phone messages between (say) the USA and Japan - then you'd want it sitting over the middle of the pacific ocean someplace. Another consideration is that most satellites have radio receivers and transmitters on board - and if they were packed too tightly, they would start causing interference on each other's transmissions. So that 410 mile gap starts to look rather smaller when you consider practical applications of the technology and there might well be a 400 satellite limit in those more useful places along that orbit. But to strictly answer the question - there is certainly space for more than 400 of them if (for example) they are sitting someplace where there isn't much market for cable TV or international phone traffic and (especially) if they aren't using radio to transmit data. SteveBaker 19:15, 21 June 2007 (UTC)

Many years ago Tom Digby pointed out that the number could easily be tripled if the added sats have ion engines (which provide a weak thrust steadily for years) pointed parallel to the axis; the thrust added to the gravity vector shifts the orbital plane enough to provide a few degrees of spacing. —Tamfang 05:02, 24 June 2007 (UTC)

Theory of Relativity Question

I'm in a bit of a puzzle about the theory of relativity because of the following... if a person was traveling 1000mph down a highway, and another person was traveling in the same direction at 100 mph

Person A
<-- (1000mph)
Person B
<-- (100 mph)

A clock would tick slower for person A right? But what i dont understand is that if this were to take place on earth, and for all intensive purposes earth was traving at around 10,000 mph in the oposite direction as the people were traveling.. then
____________
(...................)
( <--Person A )
( <--Person B )
(___________)earth --> 10,000 mph

so on earth person A is moving faster, and would be at a slower relative time, but looking at the entire picture person A is traveling 9,0000 MPH and person B is traveling at 9,900 MPH... so the clock would tick slower for person B because they are moving faster? These two ideas, yet the same situation seem to contradict eachother.

Hopefully you understand my question, its rather hard to express, but it is bothering me.

-Matt

You appear to missing the "relative" part of "relativity". It doesn't matter how fast the Earth, Solar System, or Milky Way is traveling. Person A is traveling 1000mph relative to Earth. Person B is traveling 100mph relative to Earth. So, Person A's time ticks every-so-slightly slower than Person B's. It isn't enough to measure, but that is the concept. --Kainaw ^(talk) 15:26, 21 June 2007 (UTC)

But its not just earth that is moving at 10,000 mph, the people are too, and the people are in fact moving 9,000 and 9,900 mph, and from one perspective B is moving faster relative to A, but from another perspective A is moving faster than B. So even with your answer, In the the following (identical, yet different situations) they contradict eachother.

what would make earth so special to be the only point to which to be relative to, and not say the solar system instead. or if there was a plane flying by, why not be relative to the plane, why would they have to be relative to earth, your answer doesnt really help (and depending on what object you chose to be relative to... the answers differ) for instance you could say that Person A is traveling 9,000 mph relative to the sun, and person B is traveling 9,900 mph relative to the sun... so your answer contradicts itself in the same way as my question —Preceding unsigned comment added by Demostheness (talk • contribs)

Again - you are missing the point of relativity. What if the solar system was moving at a million miles an hour in the opposite direction of the Earth? What if the Milky Way was moving at a billion miles an hour in the opposite direction of that? It doesn't matter. You take a point that is relatively the same for both objects and use that as a frame of reference. Of course, time shift is so minimal that it cannot be measured. What you need is a person traveling near the speed of light in one direction. Another person pokes along in the same direction. The person traveling near the speed of light will have slightly slower time than the person poking along. It is not possible to travel near the speed of light for any length of time "on Earth" in a straight line.

As for this contradiction that you feel you've made, you need to understand that it is not "speed". The base theory, E=mc² does not have velocity in it. It is energy. Things traveling at higher speeds have more energy. The more energy you have, the slower time moves. So, regardless of Earth, your little man A has more energy than your little man B because both are propelling themselves with reference to the Earth. --Kainaw ^(talk) 15:54, 21 June 2007 (UTC)

Demostheness, you are assuming there is absolute time, but in relativity there is no such thing. From A's point of view, B's clock as observed by A appears to be running slow. From B's point of view, A's clock as observed by B appears to be running slow. If A and B are both moving relative to some third party, then from the third party's point of view, A's clock and B's clock are both running slow. If A and B want to compare their clocks in the same frame of reference then one or the other has to accelerate/decelerate and then you get a real difference between their measured times - see twin paradox. Gandalf61 16:01, 21 June 2007 (UTC)

In this vein, there's something I've always wondered. Suppose I have a universe that's empty except for two pointlike astronauts and has the topology of a 3-sphere. They move towards each other and then synchronize their watches when they reach the same spot (from their point of view; the point here is to make everything symmetric). They then go off on their separate ways, and then meet up again. Each one can reason that the other's clock is slower, since they weren't moving and nobody underwent any acceleration, but their situation is symmetric, so their clocks will match up. How is this resolved? Veinor ^{(talk to me)} 16:17, 21 June 2007 (UTC)

Veinor, how do they meet up again if there is no acceleration? Noname

Noname is on target here... returning to the same place implies that at some point they turned and thus accelerated. Special Relativity applies only to movement along a (Euclidean) straight line. Centripetal acceleration comes into play in all other cases. Donald Hosek 16:43, 21 June 2007 (UTC)

Nobody is accelerating here; it's like on a two-dimensional sphere, where all the great circles (the 'lines') intersect. Veinor ^{(talk to me)} 16:45, 21 June 2007 (UTC)

When you start curving space, I'm pretty sure you have to invoke general relativity—inertial motion in curved space looks like acceleration to a classical observer. TenOfAllTrades(talk) 19:37, 21 June 2007 (UTC)

Exactly. Remember that there is not space and time but just one manifold spacetime and if you require those spacetime direction that are spacelike from the astronauts' frame of reference to form a 3-sphere, who knows how continuity then restricts the possible shape of the timelike direction. On second thought: isn't this just de Sitter space we are taling about. Pity that i don't know enough ART to answer your question. But I have some dim recollection that in a closed Friedmann universe it would be impossible for the astronauts to meet again as the big crunch always happen before even a light ray has finished a full trip around the spacelike 3-sphere. Simon A. 19:54, 21 June 2007 (UTC)

Ten is right. Think about Flatland creatures living on the surface of a sphere. They might argue that they're moving in a straight line and not accelerating when they traverse a great circle, but the 3-dimensional observer can clearly see that they are in fact accelerating. The same thing applies with your 3-sphere. Donald Hosek 21:01, 21 June 2007 (UTC)

And I should add that my use of the word "Euclidean" above was quite deliberate. Special Relativity only applies under Euclidean motion. Alas, outside of textbooks, the universe isn't terribly Euclidean. Donald Hosek 21:05, 21 June 2007 (UTC)

OK, then, what about on a cylinder? They have zero Euclidean curvature, so we can use special relativity. Veinor ^{(talk to me)} 06:54, 22 June 2007 (UTC)

So, what happens then? Does the curvature precisely balance the time dilation they otherwise could be expected to see? —Bromskloss 23:21, 21 June 2007 (UTC)

Messy/illegible handwritting correlated with certain segments of the population

After reading the article on Graphology I am wondering whether messy or relatively difficult to read handwritting has been positively or negatively correlated with certain segments of the population. Commonly, doctors are often cited as having rather difficult to read handwritting, more so than the general population, although I have never seen any studies about the matter. Outside of of the obvious or expected (like beginning writers or calligraphers) are any segments of the population more or less prone to exhibit poor handwritting? (segments = subject of study, higher education, hobbies, gender, profession, perfectionism... etc) I would like to avoid anecdotal evidence in the answer if possible unless it is a reference to a scientific study. 128.196.125.8 22:05, 21 June 2007 (UTC)

I tried to find some answers to your questions, but I couldn't come up with much. I did learn that the word for poor handwriting is cacography. I've found a couple of articles (here is one [1], that is pretty interesting) that reference a 2003 study at Trinity College, Dublin that found that 5 percent of doctors had illegible handwriting (not sure if that study was limited to Irish doctors). I also found a paper published in the Journal of the Royal Society of Medicine entitled "Illegible handwriting in medical records". It found, in part, that "Defects of legibility such that the whole was unclear were present in 18 (15%) of 117 reports, and were particularly frequent in records from surgical departments." That paper references several others that may be of interest to you. Looking for scientific papers on cacography, they almost exclusively dealt with doctors, so I could not find out the percentage of the general population with poor handwriting, or what other populations may be affected. --Joelmills 00:09, 22 June 2007 (UTC)

Birds

How can bird sit on a high-voltage power line and not get electrocuted? I can't find it. -^Violask8₁₉₇₆ 22:20, 21 June 2007 (UTC)

Gee I tried "bird power line electrocute" on google and got results [2] Donald Hosek 22:35, 21 June 2007 (UTC)

The non-technical answer is that the electricity takes the shortest/easiest path - so continuing to travel down that nice, straight copper wire is preferable to going the long way up through the bird - who isn't a particularly good conductor of electricity.

The technical answer is that the section of wire between the birds legs - and the bird - form a parallel circuit. The amount of electricity that travels down each path is proportional to the reciprocal of their resistances. The resistance of a couple of inches of copper wire is close to zero - let's say it's 1/1000th of an ohm - the resistance of a bird is probably 50 or 100 ohms. So the ratio of the current flowing down the wire to that flowing through the bird is going to be something like 100,000:1 - so almost no current travels through the bird and it's totally unaware that there is a problem.

Yet another way to look at it is in terms of voltage drop. The voltage drop over a couple of inches of copper is very close to zero - so the potential difference between one of the birds feet and the other is almost zero. It doesn't matter than the whole wire is at thousands of volts - the bird only cares about the potential difference.

Of course this assumes that the bird isn't touching anything else. A human who grabs hold of one of these wires while standing on a ladder or something will see the potential difference between the wire and the ground - which is thousands of volts. SteveBaker 22:37, 21 June 2007 (UTC)

Here comes yet another explanation: Think of what happens if you connect a light bulb to only one terminal of a battery. Nothing, because no current can flow through the light bulb if it isn't connected in both ends. In your question, the power line is like one of the battery terminals and the ground is the other battery terminal. The bird, of course, is the light bulb and will not be killed because no current can flow through it. Here is a movie of a person that is said to sit on a power line like a bird. —Bromskloss 23:34, 21 June 2007 (UTC)

WHen you get really high voltage wires, such as over 50000 volts the electric field around the wire is very strong and you can get corona discharge from spiky bits. Birds avoid these wires. GB 03:26, 22 June 2007 (UTC)

When a little bird lands on a relatively low voltage distribution wire, like 2400 volts to ground (4 kv phase to phase) or maybe 7200 volts to ground (12 kv phase to phase), they do not feel too much of a shock, because 1) they do not provide a path to ground or between phases and 2) the voltage is not high enough that the capacitive current from the line is too painful. Large birds, like eagles etc, sometimes land on utility poles and the current finds a path to ground or phase to phase through the bird. (Dead bird). On higher voltage, possibly 34 kv, 138 kv, or 345 kv phase to phase, birds avoid landing, because the capacitive current is too painful. Edison 04:33, 22 June 2007 (UTC)

Ok, sweet. There should be something added on Wiki to explain that...but where would it go, do you think? Oh, and thast video is pretty sweet. I'd like that job.-^Violask8₁₉₇₆ 19:49, 22 June 2007 (UTC)

I added a short note to the Electric shock article, but perhaps it should go in Power line too. --Heron 21:08, 22 June 2007 (UTC)

It should perhaps also be noted that this phenomenon is not limited to birds. Electric linemen under controlled conditions (i.e. while standing in insulated buckets carefully isolated from ground) can and do work on live wires bare-handed, although at higher voltages they have to take precautions to avoid capacitive inrush effects. I myself have held a bare, live, relatively low voltage (120V) wire in my mouth without ill effect, although as that was original research I suppose I shouldn't mention it here. (Oh, and the obligatory: "Don't try this at home, kids.") —Steve Summit (talk) 00:30, 23 June 2007 (UTC)

air vortex s and inverse square law

I heard that somewhere that air pressure applied by air vortexs that are launched from a source [e.g airzooka] conform to the inverse square law [half the distance four times the effect] is this true? and if not what law does apply to them? Robin

Well, the inverse square law generally applies to things that spread out at a uniform rate - the amount of light from a lightbulb that lands on a square foot of surface decreases as the square of the range because the light is spreading out such that at twice the distance, there is four times as much area to cover - and only the same amount of light to go around. However, a laser (more or less) doesn't spread out - so light from a laser doesn't obey the inverse square law. Same deal with sound. If you shout something - it's like a lightbulb - the energy in the sound spreads out over greater and greater area as it gets further and further away from you. But the airzooka is much more like a laser - the blast of air forms a stable toroidal vortex that hardly spreads out at all but which travels very slowly through the air (a lot slower than the speed of sound anyway) - it's like a laser beam but for very low frequency sound. So I don't think it does obey the inverse square law. It does gradually lose energy over distances of many meters - but I think that's due to the vortex gradually dissipating and becoming unstable rather than it spreading out like ripples in a pond. The relation between the energy and range might be rather complex to figure out because air turbulance is a tough phenomena to model. SteveBaker 00:59, 22 June 2007 (UTC)

I would use caution comparing an airzooka to a laser. The process of convection is not really a resonance amplification effect. However, from the very hand-wavey perspective, SteveBaker's analogy is good - the propagation of the disturbance is non-uniform. The airzooka blast is definitely not a sound wave, because it involves mass motion of air. Nimur 05:42, 22 June 2007 (UTC)

Yeah - I'm well aware of the weakness of my analogy - but it's the best way to explain why I don't think the airzooka obeys the inverse square law (which - if you've ever played with one - is pretty obvious). But the analogy breaks down if you try to use it to explain anything else! The rays of light from a laser stay parallel because the start out parallel - but otherwise, it's just light waves and they travel at the speed of light. The airzooka creates a very slow moving (maybe 2 meters/second!) self-sustaining vortex - and as you say, whilst it stays together as a coherent phenomenon, it doesn't travel in a dead straight line. If there is a light breeze, the vortex can be fired around corners! So it's definitely not a 'sound laser' - but the lack of spreading out of the energy wave warrants the analogy for the purpose of explaining the lack of an inverse-square effect. SteveBaker 11:45, 22 June 2007 (UTC)

A small correction here: the light from a laser beam does obey the inverse square law, as long as you are lot further away from the beam waist than the Rayleigh range (see gaussian beam). Laser light is no different from other light in this respect; it still obeys the laws of diffraction. Though the wavefronts are plane at the beam waist, they become spherical further out. --Prophys 12:03, 22 June 2007 (UTC)

As a minor aside, the common spelling of two vortexs is "vortices." At the very least, you might try "vortexes," which appears to be entering common usage. Nimur 05:46, 22 June 2007 (UTC)