Wikipedia:Reference desk/Archives/Science/2009 May 29

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May 29

Time quickens?

Thank you for always answering my questions.

Does time ever quicken in special relativity? Lorenz transformations shows transformation of coordinates in a reference frame S to another reference frame S'. The time is transformed from t for S to t' for S' as follows.

$t'=\gamma (t-{\frac {vx}{c^{2}}})$

where v is the velocity of S' and x seems to be the distance of the event in the direction of x-axis for S. If v and x have the same sign, that is, if the origin of S' approaches the event (The origins of S nad S' are at the same point at time 0), t' will be less than $\gamma t$, and if, for example, x =vt, t' will be even less than t ( if t is positive).

By Relativistic aberration,

$\cos \theta _{o}={\frac {\cos \theta _{s}-{\frac {v}{c}}}{1-{\frac {v}{c}}\cos \theta _{s}}}$

where v is the velocity of the source, $\theta _{o}$ is the observed direction of the light ( It seems to be from the observer to the source), and $\theta _{s}$ is the direction of emission of the light (It seems to be from the source to the observer), if the light is emitted at an angle $90^{\circ }$ or $-90^{\circ }$, the light is observed as coming form $\theta _{o}$ where $\cos \theta _{o}=-{\frac {v}{c}}$, that is, from the hemisphere at opposite side to v. If two plane sources are at both side of the observer parallel to each other and to the axis of v, and if the source is approaching the observer, at the time the wavefront is supposed to be in head to head if at rest, they seem to have already crossed each other in the direction opposite to v.

Does time quicken? Like sushi (talk) 04:52, 29 May 2009 (UTC)

Special relativity applies to observers who are moving at a constant velocity relative to one another (and where the effects of gravity are either uniform or negligible). Each observer sees time passing more slowly for the other observer - this is a symmetric observational effect. If the observers want to compare the actual duration that they each experience between two events, they will have to both become stationary in the same frame of reference, so that they have a common definition of simultaneity (see relativity of simultaneity). To achieve this, one or the other (or both) will have to accelerate, which takes us out of the realm of special relativity and into general relativity. In general relativity, there are asymmetric time dilation effects due to non-uniform gravitational fields or, equivalently, relative accelerations. So when the two observers compare the actual duration that they have experienced between events, one observer may find that they have experienced a longer duration than the other. Informally, we can say that time has passed "more quickly" for one observer than for the other. See time dilation and twin paradox for more details. Gandalf61 (talk) 12:50, 29 May 2009 (UTC)

Gandalf, your explanation is essentially correct, except at the moment when you said "will have to accelerate, which takes us out of the realm of special relativity and into general relativity." Special relativity can deal with accelerated objects within flat Minkowsky space-time. See Rindler coordinates. Dauto (talk) 14:33, 29 May 2009 (UTC)

Looking at a moving image of relativity of simultaneity, it seems like, if the relative inertial motion is resolved, that is, if the two observers become stationary to each other, the effect of special relativistic time dilation vanishes. To achieve this, accelaration is needed, but if the accelarations are symmetric, and general relativistic time dilations are the same, observers can see the resoluion of special relativistic time dilation between them? And if so, is the resolution attributed to accelaration? (Then, for an observer, the other's accelaration must look differently from his, even though they are the same.)

Like sushi (talk) 14:49, 29 May 2009 (UTC)

If the accelerations are the same, both observers will experience the same amount of time lapse. Note that there is no need to use general relativity to understand that problem since it is assumed that the two observers are moving within flat space-time. Dauto (talk) 15:15, 29 May 2009 (UTC)

I know this is a "standard" view of special relativity that's taught in classrooms, but I strongly recommend ignoring it, I think it will just confuse you. The emphasis on disagreeing observers is backwards. There's only one world, and a bunch of equivalent descriptions of it in terms of different coordinates, and the Lorentz transformation is just a translation between different coordinate systems. It's the same as

x' = cos θ (x + y tan θ)

y' = cos θ (y − x tan θ)

for converting between different Cartesian coordinate systems that are at a relative angle of θ. I've deliberately written that in an unusual way to emphasize the similarity to the Lorentz transformation as it's usually written. With y = ct, v/c = tan θ and γ = cos θ, the analogy is very close. The coordinates (x,t) describe an event, i.e. something happening at a given place and time. The Lorentz transformation turns that into (x',t') which is the location of the same event with respect to a different coordinate system.

When you write down just the t' part of the Lorentz transform, it's the same as writing down just the y' part of the Cartesian rotation. That's likely to get you into trouble, because the y' coordinate is nearly meaningless without its corresponding x'. You could say that it describes the "horizontal" line through the point represented by (x',y'), but "horizontal" in this sense is a coordinate-dependent concept. In different coordinates you would have gotten a different line, and your choice of coordinates was arbitrary, so this line is meaningless. It's very tempting to imagine that t has independent meaning, because it does in Newtonian physics, but in relativity you'll frequently get into trouble it you treat it as though it does.

Okay, think of a line with evenly spaced dots on it, and a rotation of that line:

       *
       |
       |          *
       *         /
       |        *
       |       /
       *      *

I did the best I could in ASCII—the line segments are supposed to be the same length and the * spacing is supposed to be the same in both. The point is that the stars on the diagonal line are "more compressed in the vertical direction" than the stars on the vertical line. The rotation has compressed the dots, if you like, though it's a strange notion of compression that's tied specifically to the y or y' coordinate. A rotation won't always compress the dots, sometimes it will uncompress them. For example, if you take the diagonal line and rotate it the same amount counterclockwise, you'll get the vertical line again. This is the geometric meaning of your observation that the Lorentz transformation sometimes makes t' larger than t and sometimes smaller. There is no "quickening of time" exactly in special relativity, just as no rotation can increase the y-coordinate spacing of the dots beyond the vertical case, but there are rotations that increase the spacing.

So obviously this vertical dot compression doesn't really mean anything—the two lines are the same, our definition of "vertical" is arbitrary. But can we make the compression mean something? Yes, if we're willing to bend one of the lines and make a triangle, like this:

       *
       |\
       | *
       *  >
       | *
       |/
       *

That looks awful, but I hope the point is clear: "because of the compression", if you like, there are three star spacings' worth of distance from bottom to top on the bent line, only two on the straight line. Is this a paradox? After all, the compression was a meaningless coordinate artifact a moment ago. It's not a paradox because this is really a different setup, and it's not even clear it ought to have the same name. Before we had two symmetrical lines, now we have an asymmetrical arrangement, and it's the asymmetry that determines which path is longer.

Moving along to your intersecting wavefronts. This requires at least two spatial dimensions and one time dimension, which makes it harder to visualize. Let's say that, with respect to some arbitrary inertial frame S with coordinates (x,y,t), your wavefronts are moving straight up (in the +y direction) and straight down (in the −y direction) and intersect all along the x axis at t = 0. The events where the wave fronts appear are then given by y = ±ct (+ for the upward-moving one, − for the downward). If you now replace t by a third Cartesian coordinate, z, and plot y = ±cz for some constant c, the result is an "extruded X", like this:

      \\\\\/////////////     ^ z
       \\\/////////////      |
    ... \///////////// ...   |___\ x
        /\\\\\\\\\\\\\           /
       ///\\\\\\\\\\\\\
      /////\\\\\\\\\\\\\

with the x and z axes as shown and the y axis pointing into the page. Now, rotate this shape clockwise in the plane of the page by a small angle so that the part to the right tilts down. First, the crossing of the wavefronts (i.e. the former x axis) no longer happens "at the same time" (i.e. at the same z coordinate). Points farther to the right are now at lower values of z or at "earlier times". Second, this is hard to visualize, but if you take slices of the figure in the xy plane, representing the wavefronts "at a given instant", they are no longer parallel. Rather, they are both inclined to the left by an amount that depends on how much you rotated the figure. That's aberration.

This analogy can only be taken so far. Many things end up with the wrong sign (in special relativity there are fewer clock ticks along the non-straight path, not more). And there's no analogue to the speed of light or the distinction between past and future in Euclidean geometry. But the analogy is still very helpful for understanding what the Lorentz transformation and "observers" are really about. People talk it up to be a subtle, even mystical thing, but it really, really isn't. When people talk about "observers" they are talking about planar slices through the world, and, just as in Cartesian geometry, those slices have no special meaning. You can slice a cube one way and get squares, or a different way and get equilateral triangles and hexagons, but it's just a cube any way you slice it. There is a tendency to do far too much slicing in relativity because people don't want to let go of the Newtonian world where there really was such a thing as "the state of the universe at a given moment". To really understand relativity you need to give up the idea that, in the tilted extruded X, the wavefront crossing events happen "at different times" just because they have different z (or t) coordinates. That's a Newtonian idea and it will lead you astray. -- BenRG (talk) 18:36, 29 May 2009 (UTC)

I don't know if I have understood what you (Mr. or Ms.BenRG) say, but I think I do understand that two events which occur at the same place at rest will always be observed to occur with larger time interval if in relative motion. Now it might have been better to ask if an event can be observed at an earlier time in relative motion than at rest. And I think the answer is "yes"?.

(Additionally, I have been suspecting that, if an event occurs later in relative motion than at rest, the event which has already been observed could be something not yet has occured at the time. But this is solved, if I admit that c is the ultimate speed.)

Thank you.Like sushi (talk) 04:09, 30 May 2009 (UTC)

The point is that light relative to you will always travel the same speed- c. This is a product of Maxwells law and special relativity. Maxwells law say the speed of light is c (using energy density equations). Special relativity say all physical laws apply in intertial reference frames, so the speed of light is always c in an inertial reference frame. Imagine a man walking 5 m/s north on a train going 10 m/s. His speed relative to you, an observer on the ground, is 15 m/s. However a light pulse emitted on a train going .5 c relative to you, again on the ground, is still only c (Michelson Morley experiment, neutral pions prove for waves, particles). So, imagine a train with 2 mirrors. One is on the roof of the train, one on the floor. These mirrors are seperate by a distance c*t (where t is the amount of time the light takes to travel between the mirrors). Imagine a photocell on the top mirror. Every time a blip of light hits the top mirror a photocell on it records the time. The blip is than bounced to the floor and then bounced back to the top, striking the photocell again. But now imagine the train is moving with some velocity v. As the light pulse is traveling from the bottom of the train to the top, the top mirror has moved some distance v*t, where t is the time it takes the light pulse to reach the top of the train. So, the light doesn't only travel c*t, the distance between the mirrors. It also has a horizontal component of displacement right?. The vertical component is c*t, the horizontal is v*t, and you get a triangle. But to you, an observer on the ground, light cannot move faster than c. So it takes longer than a second for the light pulse to hit the top mirror. It happens say every 1.0001 seconds (thats a hella fast train). Time seems to have slowed down to the guy on the train. To the guy on the train the same thing happens to you. However, for the Fitzgerald contractions only the reference frame of the guy on the ground is correct.

  ----------MIRROR-----                     ----------MIRROR----- 
                                                   /
.          | light pulse                          /
                                                /
                                              /
  ----------MIRROR-----             ----------MIRROR-----   

train at rest moving train, so the top mirror moves a bit to the left as the light pulse travels.

24.171.145.63 (talk) 20:28, 30 May 2009 (UTC)

At the end of the explanation above 24.171.145.63 said "However, for the Fitzgerald contractions only the reference frame of the guy on the ground is correct." That is not correct. Both observers see each other's rulers suffer Fitzgerald contraction and they both see their own ruler suffer no contraction. Both are correct. Dauto (talk) 00:31, 31 May 2009 (UTC)

What I meant is that if the train stamped a mark on the track every time its clock said 1 second, these marks would be a certain distance x apart. But since its second is longer on the train than a guy who is at rest, the marks are really more than x meters apart. The guy at rest can take out a ruler and measure the distance and he will get more then x, not x. 24.171.145.63 (talk) 03:18, 31 May 2009 (UTC)

That is correct. On the other hand, it is also correct that if the track stamps a mark on the train every second, it will be the guy on the train that will end up measuring a distance larger than x. Dauto (talk) 04:39, 2 June 2009 (UTC)

TV signal splitters for coaxial cables

What do these actually do? Mine are encased in plastic and I cannot see what any circuity inside them is, if they have any. Does it make any difference which sockets of the Y you put the input and output cables?

I am running three tvs off one aerial. Sometimes all three tvs may be on at once. The aerial signal goes through an amplifier and then is split into a TV and cable2. Cable2 goes to another part of the house, then it is split into a tv and cable3. Cable3 feeds another tv. It is not possible to provide seperate cables, but as I imagine the TV aerial sockets are all joined up in parallel rather than in series, then it should not make any difference. Is there any way of doing this better while not going to the expense of more or better aerials please, or drilling new cables through walls and floors? 84.13.164.142 (talk) 10:11, 29 May 2009 (UTC)

The key consideration with splitters is that each split reduces signal quality. Based on the architecture you describe, TV 1 is getting a better signal than TV 2 and TV 3. As a result, you should consider putting your best / most-watched TV on the TV 1 line (or switching the splitters such that the best TV becomes the TV 1 line). In my personal experience, though, TVs are rarely affected by this sort of thing -- at least with normal numbers of splitters. More critical is something like a cable modem, which should always be placed as close to the incoming signal as possible. — Lomn 15:19, 29 May 2009 (UTC)

We have a splitter article, from which it's pretty obvious the thing is a hybrid coil. From that article, you can learn that that the inner electrical component is really just a small transformer. Impdendence-balancing is pretty important for high-quality RF signal handling. While 1→2 splitters are common, there are also 1→3 and others. For your configuration, that would let you do it all at once instead of a second split on one output of the first split (see Lomn's comments for why). DMacks (talk) 15:28, 29 May 2009 (UTC)

According to the article, a hybrid coil is a kind of transformer. These splitters are small and very light, so I very much doubt they have a tranformer in them. Perhaps they have no electronics at all. 78.146.211.210 (talk) 21:00, 29 May 2009 (UTC)

Transformers needn't be large nor heavy. A power transformer usually is because it needs to handle a lot of current, be insulated against a large voltage, and/or it has an iron metal core. But for low-voltage high-frequency RF applications you can have an air core and just a few windings. DMacks (talk) 21:53, 29 May 2009 (UTC)

A power splitter can also use resistors only. This is likely what you have inside the case. Resistive splitters, however, will always give unwanted extra attenuation to your signal whereas transformer splitters do not suffer this problem. Resistors are cheaper than transformers.--ThrobbingTrousers (talk) 02:58, 4 June 2009 (UTC)

Why are cockroaches such a pest in the US but not in the UK?

While coackroaches seem to be a common pest in the US, I've never heard of any cockroach problems in the UK. Yet in the UK we have far milder winters than in places like New York, so they would find it easier to survive the cold, and we also have central heating. Is it simply that cockroaches are not a native species in the UK? 84.13.164.142 (talk) 10:58, 29 May 2009 (UTC)

I have had cockroach issues in Glasgow (so they do exist), but you're right that they seem to be pretty mild in the UK. I'm curious too as to why; my auntie in Germany had some pretty horrific cockroach infestations, which really doesn't have a greatly different climate to Scotland. 87.114.167.162 (talk) 11:02, 29 May 2009 (UTC)

I don't know where your auntie in Germany is, but Frankfurt is about 5 C hotter in summer[1][2] which is not an insignificant difference. Most sources[3] say cockroaches are originally tropical, and Glasgow is much colder and further north (even if you take into account house heating). —Preceding unsigned comment added by Maltelauridsbrigge (talk • contribs) 11:25, 29 May 2009 (UTC)

My experience - mild winters make for a reasonably nice summer. When we get a hard cold freeze that doesn't thaw until Spring, the cockroach problem is much worse. Our exterminator claims that this is because the eggs and larvae and such can survive being frozen quickly, but not being slowly frozen. Where I live (at least for the next week or so), the problem is compounded by the addition of palmetto bugs (flying cockroaches). I've fogged the trees that overhang my house and it is rather scary how many palmetto bugs fall out - hundreds of them. So, we have cockroaches on the ground and palmetto bugs in the trees and almost nothing to keep their populations in check. -- kainaw™ 12:21, 29 May 2009 (UTC)

It is also because America is a land of opportunities.

Because New York is in the US. —Preceding unsigned comment added by Wikivanda199 (talk • contribs) 15:54, 29 May 2009 (UTC)

I don't know about the UK, but when apartments in New York don't have roaches, it's often because they have mice who are eating the roaches. 207.241.239.70 (talk) 04:32, 30 May 2009 (UTC)

Automatic train control

Building a computer/sensor system which could replace a human driver for a car or an airplane is exceedingly difficult and probably not possible today (especially not if operational safety is taken into account), although we are actively working on getting there. However, at first glance, I would have thought trains would be a lot more feasible. Several subway/metro systems in the world (eg Toulouse Metro) already do this. However, from the list of driverless trains, it looks like this has never been implemented for long-distance mainline trains (ie beyond a metro system serving a city). Is there a reason for this? Is there something that make automatic control of long-distance trains more difficult / less feasible than for metro systems? Thanks! — QuantumEleven 11:06, 29 May 2009 (UTC)

I think small, closed systems like the Toulouse Metro and the Docklands Light Railway are workable because the automatic controller totally "owns" the system - it knows where all the trains are (and the system is heavily instrumented to support that). Most existing long-distance train networks are a lot more ad-hoc (a lot more like the road) with a variety of trains, tracks, signalling systems, and vexing things like level crossings. I'm sure if you built a new, modern, fully grade-separated long distance railway then an automatic system would be quite workable. 87.114.167.162 (talk) 12:50, 29 May 2009 (UTC)

(ec) I'd say that the anon above me has hit the major points. I'll also add that there are challenges associated with the time and distance to an engineer in the event of a problem. For existing systems, you can have a couple of guys with pagers at head office downtown who can be anywhere in the metro system in under an hour. A train with some sort of impending mechanical failure can often coast as far as the next station to disembark its passengers.

Long-distance routes typically have stations which are much more widely separated than those of a city metro/subway/light rail system. It is possible to strand passengers in the 'middle of nowhere', potentially between multiple working tracks. Head office can't send help for hours, so you need to have widely dispersed response teams. The line remains blocked until the train receives assistance; this can have a ripple effect on all rail service across a country. It's sometimes remarkably useful to have a 'man on the scene'. TenOfAllTrades(talk) 13:24, 29 May 2009 (UTC)

It is probably cheaper to hire people to run the trains then it would be to install the hardware and develop the software to run such a system on the current rail networks. 65.121.141.34 (talk) 13:09, 29 May 2009 (UTC)

Plus there's a comfort factor in knowing there's a person up front "in control" even if they're really not, or if the computers do a better job overall anyway. Ask an A-380 or 777 pilot who's doing most of the flying up front, for example. Arakunem^Talk 13:28, 29 May 2009 (UTC)

Yep - on a typical transatlantic flight, the pilot actually flies the plane for less than three minutes. However, if you lose both engines in a freak bird-strike accident and have to land on a handy river...you probably want a pilot on board. This could probably be solved by having a handful of well-qualified pilots stading by on the ground who could take over and fly the plane by remote control at short-notice in the event of an emergency...but the public might find that hard to swallow. SteveBaker (talk) 14:16, 29 May 2009 (UTC)

Aye, there's the rub. That darn pattern recognition in humans is still hard to beat when it comes to picking a nice soft landing spot out of an urban jungle. :) Of course with trains, your options are much simpler: Go, Go faster, Go slower, Stop. Arakunem^Talk 14:20, 29 May 2009 (UTC)

I used to think that too. I used to design flight simulators for a living - so I know a lot about what pilots have to do. But one time we had a shot at building train simulators too (we actually delivered a bunch of them to Burlington Northern Railroads). That was quite an eye-opener. While it's true that you pretty much have just the one control - what you do with it is pretty tricky. It's such a high-skilled job that the average train driver in the US earns more than the average airline pilot - which surprised me quite a bit too! There are many subtleties with driving a train that are not at all obvious to the layperson.

For example - suppose you have a long train and a series of hills to go up and down. One's natural instinct is to gun the throttle to get up the hill and sit on the brakes on the way back down again - but often, as the locomotive crests the top of the hill - the majority of the train has not yet reached the bottom of the hill - so you often actually need more power rather than less - as you reach the bottom of the hill and start going up the next one - you tend to want to apply more power - but with the rest of the train stretching back a few miles behind you, most of the weight is still rolling downhill and you may well be applying the brakes whilst simultaneously heading up a steep hill!

Also, when your train is going around a curve, it has a tendancy to want to straighten out (like pulling on the ends of a piece of string) - which would derail the cars in the middle of the train - so keeping your the tensions in the couplings between cars fairly slack (which means no acceleration) is important when going around curves.

When you stop, it's essential not to come to a nice slow, gentle stop - you actually want to stomp on the brakes fairly abruptly...what that does is to cause all of the slack in the couplings to be taken out as each carriage runs into the one in front with a nice thump. If you don't do that - so that all of the couplings are stretched out tight - then when you try to accelerate away, the locomotive has to overcome the static friction and accelerate all 100 or so carriages at once...which it probably can't do. If the couplings are all squashed up - then the engine only has to start one carriage moving at a time - which is much easier. If you see a train backing up and then starting off rolling forwards - that's a sure sign that the driver screwed up and stopped too gently!

Then, locomotive drivers are responsible for keeping a lookout for things around the track that need maintenance or are in a dangerous state - this requires them to be alert throughout the entire trip - even if they are going 100 miles across a dead flat desert at a constant speed. There are loads more things like that. It's surprising how much complexity there is in such a seemingly simple system. Even though it's essentially a one-dimensional system - it's a lot tougher than driving a car OR flying a plane. SteveBaker (talk) 22:11, 29 May 2009 (UTC)

(Steve, I've taken the liberty of putting paragraph breaks into your item above. Please try to use normal length paragraphs.)

The issues Steve discusses about controlling a train are real, but most of them -- all except the last paragraph -- relate to long freight trains. Automatic driving of that sort of train, if it was ever going to happen, might well involve sensors placed along the train to provide feedback on the speed of different cars and the tension of the couplings -- sensors which, of course, ordinary freight cars today don't have.

On the other hand, passenger trains are shorter and have a higher power-to-weight ratio, which means those driving issues basically don't arise. My guess is that if we ever saw automatically driven trains on a long-distance railway, it'd be a totally isolated line without level crossings and with only one kind of train -- a description that best fits some of the high-speed passenger train lines built since in the late 20th century. Further, these lines already have cab signaling systems; it'd "just" be a matter of taking the human out of the loop.

On a number of subways and similar urban transit systems today, driving a train normally consists of pushing a "go" button when people have finished boarding at each stop. The human driver can take over if necessary (and on some systems is encouraged to do so for part of the day, to keep in practice), but usually doesn't. On the Docklands Light Railway in London, they go one step further: there is a human on board who can drive the train, but normally he's riding with the passengers, checking tickets and giving information. That's the way I'd expect an automated long-distance railway to do it: the same person who normally works as a conductor is also able to drive the train if the automatic system fails. Then there would be just two issues: (1) getting the consent of the crew members and their unions to such an arrangement, and (2) the hazard of operating a train with nobody looking out the front. But either or both of those might still be a "showstopper".

Also, I said above the the new high-speed lines are totally isolated, but that's only true of some of them (e.g. the Shinkansen in Japan, with a different track gauge from their other railways). The TGV trains in France and ICEs in Germany mostly spend only part of their time on the high-speed lines; if you travel from Nice to Paris, for example, until you reach Marseille you're on a conventional railway and your top speed is maybe half the TGV's limit of 186 mph. If automatic driving was introduced on the high-speed line, the train would still have to carry a human driver on the less isolated, mixed-traffic line from Nice to Marseille. --Anonymous, 05:04 UTC, May 30, 2009.

Feel free to criticise my overly long paragraphs - but I should point out that you should not edit other people's posts - it's a HUGE no-no around here. K'thnks. SteveBaker (talk) 15:52, 30 May 2009 (UTC)

"Except to fix formatting errors that interfere with readability". I had to break up the long wikitext line for technical reasons anyway, and I decided that the long paragraph was interfering with readability. I admit it's a marginal call, but the main reason for the rule is to not be deceptive and I announced what I'd done. --Anon, 16:40, May 30.

I would think that pilots only actually flying the aircraft for 3 minutes on TA flights would be a bad idea, because then when there is an actual emergency the computers can not handle, the pilot will be out of practice and will not perform optimally. 65.121.141.34 (talk) 14:42, 29 May 2009 (UTC)

Eh, I doubt this is as relevant as you consider it. What does the pilot need to practice? Takeoffs and landings in adverse conditions. What is all but 3 minutes of a TA flight? Stable level operation. Pilots need to keep sharp, yes, but they'll do that better in a good sim that provides emergency conditions than by doing nothing at 35000 feet. — Lomn 15:11, 29 May 2009 (UTC)

What keeps them sharp is definitely the simulator time. The landing on the Hudson river was fairly impressive - but the guy would have trained hard at looking for suitable landing spots in busy areas after unexpected engine failures - and would have also trained for water landings - it was really just a matter of putting the two things together. And even with all that training, there was one critical switch that they forgot to throw (it's right there in the landing-over-water procedures manual) that would have made the plane sink more slowly and given people more time to get off. Airline pilots don't spend a lot of time in the simulator - but when they do - they get all of the problems thrown at them at once. You're on approach...it's night - and it's foggy - and at the last minute you're switched to a parallel runway and...oh...your undercarriage didn't come down...and wouldn't you know it - there is another plane converging on your position - and your left engine is only producing half power...GO!!! SteveBaker (talk) 22:11, 29 May 2009 (UTC)

"Forgot" isn't exactly the right word. There was a checklist for the procedure and they were following it, but the failure happened at such a low altitude that there wasn't enough time to complete it. The "ditch button" was one of the things they didn't get to. (It would be interesting to see a copy of that checklist. I wonder if it's available somewhere.) --Anonymous, 05:10 UTC, May 30, 2009.

Yes - technically, what went wrong was that the co-pilot continued to attempt to restart the engines long after the airspeed was too low for that to stand even a chance to work. Hence he left it too late to start the procedures for ditching in water. Theoretically it is the Captains' responsibility to start or end these procedures - so he should have told the Copilot to abandon his efforts to start the engines earlier. However, he was pretty busy - and it's entirely understandable. All things considered, he did it pretty much by the book. SteveBaker (talk) 15:52, 30 May 2009 (UTC)

This is a recorded message from the computer flying the aircraft. Good day ladies and gentlemen and thank you for flying AutoAirways' new automatic airplane. Our pilotless system is perfectly safe because it is impossible for an error to occur-KLIKK for an error to occur-KLIKK for an error to occur-KLIKK for an error to... Cuddlyable3 (talk) 12:00, 30 May 2009 (UTC)

If pilots only fly a plane for 3 minutes, what do they do the rest of the time? Read a book? Play I spy? Sleep in their chairs? 78.151.147.255 (talk) 00:14, 3 June 2009 (UTC)

relay setting calculation for spaj 140 c

Can any one give me an example for relay setting calculation of abb spaj 140 c relay?????????? —Preceding unsigned comment added by Parthi2020 (talk • contribs) 11:48, 29 May 2009 (UTC)

Protective relaying takes a year or 2 of study,after getting an electrical engineering degree, and cannot be conveyed in a few words. Have you studied a standard text such as "Protective relaying theory and applications" by Walter Elmore, ABB, (2004) which sells for $100? It has examples such as you seek. The setting should be sensitive and selective, both terms of art. It should trip for faults but generally not for heavy loads. It should be able to carry normal and emergency loads, with tap settings and current transformer taps correctly chosen. This is one of the simpler applications, protecting a radial feeder. I have not used this particular relay. The manufacturer's brochure shows it to be a flexible relay capable of acting as device 50, 50N, 51, 51N, and 50 breaker protection. Do you know what these mean? You could take a simple example, and set it to trip for a fault on the feeder it protects, based on the calculated fault current, such that the backup protection does not operate. It should trip for the lowest current phase to phase or phase to ground fault, at the remote end of the feeder, perhaps "buried in" the remote transformer (so that you do not leave a bit of feeder unprotected at the remote end). It should not trip before downstream fuses or other devices have a chance to clear faults downstream from them: that is coordination. If used as a 50, the operating time would be substantially constant, like a CO-6 relay. This would allow fast clearing, and might be used on an undergrounfd feeder. The 51 application would mean the operating time decreases as the amount of fault current increases, like a CO-7 relay, or a GE IAC-51. Edison (talk) 14:35, 29 May 2009 (UTC)

My protection coordination course was only two hours instead of two years, but shouldn't the relay be set to trip before any downstream fuses, to give it a chance to clear a line-to-line fault? I seem to remember basing the relay settings on the time-current curves of downstream fuses—the strategy was to have the relay re-close twice, then stay closed long enough to blow the fuse, and finally trip and stay open.—eric 15:42, 29 May 2009 (UTC)

There are many strategies for fault clearing. What does Elmore say? Edison (talk) 20:16, 30 May 2009 (UTC)

Pork

I heard that Americans extensively use Pork in their daily diet,but there is a common belief that Pig is a host to many disease causing agents like the Tapeworm!!!!!It is also said that these agents may be destroyed by high temperatures but the ova doesn't get destroyed even at such temperatures.But lot of people still eat it without any fear of infection!!!!What makes them still consume Pork extensively???? —Preceding unsigned comment added by 59.165.84.9 (talk) 14:29, 29 May 2009 (UTC)

Tapeworms such as Taenia solium are not transmitted by eating cooked pork. Only the ingestion of uncooked pork, or contaminated human vomit and faeces transmit the worms. Once proper hygiene standards are adhered to, there's little to worry about. Same as any meat, really. Fribbler (talk) 14:35, 29 May 2009 (UTC)

Taste and price? Bacon is immense, and it's not particularly expensive. Pork chops are great and not baldy priced either. Sausages are commonly pork as well. Essentially the taste, price and availability of the meat are major factors on how much of it is eaten (though the amount that sells will have impacts on price and availability too of course). Also my understanding is that Pigs can survive in quite a lot of environments thus making them a good animal to farm which helps too. 194.221.133.226 (talk) 14:33, 29 May 2009 (UTC)

Americans and many other humans also smoke, which is proven to be bad for you. Just because it might be harmful, does not stop everyone from participating in said activity. Also, I have been eating pork for many years, and have never gotten a tapeworm to my knowledge, so I can infer that the risk of eating properly prepared pork to be low enough to justify the reward. mmmmmmmmmm bacon. 65.121.141.34 (talk) 14:39, 29 May 2009 (UTC)

I think, larvae remain alive only when the pork is undercooked and not all pigs harbor tapeworm larvae. Also FYI, pork is one of the world's most commonly consumed meats. So, it is not just USA but the other part of the world also consumes it including Europe, China or India. [4] - DSachan (talk) 14:41, 29 May 2009 (UTC)

Tapeworms and trichina are rare in pork in the United States, and we usually eat our pork fully cooked, which further reduces the risk. On the other hand, outbreaks of Escherichia coli O157:H7 from beef occur with some frequency, and a high percentage of chickens are contaminated with salmonella. Your statement that "there is a common belief that Pig is a host to many disease causing agents" probably reflects more on your own culture than either the beliefs or reality of US food production. -- Coneslayer (talk) 14:46, 29 May 2009 (UTC)

Chill dudes!!!!Even I like Pork but my muslim friend abused it so much that even I started feeling lethargic towards it....He also said that even if Pigs are reared in most hygienic environments they will not forget their inherent practice of eating each others faecal matter!!!!That was the reason I posted this question...actually I wanted to ask whether Pig is really a fit to be eaten animal or not??? —Preceding unsigned comment added by 59.165.84.9 (talk) 17:22, 29 May 2009 (UTC)

Religious taboos are generally not scientifically based; religious taboos established over a millennium ago are even less likely to be scientifically based. --jpgordon^∇∆∇∆ 18:16, 29 May 2009 (UTC)

Plenty of the answers have shown that pigs are clearly a hygienic enough animal to eat. They are widely eaten around the world, and according to this site (http://www.gan.ca/animals/pigs.en.html) and many others they are reasonable clean and intelligent animals (that is clean in a hygiene senese, not religious). ny156uk (talk) 17:57, 29 May 2009 (UTC)

In terms of hygiene, cows routinely get just as dirty as pigs when they are in feed lots. And chickens will be scavenge their own dead. 65.121.141.34 (talk) 18:31, 29 May 2009 (UTC)

Indeed, there is nothing particularly unique about pigs vis-a-vis other animals in terms of how hygenic eating it is. Swine and cattle and poultry and sheep and any animal can be kept under clean or dirty conditions; the meat can be properly handled or it can be contaminated by improper butchering techiniques. In terms of the risk of disease from pork vs. other meats, there is no difference at all. If you eat beer or lamb or chicken or turkey then these are no more or less risky to eat than pork is. --Jayron32.talk.contribs 20:19, 29 May 2009 (UTC)

Unless you drive too soon afterwards. 65.121.141.34 (talk) 20:22, 29 May 2009 (UTC)

Just skimming this makes me so glad I'm a vegetarian! I don't advocate eating any sentient being, but strictly sticking to the scientific nitty-gritty, the greatest risk with any kind of meat comes not from the kind of animal it's from, but from the conditions in which the animal lived and died and how the body was handled after death. If the environment was reasonably clean and the meat was properly handled at the right temperature with clean instruments, contamination is unlikely. The more the situation deviates from this ideal, the greater the risk there is that the end result will be unsuitable for consumption. - AJ —Preceding unsigned comment added by 66.215.227.218 (talk) 21:57, 29 May 2009 (UTC)

A comment to the vegetarian: Humanity has eaten meat for ten thousands of years, even before we could be considered "humanity". Obviously this works nicely even in very unclean environments and with animals that were far from living a sterile life, so "unsuitable for consumption" is a very...cultural expression. Today our nice big brain with all the cultural ideas in it may find it "unsuitable", but our ancestors certainly ate what meat they could get, and lived fine with it. Sure, they sometimes got sick, but our immunesystem is really capable of handling most of the insults that nature has thrown at us. I don't say you should eat every meat under every circumstance, but, believe it or not, not many (biological) things you can eat will kill you. This is really only western luxury thinking. --TheMaster17 (talk) 22:26, 29 May 2009 (UTC)

Somewhat of a side-issue, but I should point out that "humans have done it for 1000s of years so it must be ok/healthy enough" is not really a valid argument. The human life expectancy for most of its history has been ~30 years, and almost doubled in the 20th century after public health improvements were introduced. So during those 1000s of years of early history,

• many people did die of what we would now consider to be avoidable causes (food poisoning and various gastrointestinal diseases),
• the short lifetimes meant most people didn't live long enough to develop and subsequently die from chronic conditions like cancer, heart disease, diabetes etc; and so we cannot assess (based on historical evidence) if their habits and lifestyles increased the risk of developing such conditions.

Again, I am not arguing against your conclusion (that eating meat is not necessarily unhealthy); just your justification for it. Abecedare (talk) 01:01, 30 May 2009 (UTC)

The low average age in history (and in most countries with a low average today) is largely because of having a high number of infant deaths (see Infant mortality and Life expectancy. If you lived passed your adolescence you were likely to live to a similar age to people today. A simple wander around any old graveyard and you'll find examples of people living well into their 70s and 80s. I've no idea whether people died of food-poisoning etc. it would seem reasonable that they contributed to early-death but I doubt their contribution was notable - especially in comparison to infant mortality and child-birth. ny156uk (talk) 08:37, 30 May 2009 (UTC)

I totally agree with Ny156uk. And I have a counter argument for Avecedare: If the contribution would have been notable, there would have been selective pressure to correct this. So we would have adapted, and today's population would still be more tolerant. Evolution is a wonderful thing: It takes care that most organisms that survive are totally capable to cope with every thing that they encounter on a regular basis. And as far as we know, bacteria were here before us, so we always had to adapt to them. And our ancestors were also omnivore for a time long enough that our body is really adapted to this (teeth, digestion etc.). There may be short and individual perturbations of the balance with bacteria (this is why some of us still get ill), but on larger scales they are corrected. --TheMaster17 (talk) 11:07, 30 May 2009 (UTC)

Regardless of whether you're actually likely to get intestinal parasites from eating cooked pork or not, there's also the possibility that these worms are in fact good for you. See http://news.bbc.co.uk/1/hi/health/7856095.stm 213.122.2.54 (talk) 19:09, 30 May 2009 (UTC)

beer freezing

Water freezes at 32F. What temperature will a can of beer freeze? I understand that the alcoholic content would make a significant difference and lower the freezing point, but would a 5% alchohol content lower the freezing point by 1 degree, or more on the order of 10 degrees? Would this be overridden by the higher pressure found inside the can? I know, a lot of questions, but I don't have the equipment to test this observationally. 65.121.141.34 (talk) 14:33, 29 May 2009 (UTC)

American-style lager#Ice beer suggests that the water in the beer will freeze separately, see fractional freezing. SpinningSpark 14:44, 29 May 2009 (UTC)

In countries with a very cold climate, I think freezing is used as an informal way to concentrate alcohol to produce a kind of spirit, instead of distillation. 84.13.52.104 (talk) 15:15, 29 May 2009 (UTC)

Dude, 65 said a can of beer, not a vat. —Preceding unsigned comment added by Wikivanda199 (talk • contribs) 15:48, 29 May 2009 (UTC)

Dude, what does that's got to do with anything? Dauto (talk) 16:41, 29 May 2009 (UTC)

He might have a point. Fractional freezing might be effected by pressure. And I assume that the can is pressurized and the vat is not? 65.121.141.34 (talk) 19:00, 29 May 2009 (UTC)

Coming back to the original question: Ethanol freezes at -174F, so the answer has to lie somewhere between 32F and -174F :-) I think we need a theoretical chemist here who can give a melting point formula for solutions. I have no clue if the relationship is linear. If this was the case, the freezing point would be lowered by 5% of 32F-(-174F)=5% of 206F which is roughly 10F, so it would freeze at 22F (without fractionated freezing, which would be irrelevant if the whole can freezes and is melted again (without taking out frozen solid). Hmm, but this is all considering normal pressure. I can't remember my physics lessons concerning pressure and melting point any more. Chemists? Physicists? Anyone? --TheMaster17 (talk) 23:09, 29 May 2009 (UTC)

It's certainly not a linear relationship. Due to the phenomenon called freezing point depression, the freezing point of a solution (in this case, water and ethanol) can be lower than the freezing point of either of the two substances alone. The freezing point of water is 0°C and the freezing point of ethylene glycol is −13°C, but when mixed in the proper ratio (as in automobile antifreeze) the freezing point can be lowered to about −50°C. See, for example, [5]. (I failed to find a good explanation of this in a Wikipedia article, though.) —Bkell (talk) 02:50, 30 May 2009 (UTC)

So the answer is: We don't know? Because there are at least three phenomenons involved: Mixing, freezing point depression and fractional freezing? So there is no "rule of thumb" for even water and ethanol? --TheMaster17 (talk) 10:50, 30 May 2009 (UTC)

No precise answer, but i can tell you that 4.7% alc. beer freezes in a typical household freezer (-18°C). But this can force the bubbles out, causing the container to explode, so the beer splatters all over the place before freezing.YobMod 11:11, 31 May 2009 (UTC)

Best solvent for a blocked printer head?

My colour cartridge printer head is partly blocked. This comment here http://www.pcguide.com/vb/showthread.php?t=49543 suggests other solvents: "Since it's dried hard you need a more powerful solvent than plain water cold or hot. Try Windex. If that does'nt work use straight household amonia. Clorox Bleach will remove the ink no matter how hard it is. Don't bleach too long or it will dissolve other parts of the cartrige as well. Heat will help, but I would not exceed 180 deg F."

What would be the best solvent for dissolving dried ink without dissolving the plastic-and-copper ink cartridge? I have a number of solvents available: water, white sprit, nail varnish remover, bleach, RugDoctor Traffic Cleaner which I think contains glycol - the same chemical used as a solvent in gloss paint. Would anything be better than water?

I am from the UK, so what would the UK equivalent of Windex be? Is ammonia available here - I've never seen any for sale? And is "Clorox Bleach" just the same as "bleach"? The cartridge is an HP78. Thanks. 84.13.52.104 (talk) 15:28, 29 May 2009 (UTC)

Windex in the US is a window cleaner and is usually blue. I would imagine the standard window cleaner in the UK will be chemically the same. 65.121.141.34 (talk) 16:32, 29 May 2009 (UTC)

Clorox bleach is just bleach. Windex is a window cleaner with ammonia in it; there are probably equivalents over there? --98.217.14.211 (talk) 17:39, 29 May 2009 (UTC)

Don't mix bleach and ammonia, though; bad things can result. —Bkell (talk) 03:00, 30 May 2009 (UTC)

Ethanol? Propanol? --Russoc4 (talk) 17:44, 29 May 2009 (UTC)

Ultrasonic cleaning using water is worth trying instead of chemicals. Cuddlyable3 (talk) 11:26, 30 May 2009 (UTC)

Unfortunately I do not have an ultrasonic cleaner. I am surprised that bleach is recommended, as it is not a solvent, but perhaps bleaching the ink colour chemicals helps. 78.147.249.77 (talk) 15:53, 30 May 2009 (UTC)

Bleach (sodium hypochlorite), especially undiluted bleach, is a reasonably powerful oxidizer. This is why it works for bleaching - the colored molecules are oxidized to a colorless form. However the oxidation reaction is not limited to dye molecules, and a large number of molecules can be oxidized. This is why sodium hypochlorite is sometimes used as a drain cleaner - oxidizing molecules tends to break them into smaller pieces which may be easier to dissolve/mechanically dislodge. Using bleach is somewhat of a scorched earth tactic, though, as in its undiluted form it is quite reactive and is slightly dangerous (especially, as mentioned above, with ammonia). If you wanted to use an oxidizer, I might start with a gentler one such as hydrogen peroxide. But before I did any of that, I'd see what the manufacturer recommends to be done for clogged print heads. If it's from a company like HP, where the print head is part of the cartridge itself, you might be best served just buying a new one. -- 128.104.112.106 (talk) 17:15, 30 May 2009 (UTC)

This webpage http://www.northlight-images.co.uk/article_pages/inkjet_cleaning.html recommends using a houshold bathroom spray cleaner, which contains bleach. I like to refill my cartridges, and now have some old ones to play with, so I will try bleach. I do not have any hydrogen peroxide, unless it is easy to buy. 78.144.254.133 (talk) 20:31, 30 May 2009 (UTC)

Try standing the head in a bath of Whey overnight. (Should be available at your grocery store or health food store.) Then rinse in water and pad onto a folded pad of paper towels. If you are in the UK you may be able to find a solvent for drafting pens marketed by the German company Rotring. That should also work. 71.236.26.74 (talk) 07:23, 1 June 2009 (UTC)

Sleep

How much amount of sleep is really necessary for a man in his 20's????Will 5 hours of sleep in night suffice??? —Preceding unsigned comment added by 59.165.84.9 (talk) 17:25, 29 May 2009 (UTC)

You should check out the article sleep. See optimal amount of sleep for adults. 152.16.223.48 (talk) 17:33, 29 May 2009 (UTC)

Old proverb: six hours for a woman, seven for a man and eight for a fool. SpinningSpark 20:01, 29 May 2009 (UTC)

What does that make the French, with their nine hour average?[6] Clarityfiend (talk) 23:10, 29 May 2009 (UTC)

No comment. --Tango (talk) 23:50, 29 May 2009 (UTC)

Yeah, but you got that number from the Daily Mail. Here is a 2006 survey by the Institut National de Sommeil et de Vigilance which comes up with an average of 7.5 hours. An average is not a very reliable figure anyway, since sleep patterns vary a lot with age, and different countries have different age profiles. --Heron (talk) 09:38, 30 May 2009 (UTC)

Twelve for a parrot, FWIW. Plus a long nap in the afternoon. Alright for some, isn't it? ;) --Kurt Shaped Box (talk) 10:04, 30 May 2009 (UTC)

Carnivores that naturally hunt for their food - Dogs, cats, lions, etc - often sleep for over 20 hours a day. If they don't need to hunt for food - they can simply sleep and conserve energy. How much sleep they need - is a different matter though. SteveBaker (talk) 15:11, 30 May 2009 (UTC)

I've heard it proposed that sleep is evolution's way of keeping us out of trouble when we don't need to be doing anything useful. --Tango (talk) 21:14, 30 May 2009 (UTC)

That does not even start to explain the dreadful consequences the following day of sleep deprivation during the night. It is much more likely the other way round, that we sleep overnight because that is a convenient time to do it. Disconnecting our brain and senses from the environment can be a dangerous thing to do when there are predators about, yet all animals do this despite it being unecessary merely to conserve energy. There simply have to be further reasons for sleep, many of which are discussed in the article, and I find it telling that bears waking from hibernation immediately have a need for sleep because they are suffering from sleep deprivation. SpinningSpark 09:17, 31 May 2009 (UTC)

There are all sorts of bizarre things about the way other animals sleep. Dolphins sleep with one half of their brain at a time - so they are never totally unconscious - which is just as well because they need to keep swimming to the surface in order to breathe. There was a hilarious piece of research I read recently that said that some roosting birds can do the same half-brain-at-a-time sleeping trick - but when they are perched in a row - only the birds on the ends of the row have half their brains awake...which suggests that they need to keep half their brain awake in order to keep one eye functioning to look out for danger - while the birds in the middle of the row can sleep with both hemispheres! SteveBaker (talk) 18:32, 1 June 2009 (UTC)

I read something similar recently about sleeping gulls (it may even have been based on the same research). When they're all flocked on a field or a car park, or wherever and dozing in the sun, it's only the ones in the centre of the congregation that sleep with both sides of the brain. I'll bet that it's the larger, stronger more dominant ones that always get to sit right in the middle. --Kurt Shaped Box (talk) 19:45, 1 June 2009 (UTC)

Fetus affected by anesthesia?

If a pregnant woman is put under general anesthesia, will the fetus be anesthetized as well, or would the placenta filter out too much of the anesthetic to make a difference? —Preceding unsigned comment added by 66.215.227.218 (talk) 21:48, 29 May 2009 (UTC)

This article suggests that the answer is ... maybe. --Sean 00:22, 30 May 2009 (UTC)

Thanks. I've since found another article that says that the fetus would be anesthetized under general anesthesia . . . at least they don't seem to get the same stress reactions and reflex responses that would seem to indicate pain during medical procedures. Of course, since the fetus can't report pain in the same way you or I would, it's hard to know for sure. —Preceding unsigned comment added by 66.215.227.218 (talk) 18:48, 30 May 2009 (UTC)

There is a remarkable phenomenon in newborns called the breast crawl (no article yet, but see [7]). When the mother is given analgesics (such as pethidine) around birth, the baby performs very poorly in breast crawling, suggesting analgesics effect the bably also. Indeed one study has found that blood plasma half life of pethidine is 3.0-4.5 hours in the mother, but as long as 13-23 hours in the infant, suggesting the impact on a fetus may actually be potentiated. Rockpocket 21:09, 30 May 2009 (UTC)

Could this rocket put a person into space?

The Skylark (rocket) can lift 440 pounds to 357 miles. Could it in theory be used to put a person into space for a brief time? Would there be so much of a problem with re-entry heating if it was not moving at an orbital speed but just going straight up and straight down? In any case, does the final stage of the rocket burn up or not as it falls to earth? 78.146.211.210 (talk) 23:14, 29 May 2009 (UTC)

I would think the payload's volume would be too small to make this feasible... But it's not given so I don't know. Clearly the available space for life support would be minimal, and I think heat shielding alone is likely to make 440 lbs too little. TastyCakes (talk) 23:24, 29 May 2009 (UTC)

Even if you could fit in the life support (not a great deal needed for just a few minutes) and heat shield (not much needed for a sub-orbital craft compared to a Space Shuttle or similar), you would need some impressive parachutes and they aren't light. --Tango (talk) 23:44, 29 May 2009 (UTC)

Taking the Project Mercury capsules as a baseline, the article says their launch weight was 4,265 lb. However, nearly half of this is fuel for the retro-engine burn to get the thing out of orbit - this would not be needed in the case of a straight-up-straight-down flight. The landing weight was 2,241 lb. A good deal of this would be engines, also not needed, and also the remains of the heatshield can be subtracted and most of the life-support. After taking all that off you might just about be getting into the right ballpark of 440 lb, but it is probably right on the limit of what can be achieved. But remember, space actually starts a lot lower than 357 miles, it is 62.1 miles by one definition. Skylark could easily achieve a manned mission to this height since payload is much greater for a lower altitude mission. SpinningSpark 01:14, 30 May 2009 (UTC)

Of course, being in space is not the same as being in orbit. A significantly larger rocket is needed to reach an altitude and stay there (in other words, to change the necessary angular momentum for an orbit that does not re-intersect the ground). Further, orbital altitudes are necessarily much higher than the ~100 km "boundary of space" definition, because atmospheric drag will decay very low orbits at a dramatic pace. Low earth orbits are typically as high as 300-500 km. Nimur (talk) 15:09, 30 May 2009 (UTC)

Wear a space suit, and a lightweight bag full of foam plus a personal parachute would suffice for reentry from orbit, let alone from a suborbital flight. See MOOSE. Edison (talk) 20:08, 30 May 2009 (UTC)

A space suit isn't light, that lightweight foam is only lightweight relative to a conventional heat shield and the parachute you would need to land all that weight at a safe speed wouldn't be light either (unless you could someone bail out of the bag and suit at just the right moment so the parachute only have to slow you down). You're not going to fit all that, and a person, into 440lbs. --Tango (talk) 20:32, 30 May 2009 (UTC)

Skylark is only 17 inches in diameter. Even if you envision a small person standing straight up the whole time it is hard to see how one could launch a person and the hardware required to get them back through reentry while using such a small rocket. Dragons flight (talk) 22:33, 30 May 2009 (UTC)

I would give it a shot, personally speaking. Sometimes the payload is larger in diameter than the rest of the rocket. Edison (talk) 02:05, 31 May 2009 (UTC)

pH change at river mouth?

I was just wondering, at the mouth of a river, would the pH change with water depth?

I know that the water containing the lower salt concentrations would remain floating up the top (usually the river water)and that the ocean water (containg more salt ions) would remain down the bottom. but does the pH vary with water depth at the mouth of the river where this water seperation occurs? if so, why?

thanks —Preceding unsigned comment added by 122.108.206.219 (talk) 23:45, 29 May 2009 (UTC)

Lots of salts alter the pH of water, so pH would also change with different amounts of different sorts of salts. It would be impossible to predict how pH will change with depth, but I would expect that in any body of water, there will be some variance in pH at different locations, whether its in the deep ocean, a fresh-water lake, or a brackish estuary like you describe. --Jayron32.talk.contribs 00:47, 31 May 2009 (UTC)