Wikipedia:Reference desk/Archives/Science/2011 April 18

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April 18

Why isn't drinking coffee when tired considered negative feedback?

I was under the understanding that negative feedback could apply to both mechanical and voluntary actions to maintain homeostasis, such as a cold-blooded lizard regulating its temperature by crawling to a sunny rock. Why isn't drinking coffee when tired considered negative feedback? The person senses he is tired, this upsets homeostasis, then he drinks coffee, which contains caffeine, which restores the person to a normal level of energy. --Gary123 (talk) 00:25, 18 April 2011 (UTC)

~ It is a feedback loop, only one that is limited by it's positive and negative power rails, which limits the output swing. A person can drink a cup of coffee to counteract the feeling of tiredness. But can a person drink an unlimited amount of coffee? No. Therefor the output driver (coffee) is limited. It hits the supply rail (a limited number of cups of coffee). Once the driver has risen to the supply rail, the output driver can't rise any further, and if the input signal still goes negative (the person becomes more tired) then the negative feedback loop can't drive the output high enough (the person can't drink enough coffee) to balance the input (the tiredness). --InverseSubstance (talk) 00:37, 18 April 2011 (UTC)

It goes deeper than that. In the case of the lizard analogy, temperature is directly corrected by solar warming. In contrast, "tiredness" is a complex symptom that is only partially counteracted by caffeine, and the latter has a number of side effects. Therefore, tiredness and coffee consumption don't approximate a negative feedback loop very well. -- Scray (talk) 02:20, 18 April 2011 (UTC)

Drinking coffee to counteract fatigue is a type of negative feedback. Looie496 (talk) 03:00, 18 April 2011 (UTC)

It is, since fatigue reduces alertness, and caffeine can increase alertness. I think the responses above were highlighting the limitations - there are limits on caffeine's efficacy, and reduced alertness is only one part of fatigue. -- Scray (talk) 03:28, 18 April 2011 (UTC)

How are each Harley-heads better than each other?

How is a Twin Camhead better than an Evolutionhead better than a Shovelhead better than a Panhead better than a Knucklehead better than a Flathead better than an F-Head? --98.190.13.3 (talk) 00:40, 18 April 2011 (UTC)

I don't know anything about this, but might help others to note that this question is about Harley-Davidson motorcycle engines. DMacks (talk) 09:35, 18 April 2011 (UTC)

"Better" is very difficult to define, especially for a work like a Harley engine, which is both an engineering product and an aesthetic piece. We have articles about most of those; there are a few cases where the newer model has a clear, objective, measurably higher performance for particular specs (such as peak power, fuel efficiency, and so forth). You'll have to be more specific if you want a more precise answer - in the meantime, consider the links available from the Harley-Davidson engine timeline. Nimur (talk) 17:26, 18 April 2011 (UTC)

Deja vu - but without the sense of experiencing that exact event

I experienced the oddest thing today: I had a strong sense of deja vu, but it had nothing to do with the action I was committing. I was petting my cat, and suddenly, I had the strong impression I'd thought about doing this before. However, the memory was one of traveling in the car as a child (I actually remember which part of the country I was in, and the scenery). Simultaneously, I also had a memory come up of being in first grade, with each of us in the class sitting at a computer using the keyboard (I remember the teacher telling us to press "ESC").

Is there a name for this phenomenon? And for that matter, can I surmise that it's just a glitch in the way that my brain stores my memories - that it stored a few in a close location and that it improperly vibrated some neuron that brought the other two to bear? Magog the Ogre (talk) 01:21, 18 April 2011 (UTC)

Don't know about the rest but if you have improperly vibrating neurons that sounds like medical advice.190.56.16.200 (talk) 01:54, 18 April 2011 (UTC)

Perhaps you previously petted the cat when thinking about those things, so there's a link between memories of petting the cat and those things. StuRat (talk) 02:04, 18 April 2011 (UTC)

I absolutely cannot imagine sitting at a computer in the first grade. Edison (talk) 03:49, 18 April 2011 (UTC)

Yeah, i was thinking the same thing, i was actually thinking "wow, it's like computer usage somehow got mixed into his memory of being in 1st gradet" until i realized that had nothing to do with it and it was just a regular memory of using a computer in 1st grade. lol. Vespine (talk) 05:21, 18 April 2011 (UTC)

Yes, to clarify: me sitting at this desk in first grade was a real event. Me petting the cat was a real event. And me going down the interstate and looking at the landscape 20 years ago is a real event. But they have nothing to do with each other, on an intellectual level, but my brain couldn't shake the feeling they were related. Magog the Ogre (talk) 05:39, 18 April 2011 (UTC)

I don't think it is unusual for memories to be triggered in strange ways, and many of us have strange glitches in memory and recall, so I wouldn't regard the incident as one requiring a medical diagnosis, or even as a problem. The strongest memory links are those related to scents and emotions, so perhaps there was some subtle link that was not noticed by the conscious mind. Dbfirs 06:29, 18 April 2011 (UTC)

When I hear a recording of spoken word (I download a lot of podcasts, etc), and listen to it. I get a strong mental image of where I was last time I heard that segment of the recording. I can trace entire journeys by listening to the podcast again. This is the odd thing that my memory does.Zzubnik (talk) 14:16, 18 April 2011 (UTC)

See Involuntary memory. Alansplodge (talk) 22:11, 18 April 2011 (UTC)

UV and Air Disinfection

HI everyone, I understand UV disinfects air and clear solution such as water, but what if I have 5mL of water contained in a sealed 10mL vial. If the vial is exposed to germicidal UVC lamp for some time, does it mean that the air inside the vial and the water inside the vial become sterile. Considering the vial is clear and 1 ml thick. Thank you zak at <email removed> --Zkoulou (talk) 06:15, 18 April 2011 (UTC)

I assume you are talking about a vial you already have. Be aware that most glass is opaque to UV, so you will not be able to sterilize it that way. Ariel. (talk) 06:17, 18 April 2011 (UTC)

Transparent plastics are even better UV absorbers than the typical glass. Unless the vial was intentionally designed to be UV transparent, it is very unlikely that you can use UV light to sterilize the contents. (You can buy UV-transparent test tubes and cuvettes, if you are thinking about future applications.) If heating is okay, you might try placing the vial in a hot water bath (assuming the vial contains a water-based solution). If you do that though, you need to be sure it is only heated by the hot water to limit the temperature it is exposed to. If heated above its boiling point, a sealed vial may explode. Dragons flight (talk) 06:42, 18 April 2011 (UTC)

P.S. I removed the email address, per standard convention at the reference desk. Any replies will be posted here. Dragons flight (talk) 06:53, 18 April 2011 (UTC)

A few years ago I saw on a documentary that in India they put water in ordinary plastic botles and expose these to sunlight for some hours (I think more than 5 hours). The water was tested and it was found to be sterilized. So, perhaps, while the bottles are not very transparant to UV radiation, the small amount of UV radiation that still makes it into the bottle, is enough to sterilize the water over a period of several hours. Count Iblis (talk) 14:19, 18 April 2011 (UTC)

If exposure to solar UV were enough to sterilize things than the whole surface of the Earth ought to be sterile, including the rivers, oceans, grasslands, etc. whenever there is no cover from the sun. Perhaps sunlight was good enough to kill off certain pathogenic bacteria that they happened to be studying? I could imagine that might be plausible for many bacteria that thrive underground or inside hosts. However, many light-adapted bacteria actually benefit from such exposure. For the broader context of the question, I will note that near-UV, such as we receive from the sun, does partially penetrate plastic. By contrast, the hard-UV used in germicidal lamps will not penetrate plastic at all. Dragons flight (talk) 15:21, 18 April 2011 (UTC)

Yes, they were only looking at some specific pathogens, they grew some cultures using the original water and the water after it was exposed in that way. Count Iblis (talk) 15:36, 18 April 2011 (UTC)

I suspect that soda bottles left in the sun get very hot, which would go a long way towards sterilizing the contents. APL (talk) 19:00, 18 April 2011 (UTC)

Micro hydro and a home's power consumption

According to this, an average Vermont home uses 4,475 kWh of electricity per year. And according to our micro hydro article, these systems can generate up to 100 kW. So, how would I figure how much that 100 kW would cover of the needed 4,475 kWh? Dismas|^(talk) 10:07, 18 April 2011 (UTC)

You would multiply by the number of hours in a year:

100kW × 24h/day × 365 days = 876,000KWh

This should power 196 typical Vermont homes (876,000KWh/4,475 kWh). StuRat (talk) 10:25, 18 April 2011 (UTC)

Okay, that's what I thought but 196 homes doesn't seem very micro to me, so I thought I was doing something wrong. Thanks! Dismas|^(talk) 13:42, 18 April 2011 (UTC)

You might also be interested in peakers, which are also "micro-sized" power plants, for comparison. Peakers usually run on coal, diesel, or gas; they typically produce power ranging from hundreds of kilowatts to a few megawatts, and are considered "small" supplemental power stations to fill in the power supply/demand curve throughout a 24-hour period. Nimur (talk) 13:59, 18 April 2011 (UTC)

Peaker plants very seldom burn coal. Coal-fired plants generally take many hours to start up and shut down, and they don't scale down well. In some places you'll see coal plants that come on line to meet seasonal (often air conditioning) demand or to fill in when other (less smoggy) plants are down for maintenance, usually over many days, weeks, or months. In those circumstances, I'd consider them as serving to adjust the baseload supply, not acting as peaker plants. TenOfAllTrades(talk) 14:59, 18 April 2011 (UTC)

200 homes is definitely in the micro range. A typical hydro power plant can supply energy for several cities... Dauto (talk) 14:25, 18 April 2011 (UTC)

A limiting factor for microhydro is the elevation of the water source (termed the "head") and the flow. If you are in relatively flat land, there may not be enough head to generate much power. If the flow is not very much, ditto. If you wish to build a dam, there are extreme licensing requirements for the dam does not flood someone else's land, or bust and drown the people downstream. Hydro requires a great deal of maintenance to keep the trash rack clean. In extremely cold weather, there are problems with freezing. A dam also has environmental consequences. 100kw does not sound very "micro." Edison (talk) 19:35, 18 April 2011 (UTC)

Thanks all! I'm doing a small paper (2 pages, double spaced) for a quiz grade and was looking into alternative energy sources for houses. So, peaker plants are a bit out of the realm of what I was looking at. As is micro hydro now that I see it provides so much power! Dismas|^(talk) 03:37, 19 April 2011 (UTC)

Don't forget the "up to" part of "up to 100kW". They might well go down to the size you have in mind. StuRat (talk) 23:30, 19 April 2011 (UTC)

There is that. The paper was handed in already, so there's no fixing it now. Dismas|^(talk) 04:01, 20 April 2011 (UTC)

galileo's theory

galileo was punished by the church for teaching that the sun is stationary and earth move around it. his opponents held the view that the earth is stationary and the sun moves around it. if the absolute motion has no meaning,are the two viewpoints not equally correct or equally wrong? — Preceding unsigned comment added by Himani gill (talk • contribs) 11:45, 18 April 2011 (UTC)

Himani gill (talk) 11:48, 18 April 2011 (UTC)helpme

Neither are truly stationary (which is a concept that has no real meaning in a relativistic universe), and either could be used as a reference point, yes. However the Copernican model was "less wrong" with regard to the relative motion of the other stars than the Tychonian model the Church preferred at that point (but there was no actual observational evidence of that until much later). But it should be noted that it was not just statements about nature that caused Galileo to be punished — it was more a question of philosophy and politics that led to his particular outcome. --Mr.98 (talk) 11:53, 18 April 2011 (UTC)

It should also be noted that the condemnation of Galileo was not universal even within the church; many important cardinals privately held that Galileo was probably correct technically, but felt that the information would "confuse" common believers as to whom to believe. Some cardinals were openly in support of Galileo, Cardinal Caesar Baronius is quoted as saying "The bible tells us how to go to heavan, not how the heavens go." Copernicus was also a priest, and so it should not be clear that every religious person rejected the notion of Heliocentrism. However, as with many such organizations with diverse opinions, the stodgy, established, conservatives won out, which is why Galileo was ultimately censored and persecuted. Within a generation or two after his death, however, few were seriously questioning his conclusions. It was just a case (as it usually is) for the stodgy old establishment to simply die off before the more revolutionary (but ultimately more correct) ideas become established thinking. --Jayron32 13:06, 18 April 2011 (UTC)

Again, it's not so much the ideas that were the real problem. It had to do more with 17th century politics than anything else. Galileo was purposefully trying to provoke the Vatican, purposefully trying to undermine their authority in a number of ways. I'm not defending the Vatican, obviously, but it's not the case that they were just being stodgy and mean or backwards. They had some very, very talented astronomers and mathematicians working for them. In any case, Galileo did not actually have any great reason to prefer the Copernican model over the Tychonian one — both accorded perfectly with observations at that time. It was, at its purest (e.g. least political), a philosophical dispute, not a scientific one. Giorgio de Santillana's The Crime of Galileo (1955) is the classic reference here; Rivka Feldhay's Galileo and the Church (1995) is a somewhat more recent spin on it. --Mr.98 (talk) 17:43, 18 April 2011 (UTC)

Also, motion is relative but acceleration isn't and if we forget about general relativity for a moment The sun's acceleration is much smaller than the earth's acceleration which means the heliocentric model is the correct one. Dauto (talk) 14:20, 18 April 2011 (UTC)

It is mathematically consistant to create a set of comoving coordinates which hold the earth to be stationary while the universe spins around us during the course of a day. It is also non-intuitive to do so; by convention it makes more sense to hold that, when two bodies are in relative motion, the smaller (the Earth) is moving while the larger (everything else in the universe) is stationary. This is also not true, but (for example) it isn't useful to consider the movement of the Milky Way galaxy relative to the Local Group when I am, say, calculating my driving time on my trip to the grocery store. We use whatever conventions regarding "moving" and "stationary" to make intuitive sense (i.e is most conventient), even if choosing what to make "moving" and what to make "stationary" is mathematically arbitrary. In other words, since a) it doesn't matter what we choose and b) we still have to choose something to be stationary, we choose that which makes the most sense for the particular situation we are working in. --Jayron32 14:28, 18 April 2011 (UTC)

Just to add that the Sun and Earth orbit around their common barycentre (ignoring the gravitational influence of other bodies). See orbit for a full explanation.--Shantavira|^{feed me} 15:12, 18 April 2011 (UTC)

And I believe that barycenter is actually beneath the surface of the Sun, so the "most correct" way to view it is that the Earth orbits the Sun, and causes it to wobble a bit as it does. (One two-body system not like this is Pluto and Charon, where the barycenter is between them, so Charon is not so much a moon, as a twin, of Pluto.) Now, if the Earth and Sun were the only objects in the universe, there would be virtually no difference in a model that had the Sun orbiting the Earth. However, the other objects lock us into the true model, since the orbital observations only make sense when all the planets, including Earth, are modeled as orbiting the Sun. The apparent retrograde motion of some planets is the relevant observation, in this case. StuRat (talk) 17:03, 18 April 2011 (UTC)

Not only is it beneath the surface of the sun, it is much closer to the absolute center of the sun than it is to the surface. --Jayron32 17:09, 18 April 2011 (UTC)

Note: You could theoretically have a model where the Sun still orbits the Earth and yet all the other planets do orbit the Sun, and this would closely match observed orbital motions. As far as I know this was never proposed. I wonder why not, as it would seem to both explain observations of the day and be more consistent with Church teachings. This model would have only broken down once the relative sizes and masses of the Sun and Earth were known, although, even then, I suppose you could just say "the Earth doesn't move because God holds it still". StuRat (talk) 17:15, 18 April 2011 (UTC)

It was proposed and it was the theory the Church was advocating at the point of the Galileo incident (not the Ptolemaic model, which was completely incompatible with the phase of Venus observation, as the Church's astronomers did admit fairly quickly). See: Tychonic system. --Mr.98 (talk) 17:43, 18 April 2011 (UTC)

Hand shaking

Hey all. I find that when I'm nervous, especially (oddly enough) when I'm playing chess, my hands become cold and shake. How can I prevent this? Thanks. 72.128.95.0 (talk) 16:35, 18 April 2011 (UTC)

Relaxation techniques ? You might want to cut back on caffeine or other stimulants you may be taking, such as some diet pills. Also, if chess makes you that nervous, you might want to find something more relaxing. (I find a short timer makes me nervous, so maybe you just need to play longer games, perhaps even correspondence chess.) StuRat (talk) 16:41, 18 April 2011 (UTC)

Probably you get overexcited, you need to find a way to calm down. Technicaly, I think it is tremor, maybe reading the article will give you some ideas (note though that unles your hands shake all the time, you probably don`t have any serious condition that article might mention as cause) ~~Xil (talk) 19:17, 18 April 2011 (UTC)

You can talk to your doctor about beta blockers – one of the treatments mentioned in the article. They're relatively benign drugs – primarily prescribed for hypertension, but also prescribed, at lower dosages, for people without hypertension to counteract some physiological effects of nervousness (like tremor, for example – the reason they've been banned from the Olympic Games). 216.139.154.103 (talk) 17:00, 20 April 2011 (UTC)

Stress balls can also work. Count Iblis (talk) 21:08, 18 April 2011 (UTC)

Shake your hands at the same frequency and amplitute, 180 degrees out of phase, the superimposition of the waves should cancel, leaving your hands motionless. Plasmic Physics (talk) 06:56, 19 April 2011 (UTC)

Yes, if the laws of physics don`t kick in, it might confuse brain and senses enough to stop, thinking that they are still shaking ;) alternatively - shaking hands on purpose acctualy seems to make blood flow to them, warming them up and probably will make your body preocupied with something other than the game for a while ~~Xil (talk) 08:18, 19 April 2011 (UTC)

UV TEA laser

In a uv TEA laser, what sort of electric discharge between the electrodes is best/necessary?. ie should it be a brush discharge, glow discharge or some other sort of discharge? How would the desired discharge be achieved practically with regard to shaping/placing of the electrodes? Not home work. Just interested in maybe making one.--92.28.80.252 (talk) 17:24, 18 April 2011 (UTC)

We have repeatedly had such questions about home-brew lasers on the desk. I will repeat verbatim a quote from last month: "Avoid using Class III and Class IV lasers in your home!" Guys, this is not something to play around with. Powerful lasers are very dangerous. UV laser light is invisible and can permanently injure and blind you. If you are interested in lasers, get in touch with an optics research laboratory and learn how to work with lasers under proper, controlled conditions. Unlike other dangerous hobbies, such as playing with firearms and explosives, UV laser damage is both invisible and instantaneous. Nimur (talk) 17:30, 18 April 2011 (UTC)

OK Im not going to make one. Im just interested in the theory of how they work.--92.28.80.252 (talk) 17:46, 18 April 2011 (UTC)

It's a glow discharge. --Srleffler (talk) 17:17, 20 April 2011 (UTC)

A question of which I do not now how to title

Relative motion of the Earth

I heard a demonstration that said during the time when it was first proposed that the earth circles the sun, which many did not believe; so they gave an example to try to disprove this new theory. (Considering the earth moves 30 km/s) "Then consider a tree with a bird, and under the bird is a worm. The bird swoops down and tries to snatch the wrom. So if the bird takes a second to get down, then it is assumed that would have moved 30 km away from the tree and the worm by time it gets to the ground." How would counter that? Bugboy52.4 ¦ =-= 18:09, 18 April 2011 (UTC)

The argument is the same as saying that if you're in a car driving down the road, and you reach for the steering wheel, you won't be able to touch it because it will have moved a hundred feet down the road while you are reaching. Looie496 (talk) 18:12, 18 April 2011 (UTC)

In fact, Galilean relativity was invented by none other than... Galileo (!) to express the mathematics of relative motion. Briefly, this formulation provides exactly the "counter-argument" that you're looking for. Galileo used the analogy of a ship - the sailors are co-moving with the ship. Nimur (talk) 18:17, 18 April 2011 (UTC)

But why does the bird move with the earth. If I was on top of a moving car, why don't I fall of? If I'm in the car, then thats because the seat stops me. Bugboy52.4 ¦ =-= 18:21, 18 April 2011 (UTC)

The seat isn't "pushing" you forward, except during the time while the car is accelerating forward. After it reaches its final speed, you and the car travel at the same velocity. You and the car both have inertia, and travel with the same velocity, so there is no net horizontal force from the car acting on you. If the car changes speed, you will either slam back in your seat, or slam forward into the seatbelt. If the car doesn't change speed, you don't change speed either, and the net effect of the contact force against the seat-back is zero. Nimur (talk) 18:29, 18 April 2011 (UTC)

Nowadays I'd refer you to Newton's Laws of Motion. An object in motion stays in motion. The bird would keep moving with the earth simply because nothing stops it from doing so. Galileo seems to have covered it in his analogy of the a ship by pointing out that if you jump on a ship you don't go flying backwards, you just go up and down relative to the deck just like on solid ground.

Once your car is moving at a constant speed you don't actually need the seat-back to keep pushing you forward. You could balance easily. (However, when the car CHANGES speed or direction the seat and the seatbelt will speed you up or slow you down with the car.) APL (talk) 18:34, 18 April 2011 (UTC)

So do you need to be in contact, what if I was floating? Or if I tried playing catchbetween two different cars? Bugboy52.4 ¦ =-= 18:39, 18 April 2011 (UTC)

You don't need to be in contact. Space shuttle astronauts are traveling at tremendous speed but they just sort of float around lazily. If you were to fire the shuttle's engines while they were floating, then the shuttle would speed up and the astronauts wouldn't. Result : Astronauts smashing against the back wall. That's why they have to sit in their seats and buckle-up before they maneuver the ship for any reason.

Playing catch between two cars would work fine on the moon where there's no air. However, here on Earth something as light-weight as a ball would probably get slowed down too much by the air. (The air outside your car is still standing still, even if you're driving through it at 55mph.) APL (talk) 18:58, 18 April 2011 (UTC)

Yup, you might be able to get away with it if you are going slower though since air resistance increases in an exponential fasion (at least in the subsonic realm). Googlemeister (talk) 19:00, 18 April 2011 (UTC)

One way I like to think about this — and if this is entirely wrongheaded, someone please correct me — is that you notice when you are in a reference frame when you come up against another reference frame. So when you're inside the car, you're surrounded only by things in your reference frame (including the air you are breathing), thus if you go at a constant speed, it doesn't feel like you're accelerating at all (and you go along with the car just the same as if it were sitting still). If you open a window, suddenly air from another reference frame is swooping in at you, and you know you're traveling through the air (or, alternatively, that the air is traveling towards you while you are sitting still). If you were on top of the car, you'd be in constant contact with the reference frame and know it quite acutely that you were traveling through a medium. If you're inside the space shuttle, you're not going to notice it moving very much at all if it is constant (and the air surrounding you is moving with it). Step outside the space shuttle and you'll be a lot more aware (especially if you let go!). Similarly, the bird is surrounded not by empty space, but by oxygen, all of which is rotating with the Earth and going around the Sun and throughout the universe etc. as well. You'd have to get to a vantage point where you were really outside of the Earth's reference frame (e.g. in space) before you'd start to worry about its speed of rotation, the fact that it is going around the Sun, etc. — you'd need contact with another inertial frame before it was acute to you that you were in an inertial frame. (You can tell if you are in an accelerating frame; not if you are an inertial one.) --Mr.98 (talk) 19:12, 18 April 2011 (UTC)

You are confusing reference frame with atmosphere.

If there was no air on Earth the bird would still continue to move with the Earth.

If you step outside the space shuttle you continue to move with the space shuttle exactly as if you were still inside.

If you are outside the space shuttle holding onto a hand-hold and then you let go, you'll stay with the ship and be able to grab the hand-hold an hour later with no problem. (Assuming you don't push-off. then you'll go flying of course.) APL (talk) 19:36, 18 April 2011 (UTC)

Yes, that's true. If we imagine a bird on the moon, the bird is moving at the same speed (more or less) as the moon itself as it glides around the Earth (and around the Sun, and the Milky Way, etc.). It's not the air itself that matters, it's the fact that it's not in contact with anything moving in another reference frame. In the Earth examples, the air is still in your reference frame. But it isn't the air that matters, per se. --Mr.98 (talk) 18:44, 19 April 2011 (UTC)

You're dead on 98. actually Galileo's ship analogy is a poor one since the air outside the ship is not moving with the ship, and a sailor who could stay in the air a little longer would be left behind.190.56.107.69 (talk) 19:21, 18 April 2011 (UTC)

The ship analogy specifies that the sailor is below decks. APL (talk) 19:37, 18 April 2011 (UTC)

And, though I assumed most readers would know it, for the record we should state that Galilean theory has been superseded by Newtonian mechanics, and later by Einstein's formulation of relativity. Both of those provide more complete and accurate explanations. Nimur (talk) 21:10, 18 April 2011 (UTC)

I added a more useful (sub)title. StuRat (talk) 19:33, 18 April 2011 (UTC)

If I was holding on to a rocket and I would let go, wouldn't I get left behind considering its moving? Bugboy52.4 ¦ =-= 19:57, 18 April 2011 (UTC)

First of all, If you're on Earth then gravity and air complicate the question, so lets assume this is happening in space.

If the rocket is firing, then it is speeding up.

If the rocket is speeding up, then you need to hold on or you'll be left behind.

When the rocket engine stops, the rocket will still be coasting at a very fast speed, but it won't be speeding up anymore.

If it's not speeding up, then you can safely let go. Both you and the rocket will continue to drift in the same direction at the same speed forever. Unless you push off from the rocket, you'd be right next to it forever.

Hope this helps. APL (talk) 20:11, 18 April 2011 (UTC)

And again, it's worth emphasizing emphatically that the question is whether you are accelerating (speeding up or changing direction) or not. If you are not accelerating (in an inertial reference frame), then you're fine — you and the rocket are by definition going at the same speed in the same direction. If you are accelerating (turning or changing speed), then you're not in an inertial reference frame at all, but an accelerating frame, and all bets are off! --Mr.98 (talk) 18:44, 19 April 2011 (UTC)

A classic thought-experiment to help unpick this is to consider a fly in a moving car. When you drive at 10 mph, the fly can fly from the back of the car to the front in a few seconds. When you drive at 70 mph, the fly can still fly from the back to the front in a few seconds. Why? Does the fly have the ability to fly faster than 70 mph? But then, the fly is no quicker flying front to back: when the car travels at a steady speed, it doesn't matter to the fly how fast the car is travelling relative to the outside world, because everything in the car, including the air and the fly, are moving with it. The fly only has problems when the car speeds up or slows down: when the car accelerates, the fly has to catch up. It can do so simply by landing on part of the car, because the acceleration is not typically great enough to overcome the simple ability of a fly to stand on something. 86.164.75.102 (talk) 23:20, 18 April 2011 (UTC)

Might be worth mentioning that objects of different mass, such as a rocket ship and a person "fall" at the same rate. The only thing that makes this seem like not the case is wind resistance. On the moon, a feather and an anvil would hit the ground at the same time if dropped from the same height. Secondly, falling due to gravity is not affected by velocity perpendicular to the "ground". For example, a bullet fired horizontally will hit the ground at exactly the same time as something simply dropped from the same height, it will just be a lot further away. (again ignoring wind resistance). Vespine (talk) 00:15, 19 April 2011 (UTC)

Another query: Storage of Information

The only two subjects that have rely left me confused are those on the brain and computer chips (I have studied even quantum mechanics, but that's not as hard as these, at least for me). How is information stored in these two substrates: How does the brain store such information as memory, and how does a computer do the same? Does it store this information in the form of matter or energy? Bugboy52.4 ¦ =-= 20:07, 18 April 2011 (UTC)

For computer ram see Dynamic_random_access_memory#Operation_principle.

For computer storage see Hard_disk_drive#Magnetic_recording.

The mechanisms of the brain are not entirely known yet. It looks like short term memory is stored electrically, but long-term memory seems to involve actually re-wiring neurons. But there's a summary here : Long-term_memory#Biological_underpinnings_at_the_cellular_level

Hope this helps. APL (talk) 20:16, 18 April 2011 (UTC)

There's also Neuroanatomy of memory for a higher level look at the nuts and bolts. Clarityfiend (talk) 00:57, 19 April 2011 (UTC)

You may find Magnetic_core_memory easier to grasp than its modern analogues. Information is quite literally stored by the physical configuration of little magnetic donuts. Note that the analogy between computer memory and human memory is quite weak. SemanticMantis (talk) 20:20, 18 April 2011 (UTC)

How is the magnetic core memory any more physical than a hard drive memory? Dauto (talk) 20:46, 18 April 2011 (UTC)

You can see the physical location of bits in iron-donut core-memory without a microscope; but you can't actually see the bits, which exist as the presence or non-presence of an invisible magnetic field. Most modern electronics rely on "invisible" principles of physics (like voltage and current), and therefore require a little abstract thought. Very modern electronics are ultra-miniature, adding another level of confounding abstraction. Nimur (talk) 21:06, 18 April 2011 (UTC)

In other words, by more physical you actually mean more visual? Interesting... Dauto (talk) 21:13, 18 April 2011 (UTC)

Nimur was not the one who made the "physical" comment. APL (talk) 21:24, 18 April 2011 (UTC)

I know. But he answered my question with a very puzzling explanation. Dauto (talk) 21:33, 18 April 2011 (UTC)

Nimur has answered basically how I would have. I was trying to help the OP by giving an example that I think is easier to understand than modern RAM, in part because of its macroscopic nature. I didn't mean to imply that the physical position of the cores changed when the bits they encode change. If the OP is interested in cases where information is stored by physical configurations, compact discs and vinyl records are some common examples, and Mercury_memory is a type of physical memory that has used by digital computers when RAM technologies were not readily available. SemanticMantis (talk) 21:49, 18 April 2011 (UTC)

Tachyons. —Preceding unsigned comment added by 165.212.189.187 (talk) 13:21, 19 April 2011 (UTC)

transmutation

Would bombarding the nucleus with alpha particles move the element up the table of elements and would high voltage electricity have a effect on this? —Preceding unsigned comment added by 82.38.96.241 (talk) 20:52, 18 April 2011 (UTC)

It can. Neither alpha particle nor nuclear transmutation goes into it much, but for example californium is made that way from curium, which is made that way from plutonium. The high voltage electricity might be used to deflect alpha particles in some desired direction, or to power the magnets of a cyclotron; indirectly it gives the particles the kinetic energy needed to react successfully. Wnt (talk) 22:06, 18 April 2011 (UTC)

The OP may want to do some research into the work of Glenn Seaborg. The Wikipedia article and section Glenn_Seaborg#Pioneering_work_in_nuclear_chemistry hints on some of his work, but it roughly amounts to smashing alpha particles into large nuclei and hoping they stick. --Jayron32 00:29, 19 April 2011 (UTC)

Bouncy bicycles

Pneumatic bicycle tyres have less rolling resistance than solid tyres, both according to what I saw on a pop science program on TV, and from personal experience when I once used un-puncturable solid tyres which required much more effort to pedal.

Bearing that in mind, would it require less effort to pedal my bicycle if I replaced the seat post with a springy suspension seat post? I imagine that less energy would be wasted moving my body up and down over rough surfaces. Thanks. 92.29.122.67 (talk) 20:54, 18 April 2011 (UTC)

It depends on the type of terrain you're on. Typically, road-bikes are rigid; mountain bikes have suspensions, and we have an entire article on bike suspensions discussing all sorts of detail. Nimur (talk) 21:00, 18 April 2011 (UTC)

The reason the terrain matters is that, depending on the magnitude and wavelength of the bumps, a given fixed suspension may either reduce or increase motion of the passenger. So, with anything less than an active suspension system, you take your chances. In the case of a bicycle, I suspect that the weight of the suspension system would be more of an energy drain than you would expect to ever get back. I'd also be skeptical on saving energy, since your torso will then move up and down relative to your feet on the pedals, and your hands on the handlebars. This will require muscle movements to adjust for the constantly changing position. So then, why do mountain bikes have suspensions ? Mainly to reduce injuries, I suspect. StuRat (talk) 22:12, 18 April 2011 (UTC)

You are already making muscle movements to stay balanced on your seat, and even for just standing up. The latter is why standing up is more tiring than sitting down. 92.24.189.51 (talk) 10:31, 21 April 2011 (UTC)

But not nearly the same amount of muscle movements. There have been several exercise devices based on this, like trying to balance on a moving ball while seated. StuRat (talk) 09:59, 22 April 2011 (UTC)

The information here about bicycle suspension suggests that a suspension seatpost would probably be an improvement, particularly as I recall how much hard work and effort solid tyres required to pedal. 92.15.15.88 (talk) 08:33, 19 April 2011 (UTC)

Ethanol Efficiency.

An unsourced article stated that if the total energy imput was calculated to buy,plant,water,fertilize,harvest,process etc to produce a gallon of ethanol, it would exceed the energy available in that gallon produced.It did not take into account the associated loss of food production it replaced.Would someone care to comment? John Cowell.124.186.141.14 (talk) 22:33, 18 April 2011 (UTC)

Please don't use this forum as a discussion site. If you have a question, please ask it. Are you wondering whether the claims are accurate? Looie496 (talk) 22:52, 18 April 2011 (UTC)

YES! —Preceding unsigned comment added by 124.186.141.14 (talk) 23:00, 18 April 2011 (UTC)

It depends on how that ethanol is produced. Ethanol from corn kernels seems like it might fall into that category. Other sources, like sugar beets, seem better. Even better yet is if normal waste products or low value crops could be turned into ethanol, such as silage. The corn kernel, for example, is only a tiny portion of the total corn stalk, and is also useful as food, so using it to make ethanol in not wise. As far as whether it's good for the environment to use ethanol, other factors also matter, like whether fossil fuels are used to power the tractors and other equipment, or if they can be powered from more Earth-friendly technologies. StuRat (talk) 23:04, 18 April 2011 (UTC)

The answer you seek is here. Even if what you said was true, which was probably the case not that long ago, that's not an argument to "not pursue" a line of research. Lots of technology starts off as "un-viable" and requires a lot of R&D to become commercially useful. Just look at fusion power research, at the moment it's not creating ANY net energy at all, but there are still groups researching it in hope that one day it might become viable. Vespine (talk) 23:14, 18 April 2011 (UTC)

I certainly agree that research should continue, but the current issue, at least in the US, is that the government is subsidizing ethanol production from corn kernels, which seems unwise, since it's such an inefficient process. Perhaps those funds should instead be used to further research into better ways to make ethanol. To continue with your analogy, would it make sense for the government to subsidize large, production scale, fusion reactors, even if they used more energy than they produced ? StuRat (talk) 19:08, 19 April 2011 (UTC)

Ethanol fuel energy balance (linked from the ethanol fuel article cited above) looks like a decent stab at the topic. Wnt (talk) 03:17, 19 April 2011 (UTC)

The corn grown here is not used to make food, at least not directly. It is fed to cattle and hogs, or turned into ethonol. Losing out on food is a tiny problem in the USA. Googlemeister (talk) 15:10, 19 April 2011 (UTC)

Using corn for ethanol can affect food prices in many ways:

1) Corn prices go up.

2) Products containing corn products (like corn syrup) go up to.

3) Animals fed corn meal go up.

4) If land used for other crops is then switched to corn production, the lower supply of those other crops causes their prices to rise.

The result is that, if all our cars were 100% (corn kernel) ethanol powered, our food prices would skyrocket. So, we need to find a better way to make ethanol or give up on that goal. StuRat (talk) 19:00, 19 April 2011 (UTC)

We should avoid getting into a forum, but to ask a question: why does the U.S. have to put up with a 1.3-fold efficiency when Brazil gets an 8-fold efficiency? Do we really have no where to grow sugar cane? So much land in Louisiana and other Gulf states is being lost to rising seas and slumps over depleted oil wells and loss of sediment to various construction-related causes, seemingly without opposition. (Just look at the online maps of those states versus the actual satellite photo) Why isn't it cost effective to rescue some of the deluged land Holland-style and grow sugar cane on it for fuel? Wnt (talk) 19:45, 19 April 2011 (UTC)

That would cost billions, and would likely fail the next time a hurricane hit. (Holland doesn't get many of those.) Sugar cane could also be grown in Florida, but environmentalists wouldn't like to see us spoiling "pristine wetlands" (AKA stinky, mosquito filled swamps). Brazil has no problem with spoiling the Amazon basin. StuRat (talk) 23:24, 19 April 2011 (UTC)

Sugar cane is grown pretty intensively in the Rio Grande Valley of south Texas, and in Hawaii, as well as Florida. There are no other places with suitable climate, I believe. If not for political considerations, we could probably get a substantial amount from Cuba, where it is a major crop. Looie496 (talk) 02:26, 20 April 2011 (UTC)

Binding energy

In our article section Nuclear fission#Origin of the active energy and the curve of binding energy, it says the specific binding energy of elements like nickel and iron is greater than the specific binding energy of uranium. Doesn't it mean to say that it's lower, and that's where the potential energy difference lies? Comet Tuttle (talk) 23:03, 18 April 2011 (UTC)

The definitions provided in the binding energy article (referenced from the article you cite) suggest that the binding energy refers to the energy needed to dissociate the nuclear particles. Thus, if nickel and iron have higher binding energy, it will require more energy to separate nucleons. Doesn't that seem consistent? -- Scray (talk) 23:51, 18 April 2011 (UTC)

Hm. Yes, it makes sense, but I'm confused over where the energy comes from when a nuclear fission event occurs. If the specific binding energy (meaning "binding energy per particle") of a 238-particle nucleus is lower than the specific binding energies of the two fission products — suppose they're 38-particle and 200-particle nuclei, with no stray neutrons emitted — then it seems to me that the fission product nuclei have gained energy (per particle) as a result of the fission, which is the opposite of what occurs. Where's the defect in my understanding? Comet Tuttle (talk) 00:08, 19 April 2011 (UTC)

I am sure others have more updated nuclear physics knowledge than mine, but one thing I do recall that has always been true - always consider entropy, because it always wins. -- Scray (talk) 00:17, 19 April 2011 (UTC)

The article you linked indicates that electrostatic repulsion is the principal repulsive force in the nucleus. From a simplistic point of view, wouldn't that force provide substantial energy for fission? Here's a quote from farther up in the article you cited: "Once the nuclear lobes have been pushed to a critical distance, beyond which the short range strong force can no longer hold them together, the process of their separation proceeds from the energy of the (longer range) electromagnetic repulsion between the fragments. The result is two fission fragments moving away from each other, at high energy." -- Scray (talk) 00:27, 19 April 2011 (UTC)

About 80% of the total energy liberated in an individual fission reaction can be calculated just by taking into account the kinetic energy from the electromagnetic repulsion. The rest is from some of the spontaneous radioactive decay that occurs during the fissioning, instantaneous gamma rays produced in the fission, and the kinetic energy of the neutrons that pop out. --Mr.98 (talk) 15:35, 19 April 2011 (UTC)

(ec) I find it makes more sense when you don't think of binding energy as a "thing" but as the balance between the strong and electromagnetic forces. In uranium fission, you are taking a nuclei that is just on the line of being stable and wobbling it so that the electromagnetic forces suddenly outweigh the nuclear force (because the wobbling makes it act as if it were two distinct positively charged nuclei that happen to be right next to each other). You can calculate almost all of the fission energy by treating them as two very positively charged particles repelling from one another. That act of repulsion is the release of that binding energy, in a sense — in the same way that the springing of a spring is a release of stored mechanical energy. Looking at the "curve," you have a net energy release whenever you move up — whenever the binding energy per nucleon increases (on either side of the curve). --Mr.98 (talk) 00:22, 19 April 2011 (UTC)

The binding energy is always negative; elements in the middle of the periodic table have a "lower" per-nucleon binding energy inasmuch as −2 < −1, but a "higher" per-nucleon binding energy inasmuch as |−2| > |−1|. It's a linguistic inconsistency, not a physical one. -- BenRG (talk) 00:32, 19 April 2011 (UTC)

That's one of the problems I always have in this sort of discussion. Lower can mean both "more negative" or "smaller in magnitude". For example, a process which has a ΔH = -1000 KJ can be said to be more exothermic (release more energy) than one which has a ΔH = -500 KJ. However, we could also say that the first process has a lower enthalpy, being that -1000 is a lower number than -500. The trick is to be very precise in ones language; when we say that something has a higher binding energy, do we mean it has a greater magnitude of energy (say a larger absolute value), or do we mean that it has a more positive (i.e. farther to the right on the number line) value. It's a constant source of confusion, and you are correct that this is a linguistic problem that unfortunately has yet to be fully fixed. --Jayron32 00:37, 19 April 2011 (UTC)

You may find it helpful to think about the binding energy as being equivalent to (or at least precisely proportional to, depending on your preferred units) the mass defect of the given nucleus/nuclei. The mass defect is the difference in mass between the unbound, free, fully-separated nucleons (protons and neutrons) and the mass of the fully-assembled nucleus; the atomic nucleus will always have a slightly lower mass than the sum of the masses of its constituent particles. The mass defect is exactly equal to the total binding energy of the nucleus (by E=mc²), and it is the amount of energy that would be released if one were to assemble a nucleus 'from scratch'. As Jayron32 notes above, there can be some confusion about sign versus magnitude when comparing these values. It's important to realize that the binding energy isn't energy that a nucleus has or carries around with it. The binding energy is energy that was released when the nucleus was formed, and which you would have to put back in in order to break it up.

In the case of the fission of a heavy nucleus, we can make several equivalent statements:

• the total binding energy of the fission products will be greater than the binding energy of the initial nucleus;
• the binding energy per nucleon is higher in the products than in the initial nucleus (since we didn't change the number of nucleons);
• the combined mass defect of the products is greater than the mass defect of the original nucleus;
• the total mass of the products is less than the mass of the original nucleus; and
• the total energy (E=mc²) of the products is less than the total energy of the original nucleus.

Hope that helps. TenOfAllTrades(talk) 13:16, 19 April 2011 (UTC)

I remain confused, unfortunately, and now must ask a broader question, which I'll do below under the title "Fission2". Comet Tuttle (talk) 20:53, 22 April 2011 (UTC)