Wikipedia:Reference desk/Archives/Science/2009 February 2

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February 2

Blood tests

Over a lifetime annual physical exams and diagnostic lab work will have taken blood from the elbow veins many, many times. Is that ever a concern in non-immunodeficient individuals? Thanks. Imagine Reason (talk) 01:32, 2 February 2009 (UTC)

Do you mean, could someone be harmed by removal of blood from the same location? The venipuncture article has information about this procedure. The risks are quite minimal, and relate to the puncture of skin, the vein, and any inadvertently-punctured structures nearby. Removed blood must be replaced through hematopoeisis and other processes, but this is a systemic, not local effect. In other words, if you have blood removed always from the left antecubital fossa, this has no special effect on the blood present in the left arm 10 minutes later - circulation takes care of that. By the same token, the blood tests taken from one arm are expected (within certain levels of approximation) to represent the rest of venous blood. Perhaps I misunderstand your question, though. --Scray (talk) 02:10, 2 February 2009 (UTC)

I guess there might be a question about scarring or weakening of the vein itself...but I have no idea. SteveBaker (talk) 02:40, 2 February 2009 (UTC)

Even under ideal conditions, there is always a small risk of infection associated with venipuncture. If proper precautions are taken and the individual has a reasonably healthy immune system, this risk is quite small. (I don't have statistics handy, but bear in mind that otherwise healthy individuals are permitted to donate blood, platelets, or plasma hundreds of times, generally without incident.)

Repeated insertion of intravenous lines can lead to local scarring of the vein, making it more difficult to insert a needle in the future. This is sometimes a problem for repeat blood donors, and more often a problem for intravenous drug abusers or for individuals who have chronic illnesses requiring frequent, recurring blood draws.

Problems may also arise in individuals whose blood clotting is severely compromised. TenOfAllTrades(talk) 04:33, 2 February 2009 (UTC)

Repeated access to the blood stream is a big problem for hemodialysis patients. That's why they get AV grafts to help with any scarring of a natural vein. --Ayacop (talk) 11:14, 2 February 2009 (UTC)

Snow fleas

Why do snow fleas cluster on the surface of snow? There can be little of nutrient value there. They particularly seem to cluster on the sides of indentations in the show surface, such as footprints. Is there any explanation for this? Thanks. Marco polo (talk) 02:05, 2 February 2009 (UTC)

There can well be all kinds of algae (see Snow algae and Chlamydomonas nivalis) thriving in the thawing snow. There can also be a lot of microscopic detritus adhering to the surface of the snow, especially in the depressions of the surface. I would guess snow-fleas gather to feed on that. --Dr Dima (talk) 03:05, 2 February 2009 (UTC)

Dwarfism due to gas attack?

Are any of the chemical weapons used in the First World War known to cause reproductive genetic damage? Could such damage cause dwarfism? I recently met a man, approximately five feet tall andwith a prepubescent-sounding voice, who said that because his grandfather sustained a gas attack while serving in the war, his mother and all her siblings have dwarfism. A mutual friend describes him as a "storyteller" but confirms he is 25 years old. NeonMerlin 02:48, 2 February 2009 (UTC)

To establish it as a credible fact, there would need to be more than a single incident. I found this book, Veterans at Risk, by the Institute of Medicine (U.S.). Committee to Survey the Health Effects of Mustard Gas and Lewisite, Constance M. Pechura, David P. Rall. "Sulfur mustard causes chromosome breakage and induces sister chromatid exchanges in a wide variety of cells. Epidemiologic studies have also led the International Agency for Research on Cancer to classify sulfur mustard as a human carcinogen. These observations underscore the potential of this compound to induce genetic damage. They also suggest that sulfur mustards could be a reproductive toxin." Dwarfism does not appear specifically mentioned. Nimur (talk) 03:37, 2 February 2009 (UTC)

Wouldn't it have been quite cruel by the grandfather to father more dwarfs after he saw what his first offspring became? That's why I don't believe this. --Ayacop (talk) 11:08, 2 February 2009 (UTC)

Try not to be too judgmental of other peoples' reproductive decisions. Families affected by genetic conditions are often in an excruciatingly difficult situation trying to decide whether or not to have additional children. To say that it was "cruel" for the grandfather to try to have more children after the birth of an affected child is not warranted. We don't know the circumstances of his situation. --- Medical geneticist (talk) 14:49, 2 February 2009 (UTC)

Plus, that presupposes that life as a dwarf is so absolutely horrible that it would be better if they'd never been born. Do you think that if you asked the siblings whether they considered their father cruel to bring them into the world, they would reply that they'd rather not have been born? If you had an ugly child, would you consider it cruel to have anymore when you saw what the first 'became'? Do you think the man NeonMerlin met should never have been born? I find it very hard to get my head round this point of view. After all, he wasn't taking non-dwarf children and 'making' them dwarfs. 79.66.57.25 (talk) 18:12, 3 February 2009 (UTC)

An alternative explanation could be that the grandfather sought some sort of external rationalisation for the fathering of dwarf offspring as he may have been unable to accept that it was a genetic anomaly within himself. Richard Avery (talk) 11:36, 2 February 2009 (UTC)

Achondroplasia, the most commonly recognized form of dwarfism is caused (in the vast majority of cases) by a de novo mutation of a particular nucleotide of the FGFR3 gene. It is plausible that chemical agents that are known mutagens such as mustard gas could increase the rate of spontaneous mutations at mutable sites such as this. However, I couldn't find any references citing such an event. I doubt that the OP's 5' tall friend could have achondroplasia, which is usually associated with much shorter stature (around 4'4" for males) and a very recognizable appearance (disproportionately short limbs vs. trunk). Probably something else leading to proportionate growth restriction, which may or may not be true dwarfism. --- Medical geneticist (talk) 15:17, 2 February 2009 (UTC)

Wind powered

Can you build a machine that moves downwind faster than the wind, without using anything but the wind as a source of energy? Nadando (talk) 05:10, 2 February 2009 (UTC)

I can imagine a machine that operates in two alternating modes. First, it affixes itself to the ground and deploys a windmill to collect and store energy. It would then retract the windmill and use that stored energy to propel itself. Given a sufficiently efficient design (large, lightweight, and reasonably streamlined), the average velocity could exceed the original wind velocity. It might even be possible to take that design a step further and design a machine for continuous operation by having multiple parts that cycle between wind-collection and streamlined advancement. -- Tcncv (talk) 06:29, 2 February 2009 (UTC)

From the article, sailing - "When moving, the motion of the boat creates its own apparent wind. Apparent wind is what is experienced onboard and is the wind that the boat is actually sailing by. Sailing into the wind causes the apparent wind to be greater than the true wind and the direction of the apparent wind will be forward of the true wind. Some extreme design boats are capable of traveling faster than the true windspeed." Nimur (talk) 06:38, 2 February 2009 (UTC)

Here's a fun video which I found while searching... Under the ruler faster than the ruler. Nimur (talk) 06:49, 2 February 2009 (UTC)\

That's not what he asked. Many sail boats are capable of traveling faster than the true windspeed. The question is whether you can go downwind faster than true windspeed. The traditional answer to this problem is no. When the boat matches the windspeed the relative airspeed drops to zero and the boat can not accelerate. APL (talk) 15:55, 2 February 2009 (UTC)

You could split the energy 'generation' by using windmills that transfer generated energy by microwave laser to the ship. In effect, the amount and with it the speed is thus only limited by that of light. --Ayacop (talk) 11:02, 2 February 2009 (UTC)

Given a straightforward interpretation of the question (can a sailboat or the like move downwind faster than the wind?), the answer is no. Once your machine reached windspeed, it would have no further thrust. However, it may be worth noting that sailboats move fastest when the wind is abeam (that is, at right angles to the direction of motion). It might be possible under those circumstances to exceed wind speed in a non-downwind direction. — Lomn 14:12, 2 February 2009 (UTC)

removing duplicated posts... Nimur (talk) 14:29, 2 February 2009 (UTC)

Yes but it wouldn't go faster all the time. A windmill gathers the electric for a while, then it takes off faster than the wind. Wind is a force of gravity and convection. If you can beat gravity downwind or turn it sideways, you may do it. ~ R.T.G 14:17, 2 February 2009 (UTC)

Actually wind has much more to do with pressure gradient force than any of those things </nitpicking> -RunningOnBrains 00:34, 3 February 2009 (UTC)

Certainly it's possible to sail a yacht faster than the prevailing wind. If you have a decent keel - you can aim the boat at (say) 45 degrees to the wind - so the wind is coming from behind - but at an angle. Suppose the wind is blowing from the South - and you are sailing towards the North-East. If your sail was 100% perfect and your boat was infinitely slippery - then for every kilometer the wind moved northwards, you'd move a kilometer northwards AND (because the keel is preventing you from being blown sideways) a kilometer to the east. Your net speed is therefore about 1.4 times the wind speed. Sailing ever closer to 90 degrees to the wind would theoretically make you go yet faster. The practical limit of how close you can get to that ideal depends on the design of the yacht. Land-yachts (which have wheels instead of a keel and have a MUCH lower coefficient of drag than a water-bound yacht can get up to 100mph in a mere 30mph wind! SteveBaker (talk) 14:47, 2 February 2009 (UTC)

OK, but he asked specifically about sailing downwind. Ever since I first heard this problem years ago, I've wondered if a solution like Tcncv's solution above could work, alternating between energy storage and energy use. I can't see any reason it wouldn't in theory, if you could get good enough parts. (If continuous down-wind motion is required, you could imagine all sorts of crazy schemes, imagine two pontoons alternating between stored energy and energy use and leap-frogging each other. Between them is the passenger compartment being dragged along by ropes at the average speed.) APL (talk) 16:06, 2 February 2009 (UTC)

Perhaps this isn't the paradox that physics professors would have us believe. Discussions here, and here. Personally, I fully don't understand how these devices are claimed to work. I'm not sure that they do, or if they do that they work for the reasons provided. APL (talk) 16:36, 2 February 2009 (UTC)

There seems to be very few videos of these devices actually going against the wind 'in the wild' as opposed to a treadmill. Even then they don't seem to make any attempt to measure the speed of the craft or the wind. I suspect wishful thinking of the same sort that plagues perpetual motion inventors. APL (talk) 16:43, 2 February 2009 (UTC)

Rule #1: Anything at all that you find on peswiki.com is crap...anything...no exceptions!

Rule #2: All videos on Youtube that purport to break the laws of physics are faked.

The video of the craft moving rapidly along the road with the little white flag flapping out of the back was probably running downhill. In the shots of the treadmill tests, how do you know there isn't a big fan just off-camera? Scepticism is required here. We KNOW these machines can't possibly work (see Rule #1 above) - it's just a matter of why. Since we know for 100% sure that people WILL fake videos on YouTube (see Rule #2 above) - it's just down to the tedious business of figuring out why. Because you don't know what's happening off-camera, it's a trivial matter to do this stuff. SteveBaker (talk) 03:03, 3 February 2009 (UTC)

OK - so here's another idea. Let's suppose you have a yacht with a big sail and a very slippery hull. We imagine it being pushed through the water VERY close to the wind speed with the wind right behind it. We imagine that if we were able to go any faster than the wind then the sail would collapse and turn into a parachute - slowing the yacht down again - so there would be no way to collect more energy. But we forget that the boat is now moving very fast compared to the water. So could we not deploy a turbine to collect energy from the water that's rushing past at high speed - and use that to produce a little more power to overcome the drag of the 'parachute' sail? I rather doubt it because in order to move at the same speed as the wind (and parallel to it's motion) you need a zero-drag hull - and as soon as you increase the drag by extracting energy from the water - you're going slower than the wind.

OK - so if we are talking about the speed of the yacht in a direction parallel to the wind velocity - then what I said before doesn't work. So can a yacht (or anything else for that matter) move faster than the wind IN THE DIRECTION THE WIND IS BLOWING. I think the answer is a definite "NO" - but it takes some jiggling around with frames of reference to prove it.

It's easier if you forget the wind and think in terms of the frame of reference of the yacht. From the point of view of a yacht with a really HUGE sail and a super-slippery hull that's moving at almost exactly the speed of the wind - and parallel to it - then from the perspective of a guy sitting on the deck, you have (let's say) negligable forward wind speed - but lots and LOTS of water coming towards you. Your hull is so slippery that despite the huge speed of the water - it produces negligable drag - so you can just sit there - despite the negligable wind-speed relative to you. So to move forwards (ie to move faster than the wind - as a "stationary" observer would say)...you have to turn that backwards water motion into forward motion of the boat...but you have to do it in such a way that the drag-force of the water on the front of the boat doesn't increase by as much as the thrust you're generating from it.

I don't see how that's possible - it seems like it would be a violation of conservation of momentum. You can't have the yacht move forwards without the water moving backwards even faster than it is now.

We can go one step forward from there. Since the wind speed is negligable when viewed from the yacht's perspective - we could pretend the yacht was sitting there in a vacuum - with just the water flowing past it's hypothetical zero-drag, zero-friction hull. If it could somehow move forwards against the flow - then we could transfer our mental coordinate system to the frame of reference of the water itself. It would appear that our boat is moving at uniform speed relative to the water (which it can do - per Newton's laws - providing it has a zero-drag hull) - but then suddenly it accelerates without any source of motive power! Where did the energy come from? SteveBaker (talk) 19:57, 2 February 2009 (UTC)

If you use the difference between wind speed and water speed to produce energy, the boat will have a limitless source of energy, and the boat can have arbitrarily low friction, so there's no limit to how fast it can go. Interestingly, even if you only use the wind and the boat (or airplane, which would work better for this case), you can still go faster than it. Imagine the air is stopped and you are moving, which is equivalent. Turn around. Of course, that limits your top speed. — DanielLC 22:59, 2 February 2009 (UTC)

You didn't read a single word of what I just wrote - did you? It's a matter of conservation laws. The boat has to (at some point) travel at the same speed as the wind. At this point the air is stationary relative to the boat. So you have a boat travelling through water - and according to Newton's laws - it'll continue at a constant speed until acted on by an external force. Where is that force coming from? It can only be from the water (since the air is effectively stationary relative to the boat). In order to extract energy from the water, you need to change it's momentum. But conservation of momentum says that you have to push the water YET FASTER behind the boat in order to allow it to move forwards. So what concievable thing could SPEED UP the water and use that to make the boat go faster? Nothing - because to do so is to violate all three laws of thermodynamics. So this CAN'T work...it just can't. SteveBaker (talk) 03:03, 3 February 2009 (UTC)

The question was about traveling downwind. I don't think there is any known or theoretical craft that can do that. APL (talk) 00:09, 3 February 2009 (UTC)

I'm pretty certain it should be possible to make a land vehicle that goes downwind faster than the wind. I'm not certain if that video where they claimed it was happening really had a machine that would do it as it would be quite difficult to achieve. The under the ruler faster than the ruler video shows the principle. You just have to use the wind instead of the ruler. An interesting aspect is that using a turbine it should go in the opposite direction to which one would expect when the wind blows on it, I couldn't see in the video if that was happening. Dmcq (talk) 00:08, 3 February 2009 (UTC)

These are two VERY different scenarios. By going faster than the wind, you actually introduce forces in opposition to your movement, as opposed to the ruler, which is still providing a forward thrust to the cart.-RunningOnBrains 00:34, 3 February 2009 (UTC)

They are not different scenarios. You might see it easier if you attach little paddles round the big wheel in the video and only blow over the top wher the ruler went. The paddles at the top would still go slower than the wind even though the vehicle goes faster. The same would happen with a turbine suitably geared. This is why the turbine would appear to be going round the wrong way. Dmcq (talk) 01:12, 3 February 2009 (UTC)

Several people have pointed out scenarios where one can reach speeds faster than the wind by traveling at an angle to the wind. If you can do that, then all you need is a way to efficiently turn. Carry that extra momentum with you as you turn to the same direction as the wind, and you will for a while be able to coast at a speed faster than the wind. Once you lose momentum, turn back to an angle to pick up speed again. Lather. Rinse. Repeat. Assuming the vehicle can turn efficiently enough and coast at very low drag, it should be possible for one to travel with an average velocity greater than the wind. Dragons flight (talk) 00:49, 3 February 2009 (UTC)

There is no denying that you can attain a SPEED greater than that of the wind. Yachts do it all the time. The question is whether you can move fast IN THE DIRECTION THE WIND IS BLOWING (ie: If the wind is blowing from the south, can you move northwards faster than the wind. Zigzagging does indeed let your speed be much higher than the wind - but your net progress IN THE DIRECTION OF THE WIND won't be any faster at all. SteveBaker (talk) 03:03, 3 February 2009 (UTC)

Steve, if you are moving faster than the wind speed at a angle and you turn to the direction of the wind, then conservation of momentum will now have you moving temporarily faster that the wind in the direction of the wind, at least until friction and air resistance slow you down to match the wind speed. This may not be practical in a boat, but I am fairly sure one could achieve useful results with a sail car. The turning allows one to achieve temporary bursts of speed in the direction of the wind that exceed the speed that the wind blowing behind you could support. In other words, by zig-zagging under the right conditions, one can achieve an average speed in the direction of the wind that is faster than the wind. Dragons flight (talk) 06:36, 3 February 2009 (UTC)

Momentum is a VECTOR quantity - not a SCALAR. SPEED is not the same thing as VELOCITY - and momentum is MASS x VELOCITY - not MASS x SPEED. Hence when your boat turns a corner - momentum isn't being conserved so some energy input is required. Bottom line is that to turn a yacht you need either a winch or some big muscular sailors hauling on ropes. That's an energy input - and once you have that, it's not a closed system anymore - so all bets are off. SteveBaker (talk) 16:14, 3 February 2009 (UTC)

Actually, I think the key factor is the water. By turning the boat (which requires a small amount of energy input, admittedly) you change the hydrodynamics and the water pressing against you is what changes your velocity. Consider a glider - the energy required to move the flaps (or whatever part of the glider it is that makes it turn) is minuscule compared the the energy required to turn the glider around, that extra energy comes from the air (well, I'm not sure there's an actual transfer of energy, but in any case it's interaction with the air that changes the momentum). --Tango (talk) 16:22, 3 February 2009 (UTC)

Nearly all of the force to turn a car, boat, plane, etc. is provided by the ground, water, air, etc. Just as throwing a ball against a wall, causes it to bounce back with no additional expenditure of energy, pushing against the ground can change one's direction of travel at virtually no cost. Also, when traveling at an angle to the wind in your boat, you are already using the force of the water to provide the cross-wind acceleration for your boat, so it was never a closed system to begin with. Dragons flight (talk) 17:57, 3 February 2009 (UTC)

Are sailboats and yachts limited to wind speed when heading directly downwind? Yes. However, the original question involved a "machine that moves downwind", and is not limited to a sailboat (with relatively few moving parts). We are free to design a much more complicated machine, one such design I put forth below.

• The machine would consist of multiple Wind Energy Collection Assemblies (WECAs) that would take turns collecting, converting, and supplying energy to the base assembly that would use that energy to drive its propulsion system.
• Each WECA would contain a wind collection device, such as a turbine, plus a ground support structure (legs) that would brace the WECA against the ground.
• Two or more WECAs would be mounted on a tank-track-like system that would rotate such that the top of the track remains stationary relative to the ground, while the bottom advances at 2x vehicle ground speed.
• Each WECA would have a deployed and retracted configuration. While a WECA is on the top of the track, it would deploy its support structure and wind energy collection device. As the back end of the machine reaches a WECA. that WECA would disengage its ground supports, retract its wind collection device, and configure itself in a compact arrangement for transport to the front of the machine, where it will redeploy for another cycle.
• Energy collected by the WECAs would be used by the base assembly to power the machine's propulsion system, such as power driven wheels, and to replenish any energy lost by the other moving parts due to mechanical friction.

Would it work? The ground supports will transfer the reactive force from the wind energy collection to the ground, so there is no force acting against the WECA track. With an even number of WECAs on the track, conservation of momentum will allow the WECAs to rotate through the system with no sustained energy requirements. The mechanism for deploying and retracting the WECA could be designed using energy recovery techniques. A streamlined enclosure would be designed around the propulsion system and the lower part of the WECA track (including the advancing WECAs) to minimize air resistance. Note that the energy gathered is dependent on the gradient between the wind and the ground, and does not depend on vehicle speed.

Energy collection would be limited by the size of the deployed wind collection devices, which could be arbitrarily large. Some energy collection opportunity would be lost during the time its takes for each WECA to reconfigure itself between deployed and retracted. Energy losses would include vehicle aerodynamic drag, the aerodynamic drag of the retracted WECAs being transported internally, ground friction, and the mechanical friction from the sum of all moving parts. With good engineering, this loss could be kept to a minimum (theoretically approaching zero). The difference collected and lost energy is available for propulsion, and could accelerate the machine up to a speed at which the energy losses match the energy collected. Theoretically, with a machine of sufficient scale and engineering perfection, there is no upper limit.

Could it be be built? I believe it would be quite an engineering challenge, but I think the concept is sound. Have I missed anything? -- Tcncv (talk) 03:56, 3 February 2009 (UTC)

Yeah - you've missed EVERYTHING! As the vehicle accelerates to a speed equal to that of the wind - there is a point when there is zero relative wind-speed. To someone standing on top of the vehicle, it would be exactly like the wind just stopped blowing. At that moment, where do you collect the energy from to accellerate the vehicle to higher speed? It's exactly like saying that I could make my car go faster by collecting the air that's pushing against the front of the car at 50mph and use that to make the car go faster. You simply can't do that - it's a perpetual motion machine and the laws of physics won't let you do that. This whole thread is just a matter of people wildly speculating without looking at the very basic underlying physics. There is simply no such thing as perpetual motion. I don't care how clever you get with WECA's and other bullshit - I don't even have to read your long explanation. You can't beat the laws of thermodynamics no matter how hard you try - period. Just give it up. 16:06, 3 February 2009 (UTC)

Right. From the reference frame of the vehicle, the wind has stopped moving. But now the ground is moving BACKWARDS. THAT is what you harness. Get out of the mindset of thinking that your energy comes from air moving relative to the vehicle. It doesn't. It comes from the GROUND and the AIR moving RELATIVE TO EACH OTHER. Maelin (Talk | Contribs) 16:44, 3 February 2009 (UTC)

Maybe you missed the part where the wind energy collection is performed when the assemblies are stationary relative to the ground and in fact braced against the ground, so it is indeed able to collect energy from the wind while as other parts of the machine advance ahead of the wind. But it seems that the main attention of this topic is focused on the internet hoax, so I'll just sit back and watch those discussions. They are quite entertaining. -- Tcncv (talk) 01:33, 4 February 2009 (UTC)

It certainly is possible. Here is a video of some fellows testing such a device under fairly careful conditions. They first show the video of someone performing the experiment on a road, and then produce a replica of the device on a treadmill. Maelin (Talk | Contribs) 15:52, 3 February 2009 (UTC)

You can't tell anything from those videos. Remember: a YouTube video does not constitute scientific evidence - people make fake YouTube videos all the time - and we get asked about them here all the time. The device running down the road is probably going downhill - the road looks level - but if the camera is mounted horizontally in the car and the car is going down the same hill - then that's exactly how it WOULD look. In the case of the treadmill, there is probably a fan blowing on it off-camera. Please don't fall for that kind of crap. This is a classic "free energy" nutjob thing - it's been around for years - along with all of the other crazy perpetual motion machines. SteveBaker (talk) 16:06, 3 February 2009 (UTC)

Here they take a good surround shot of the setup while the device is running to show that there are no fans or other such cheats going on. There is a whole sequence of these videos under this user account showing how they made the device and how it works. Maelin (Talk | Contribs) 17:20, 3 February 2009 (UTC)

I've just had a good look at that video and it all seems to be in accord with how I would expect such a machine to work and the fan is turning the way I thought it would rather than how a hoaxer would do it and th proportions seem about correct. I believe it is all kosher. I'm surprised it worked quite so well, perhaps there is hope for one that would actually work in 5 knot winds on water. Dmcq (talk) 16:22, 3 February 2009 (UTC)

How do you account for Steve's point that when travelling at wind speed, there is no wind? Where does the energy come from to get you past that point? Are you trying to collect energy from the wind going backwards once you are faster than wind speed (having used stored energy to get there)? Doesn't the added drag from such a system at least balance the energy gain? --Tango (talk) 16:27, 3 February 2009 (UTC)

When travelling at wind speed the propellor is driven by the wheels to cut into the wind and pull the cart forward into the wind. Drag will eventually limit the speed but is not enough at that point to overcome the energy generated using the difference between the air speed and the ground speed. Dmcq (talk) 16:36, 3 February 2009 (UTC)

The energy comes from the fact that the air and the ground are moving relative to each other and that for all intents and purposes they are immutable things. The fact that it's air just confuses matters - you'd be better to look at those charming "Under the ruler faster than the ruler" videos (first one here) to see how such a system is perfectly plausible. Maelin (Talk | Contribs) 16:42, 3 February 2009 (UTC)

Ok, I'm getting there... this is related to that weird "the bottom of a wheel is stationary" thing, isn't it? While I know and understand that it's true, it's very weird... In the case with the ruler, limiting friction with the paper is pushing the cotton reels forwards, while dynamic friction with the ruler is pushing it backwards, but with less strength, this gives a net forwards force. Very weird... --Tango (talk) 17:12, 3 February 2009 (UTC)

No, I'm not there... Why is the friction with the ruler less than with the paper? I can see from the video that it must be, but I can't see how... --Tango (talk) 17:24, 3 February 2009 (UTC)

You could actually have teeth on the ruler ground and bobbin rather than depending on friction. Dmcq (talk) 18:16, 3 February 2009 (UTC)

Well, that's just friction taken to a macroscopic scale, it's the same principle. --Tango (talk) 18:30, 3 February 2009 (UTC)

I have now done the experiments shown in under the ruler faster than the ruler vdeos (with a Teddy bear and Upsy Daisy as spectators) and I can confirm they with just a little tweaking they work very well. Dmcq (talk) 20:57, 3 February 2009 (UTC)

If these craft perform as advertised, why does no one race them against a similar cart with a square-rigged sail? Why does no one measure their speed and compare it to wind-speed.

Treadmill tests are unconvincing. Out of context experiments testing false analogies are one of the pillars of free energy self-delusion. (Yes, I understand that in this case the energy is not free.)

I understand that energy can be extracted from the fluid moving at a different speed, but in doing so you pick up momentum from the fluid. The energy you extract from the fluid is not free.

At the zero-wind point, any energy extracted from the wheels clearly comes out of the craft's forward velocity. The idea that that same energy could somehow be put back into the craft for a net acceleration does not make sense. APL (talk) 20:54, 3 February 2009 (UTC)

The idea is that the energy comes from the ground, which is moving relative to the craft (it's in contact with the ground, so the whole Earth has to be included if you want a closed system, and there is plenty of energy available there). I haven't quite got my head round it yet, but I'm certainly not as sure as I was that it's impossible. --Tango (talk) 22:16, 3 February 2009 (UTC)

I understand that, but (from the craft's perspective) how do you extract that energy without picking up equivalent momentum from the moving ground? Or, to put it another way (From the grounds perspective) how does the craft extract energy from it's own wheels without slowing the wheels, and it's own forward velocity?

It's obviously possible to extract energy from the ground-air system. But if both portions of the ground-air system are going against you, I don't see how you're going to extract the energy without picking up an equivalent amount of momentum in the 'wrong' direction.

And most importantly, if all these people have built devices that claim to do a certain thing, why is there no evidence, even on YouTube, that they can do that certain thing? Why are there only videos of them doing other things? Nobody else finds that suspect? APL (talk) 13:56, 4 February 2009 (UTC)

This video shows how you can take advantage of two things moving in the same direction (relative to you) to move in the opposite direction (or equivalently, how to move in one direction faster than either of the two things you are using for power). This is a set of videos of people who have built devices that do precisely that using the wind. Air moving over stationary ground (wind) is equivalent to the ground moving under stationary air (a treadmill in a closed room). Maelin (Talk | Contribs) 15:36, 4 February 2009 (UTC)

I think I'm going to satisfy my curiosity by building one of these things and buying myself a hand-held Anemometer.

If it works at all then I'll add an R/C servo to the back wheel. That'll make it worth the effort even if it doesn't break the faster-than-wind barrier.

I'll report back in a week or two. (Longer if I can't find a parking lot without snow on it.) APL (talk) 18:01, 4 February 2009 (UTC)

do insects have kidneys?

… and if it has a different name, which is it? I'm sure they have, as they also have blood which must be cleaned somehow. The insect article anatomy picture doesn't show anything, nor does the kidney article have an evolutionary section. --Ayacop (talk) 10:55, 2 February 2009 (UTC)

Do they have blood? they dont have a circulatroy system. And the insect article says that they have no closed veins or arteries. So I dont see how there could be a fluid analogous to blood from larger organisms. —Preceding unsigned comment added by 129.67.37.225 (talk) 11:43, 2 February 2009 (UTC)

The equivalent of blood in insects is hemolymph. I believe it does carry waste away from cells, so I guess there must be some way of filtering it. I've found a paper ([1]) talking about it. I haven't researched it in detail, but there is clearly something that serves an equivalent purpose to kidneys in insects. --Tango (talk) 11:51, 2 February 2009 (UTC)

See Malpighian tubule system. Gandalf61 (talk) 12:04, 2 February 2009 (UTC)

Do not expose to direct sunlight

Does that mean "while switched on"? ~ R.T.G 13:49, 2 February 2009 (UTC)

No. APL (talk) 13:51, 2 February 2009 (UTC)

Well its short but i sort of believe it :) ~ R.T.G 14:18, 2 February 2009 (UTC)

It depends, though. If it's a camera with a sensitive image sensor, it might not do to point it at the sun: but if the shutter is closed because it's off, nevermind.

It would help to know what the heck you're talking about...but unless it's a bomb with a solar-powered detonator...it's hard to imagine anything that would be damaged by sunlight exposure ONLY when it's switched on. SteveBaker (talk) 14:37, 2 February 2009 (UTC)

Many plastics break down in the presence of UV light from direct sunshine and many types of paper yellow and weaken; such products should always be kept out of direct sunlight. StuRat (talk) 17:47, 2 February 2009 (UTC)

Yes, indeed - but why only "while switched on" - as the OP asks? SteveBaker (talk) 18:57, 2 February 2009 (UTC)

I just made a general comment about why to avoid direct sunlight, which applies when on or off. My comment wasn't an answer to you, it only appeared to be because you hadn't indented your answer to the Original Poster. StuRat (talk) 21:59, 2 February 2009 (UTC)

In my student days, I turned on a sensitive detector while the lights were on in the observatory, thereby annoying my astronomy professor. Clarityfiend (talk) 19:32, 2 February 2009 (UTC)

Turning it on and off shouldn't have made any difference. Un-capping it in sunlight would be the problem. SteveBaker (talk) 20:22, 2 February 2009 (UTC)

Maybe they are trying to avoid the equipment becoming toast or starting a fire.76.97.245.5 (talk) 20:56, 2 February 2009 (UTC)

LCD screens can be damaged by prolonged sun exposure. I can only find forum postings as sources [2], but I'm pretty sure it's true.-RunningOnBrains 00:41, 3 February 2009 (UTC)

Well, my lcd monitor can get some direct sunlight from behind it and I just was switching it off or closing the blinds and it is only temporary but I had to worry if it was still getting damaged (and my computer is in the same situation). Sometimes the sun is hot around here, even when its been snowing, so hopefully I don't wreck my little monitor or my nice computer. Although not using as much energy as CRT, LCDs do cope with more localised heat but that doesnt mean that UV doesn't degrade it. ~ R.T.G 12:10, 3 February 2009 (UTC)

The degradation due to sunlight is the same no matter whether it's turned on or off...it's a pretty tiny effect though. I wouldn't worry about it personally. SteveBaker (talk) 15:14, 3 February 2009 (UTC)

OK thanks Steve ~ R.T.G 16:36, 3 February 2009 (UTC)

"Pretty minimal"? Depends where you live. [3] Gwinva (talk) 22:11, 3 February 2009 (UTC)

wind power

Is there any wind power or wind mills to generate current for a individual home(220v) in a city?Prince sha (talk) 14:32, 2 February 2009 (UTC)

If you google for "domestic wind power" you'll find a ton of small windmills suitable for powering an individual home. Their effectiveness in a city may be a little tricky though - adjacent buildings could do some very strange things to your wind-flow. Also, in a confined space you may find that your local planning laws prevent you from putting one up. SteveBaker (talk) 14:40, 2 February 2009 (UTC)

Plus small single domestic installations rotate fast and make a lot of noise. However tiny top up wind turbines to provide a small amount of electric power are possible although uneconomic. [4] discusses one put up by the leader of the opposition in the UK. --BozMo talk 14:47, 2 February 2009 (UTC)

Wow! In the study, one of the domestic wind turbines didn't generate enough electricity to power it's own electronics! Ouch! The average turbine produced 214 watt/hours per day...so three or four lightbulbs. They say that on average people are saving 33 UKpounds per year from their 1500 UKpound investment. Since I very much doubt they'd survive 45 years of continual service - they'll never pay for themselves. However, the study also showed that one of these turbines that they studied DID produce enough electricity to run the house because it was situated on top of a 10 storey building. So it all CRUCIALLY depends on where the thing is situated...because anything from using more electricity than it makes(!) to running your entire house is a pretty huge range - and if the average is a 45 year payback time - then you'd better be installing it somewhere very much better than the average household is managing to do! SteveBaker (talk) 20:21, 2 February 2009 (UTC)

Hand over your geek card, Steve... It's "watt hours", not "watt/hours" - it's power times time, not power divided by time. And three or four lightbulbs might use around 214 watts, not 214 watt hours (unless they were on for an hour). You're a disgrace... --Tango (talk) 16:09, 3 February 2009 (UTC)

Old-fashioned windmills have wide, slow-turning, blades. Modern wind-powered generators have narrow blades. Would wide, slow-turning blades be best for low wind velocities? It seems that the wide blades would generate more torque, which would partly compensate for their slower speed of rotation. A wide-blade windmill might also make less noise. —Preceding unsigned comment added by 98.17.34.148 (talk) 17:35, 2 February 2009 (UTC)

It's probably safe to assume that the manufacturers already tried that. It's certainly notable that airplane and helicopter propellers (which are really just backwards windmills) are all very skinny - but boat and submarine propellers are always very fat and short...and so are desk fans and airconditoner fans. SteveBaker (talk) 20:21, 2 February 2009 (UTC)

For mounting on the roof of a city building, such as a 3 story apartment building, consider a vertical axis wind turbine. They are more attractive than the horizontal axis ones, and can be mounted much like a TV antenna, resting on a plate on the roof and guyed to the wall of the staircase projection. Installation cost/payback is the question. Edison (talk) 20:25, 2 February 2009 (UTC)

Savonius wind turbines are easiest to DIY. Although they are less efficient the low installation cost makes them one of the more economical choices for a home installation. AFAIK they are also less fragile in case of occasional high winds, which can break DIY horizontal axis designs. Studying wind patterns throughout the year to find the best location for installation is definitely indicated. Just putting it on the roof of a shed because it looks good there isn't the thing to do. 76.97.245.5 (talk) 21:13, 2 February 2009 (UTC)

Many are available for that and as I live in a windy area myself I was enquiring about it and a very good expert swore to me black and blue that they need to be way up in the air, that rooftop ones are more or less crap. Now I looked into it a lot and if you have a lot of wind I reckon you could get you lighting and stuff but to be fair to the guy (the guy involved in Scoraigwind as it happens), to power your cooker and fridge etc. you need wind strong enough to force the blades against strong magnets. You need something like 20 metres clearance for the best wind and in a city, the houses slow the wind down, nevermind tall commercial buildings and apartment blocks. ~ R.T.G 16:42, 3 February 2009 (UTC)

In saying that, if you can put one up in the air (citys wont let you do that) you can make them out of car alternators with a basic mechanical, carpenting skill. Look at http://www.scoraigwind.co.uk it gives stacks of info and links ~ R.T.G 16:45, 3 February 2009 (UTC)

People who "Don't like music"

I have a friend who has claimed over the years that he "doesn't like music" and I believe him. Oddly enough however if you spend time with him you'd think he's hyperactive and he will ALWAYS be humming or singing a song - usually with the lyrics corrupted to something else - or just repeating a tune to a popular song. This might seem contradictory, but I gather he doesn't intentionally listen to music, but if he hears any it will get stuck in his head.
Notably, he has a form of dyslexia, an above-average IQ and above-average level of general knowledge. He strictly only listens to talk radio.
Based upon his remarks I assume he has no "appreciation" of music, and has no favourite songs".
Do you have any references or explanations for this type of thing?
From my unhelpful googling for other cases, someone suggested checking if he's tone deaf or has other audio-perceptual problems - but the other day in the car he managed to hit the requested note when repeating a song -- and all his other songs tend to be in tune.
Rfwoolf (talk) 17:06, 2 February 2009 (UTC)

If it comes to that, a lot of people don't like classical music.

Maybe he doesn't like music precisely because it gets stuck in his head, which can be annoying. StuRat (talk) 17:43, 2 February 2009 (UTC)

I have no explanations, but I can do anecdotes, if that helps: I have nothing against music, but I rarely make an active choice to listen to it. It just doesn't interest me. I have great difficulty making out the lyrics in songs, but I don't know if that is cause or effect. I hum and sing to myself about as much as anyone else, I think. I am a mathematician and am quite interested in music from a technical perspective, and I do appreciate music when I do listen to it. I have, at various times, played the recorder, piano and trumpet, although never had the dedication to get anywhere with them. I'm not dyslexic, but I do have an above-average IQ and level of general knowledge (like most Ref Deskers, I guess!). I don't know if any of that helps. --Tango (talk) 18:40, 2 February 2009 (UTC)

One explanation may be rooted in human communication. Research found that when people are talking they are trying to reach a common "hum". Individuals higher up in the hierarchy literally "set the tune" with lower level people "chiming in", (Our language developed phrases for this before science found what was going on.) So people who are attuned to communications are more prone to being influenced by ambient muzak. (I bet you can find the study wigh some googling. I'm afraid I'm a bit too busy to dig it up.) There's even an association that has been trying to limit the use of muzak. My sympathies to your friend. It must be somewhat like hearing the phone ring running to pick it up and finding it was on TV. 76.97.245.5 (talk) 20:36, 2 February 2009 (UTC)

Thanks for the interesting comments. The idea about communication is interesting and I will continue to monitor his communication skills and communication behaviour. I'm still trying to fathom through what you're trying to say though - are you saying people that tend to "set the tune" would be put off by music, while those of us that "chime in" would like to 'chime in' to a song? Or are you saying conversely that those of us that are better at communication in terms of oration, diction, tone, emphasis, expression etc would appreciate music more than those who lack the aforementioned? (I tend to think that a person's taste in music could be linked to that -- people that favour rap music for example, compared with those that favour classic music, or those that favour jazz, or heavy rock - surely it is about expression, and therefore no matter how tone-deaf, monotone, tone-deaf, or communicationally unadept an individual is they will always find something in the musical spectrum that they like??). I will keep thinking. Thanks again Rfwoolf (talk) 22:08, 2 February 2009 (UTC)

For myself I can't see why people would want to listen to a piece of music more than once or a couple of times. There may be a few films or books one wants to read more than once but going around listening to the same bit of music 50 times just seems weird to me. Dmcq (talk) 09:42, 3 February 2009 (UTC)

"Music is a drug" is a common layman's cliché. Pleasurable drugs are often readministered, so I would guess pleasurable music is "readministered". --Mark PEA (talk) 19:54, 3 February 2009 (UTC)

First time listening to anything worth relistening to is much less good than the follow ups; as you start anticipating the patterns and experiencing more of the details it just gets better and better. It can feel a bit like the music is slotting into your mind: as you relisten, the slot for it becomes a better fit. 79.66.57.25 (talk) 00:14, 4 February 2009 (UTC)

Dmcq, you seem to inhabit a different world than the bulk of humanity. The manufacturers of CDs, MP3s, musical instruments (and previously LPs, EPs and cassette tapes), have a ready-made market, because music is one of the great unifiers. There are some pieces of music I've listened to literally thousands of times, and hope to listen to them thousands of more times. Why? Mainly because I enjoy them, and they give me pleasure. Sometimes it's because they just happen to be playing on the radio while I'm tapping away on my keyboard; but I never think "Oh, I've heard that before, so I'll put a CD on". No, if I like it, I like it. (And even if I did do that, I'd still be choosing a CD I'd heard before; because we don't buy CDs just to listen to them once and never again; that would be extremely poor value for money.) I might think "I don't like that particular piece playing now, and it's going to go on for another hour or more, so I'll turn it off and put something of my own choice in the CD player or the record player" (yes, I still have lots of vinyl LPs). Similarly, I've played many pieces on the piano literally thousands of times, and will continue to play my favourites thousands of more times. Naturally, every performance is at least subtly different from every other, just as every recording of the same piece is at least subtly different from every other. But leaving that aside, if there were only 1 recording of some work I really love, I'd happily listen to it over and over and over. Books are different. I might re-read a favourite book, but usually with a gap of at least a year since the last reading. Having pictures on your walls is analagous. They're there because you like them. You don't walk into your loungeroom and think "I've seen this picture before, so I'll take it down and put a different one up" - do you? And you'd then have to change it again the next day, and the next day ... No, it keeps on giving you pleasure, so you keep it there so it can continue to keep on giving you pleasure. That might stop if you decide to redecorate your room, but that's a different matter. So, if the bulk of humanity likes listening to the same music many, many times, but a small %age of people don't, who are the "weird" ones? -- JackofOz (talk) 00:59, 4 February 2009 (UTC)

If someone doesn't listen much to music, it doesn't nessecarily mean they don't like music. I don't listen much to music and I can make my own complex tunes (with unknown original sources!) ~AH1^(TCU) 17:41, 7 February 2009 (UTC)

Earthing

What is the reason behind earthing a electric neutral wire.If earth is used for conducting electricity then why does a wire needed for it? —Preceding unsigned comment added by 120.89.115.198 (talk) 18:22, 2 February 2009 (UTC)

The earth is only there for safety purposes, it makes sure any electricity that gets to the wrong place (eg. the metal casing of the device) goes safely the earth rather than shocking anyone that touches it. The neutral is there is complete the circuit in ordinary circumstances. --Tango (talk) 18:33, 2 February 2009 (UTC)

In some countries the "neutral" isn't earthed, but is "negaitve live". It is a perfectly valid way of doing things, but isn't the UK way. -- SGBailey (talk) 18:46, 2 February 2009 (UTC)

Which particular countries are you thinking of? I would be interested in how they do it.--GreenSpigot (talk) 19:40, 2 February 2009 (UTC)

Also in those countries, is the negative live always negative or does it have alternating voltage on it?--GreenSpigot (talk) 20:13, 2 February 2009 (UTC)

May be one of those urban legends. Or I may have been thinking of US 240V supply. In any case I haven't found a reference, so forget it. -- SGBailey (talk) 21:49, 2 February 2009 (UTC)

(edit typos)In the early days of telegraphs and telephones, it was discovered that substantial money could be saved by running only 1 wire per circuit from terminus to terminus, and using a buried metal plate or ground rod to connect to the earth at each terminus. When 2 wires were used per circuit, it was called a "metallic" circuit. There was less noise and crosstalk with a 2 conductor metallic circuit. Some high voltage DC transmission lines have a + and - conductor, but in an outage of one conductor, they can operate with the other conductor and a ground conductor. In your home, the grounding of the neutral is a safety measure. Without it, a transformer failure or a broken high voltage line falling on the wires to the house could result in the full high voltage being on all the wires in the house, electrocuting anyone who came near anything electrical. What is a "negative live? Maybe they refer to providing 2 wires with 240 volts between them. In countries I am familiar with, there would still have to be a ground conductor from the utility to the building to ground the switch/breaker/fuse enclosure, so that an insulation failure does not energize it at 240 volts to ground. This would be true even if it is 3 phase service or 240 volt service without a neutral. Edison (talk) 20:21, 2 February 2009 (UTC)

AFAIK, Neutral is earthed by the the electricity company at the substation. Earthing neutral in your home sounds a bad idea that would unbalance the system. Earthing earth should be done locally (and at the sub-station). -- SGBailey (talk) 21:49, 2 February 2009 (UTC)

That's my understanding as well (at least for the UK). --Tango (talk) 22:48, 2 February 2009 (UTC)

In a typical U.S. home, we get a single "240VAC" phase delivered on two wires plus a ground wire to the Electrical distribution panel in the home. At the distribution panel, ground wire is tied to anther ground wire that in turn runs to a long copper rod that is driven into the earth outside the home. In the panel, the house neutral and the safety ground are both connected to the panel ground. approximately half of the home's "120VAC" circuits use one of the two hot wires and neutral. The other half of the "120VAC" circuits use the other hot wire and neutral. the "240VAC" circuits in the house are connected to both hot wires. -Arch dude (talk) 00:08, 3 February 2009 (UTC)

From our article on ground and neutral, it seems that both European and North American installations generally link earth and neutral wires at some point between the final distribution transformer and the end user's fuse or breaker panel. Separate earth and neutral wires are only maintained inside the residence. TenOfAllTrades(talk) 00:02, 3 February 2009 (UTC)

So ArchDude, in the US home you get 2 phases: each of 120V balanced about ground. Is that what you are saying?--GreenSpigot (talk) 01:04, 3 February 2009 (UTC)

Nope, it's single phase, or split-phase. You could say one line is ∠0° and the other ∠180°, but that's just a matter of how you are measuring, there is still only one waveform. It's "balanced" if there are equal loads on each line and there is no current through the neutral. The neutral wire between the breaker box and the pole transformer carries a current equal to the difference in the currents of the other two lines (see Kirchhoff's Current Law). Typical distribution systems in the U.S. are "wye circuits" and are grounded at the breaker box, the pole transformer, each pole, and the step-down transformer at the substation. Most transmission lines, and i think some distribution systems are "delta circuits"—no neutral wire so no connection to ground.—eric 02:57, 3 February 2009 (UTC)

Also, SGB seems to be contradicting him/herself by at once saying that earthing the neutral in the home is a bad idea and then proposing that it should be done locally (in addition to the substation)--GreenSpigot (talk) 01:10, 3 February 2009 (UTC)

see Earthing system.—eric 03:03, 3 February 2009 (UTC)

As noted in previous Q&A, in U.S. residential wiring the power company provides one phase with 120 to ground from each hot wire and 240 between the hot wires. The neutral and ground conductors are bonded together at the transformer and then at the house main breaker panel. In the house, the neutral carries the imbalance between the 2 hot conductors back to the main panel, if 2 hot wires and a neutral are used as a set. More modern wiring provides a neutral for each hot wire, since with electronic loads the neutral currents do not cancel like they tended to with resistive or reactive passive loads. There are some customers in downtown areas who get 120/208 volt service, since they actually are provided with 2 phases 120 electrical degrees apart. Each hot wire is 120 volts from ground/neutral, and the voltage between them is 208 volts. This system is for the benefit of busnisses who need three phase service, and who just have the third phase brought in. Edison (talk) 16:11, 3 February 2009 (UTC)

FWIW: In the UK. "Protective Multiple Earthing" used earthed neutral both at the last transformer and within the dwelling. Where this is not installed, or is faulty, I have been able to light a 6v flashlight bulb between neutral and earth (but don't try this, just in case!). Dbfirs 10:39, 5 February 2009 (UTC)

Would lens flares ever have motion blur?

I was just curious if real world lens flares would ever have motion blur? --70.167.58.6 (talk) 19:32, 2 February 2009 (UTC)

Motion blur is a consequence of the recording technique you are using - film, digital photosensors, etc. If the light source is moving - then the lens flare must also move - and the recording media can't tell whether the light that's hitting it is a lens flare or light from an actual object - so, Yes - lens flares do exhibit motion blur in things like cameras, etc. Human eyes don't exhibit lens flare per-se - although you do get effects like starbursts from bright light due to the behavior of our eyelashes. Our brains visual cortex "hides" motion blur from our consciousness - so we don't really "see" it as such - although if you could monitor the output of a rod or cone cell, it would have to be there because no image sensor can ever be infinitely fast. SteveBaker (talk) 20:06, 2 February 2009 (UTC)

[medical Q] identify a tool

What is this tool called? It's, I think, informally called a "stripper" and is used, if I understand it correctly, to clamp an IV (infusion) tube and roll the tube through the tool in order to push the tube's contents down along the tube and out.—msh210℠ 22:00, 2 February 2009 (UTC)

Yes, "stripper" is correct. The same tool is pictured in online catalogs here and here. --Scray (talk) 04:00, 3 February 2009 (UTC)

So much for "Wikipedia has an article on everything. Nimur (talk) 17:20, 3 February 2009 (UTC)

Thanks much, Scray.—msh210℠ 17:53, 3 February 2009 (UTC)

@Nimur, see Stripper (tool). Created tiny stub now, added to disambiguation page, and added a redirect from Tube stripper. Everyone, please improve it. --NorwegianBlue ^talk 21:41, 3 February 2009 (UTC)

Black dwarf sun and planets drfiting

The time the sun becoems a black dwarf, is it trillion of years. Since sun will have lost about 90% of gravity by the time it becomes a white dwarf, is this possible Uranus and beyond would have flown off it's orbit? Why will planets eventually drift away and left with black sun with no planets around it?--69.226.46.118 (talk) 23:22, 2 February 2009 (UTC)

According to Stellar evolution#White dwarfs, the Sun should only lose about 40% of its mass. The planets will move into higher orbits, but they won't escape completely without some kind of external interaction (another star passing close by, perhaps). Baring such interactions, the radiation of gravity waves will cause a loss or energy and the planets will very slowly spiral in (over the course of trillions of years, if not longer). --Tango (talk) 00:11, 3 February 2009 (UTC)

• They said on Formation and evolution of the Solar System eventually sun will just be left with no planets orbiting around it. And what does and the planets will very slowly spiral in (over the course of trillions of years, if not longer). mean?--69.229.108.39 (talk) 00:53, 3 February 2009 (UTC)

  General relativity predicts that orbiting objects will emit gravity waves. This means there is a loss of energy so the planet will fall towards the sun (just friction causes a ball rolling round inside a funnel to fall towards the middle). On the scales of planets orbit the sun (rather than, for example, neutron stars orbiting each other), this effect is tiny, so it would take trillions of years for the planets orbits to decay noticeably. --Tango (talk) 15:12, 3 February 2009 (UTC)

Talking about 90% of gravity is awkward since the force of gravity changes over distance. It would be better to talk in terms of mass. Assuming a circular orbit for Uranus, the mass of the sun required to keep it in place at a certain distance r and orbital velocity v is

$m_{sun}={\frac {r{v}^{2}}{G}}$

where mass is in kilograms, r is the distance from Uranus to the earth in meters, and v is the velocity of Uranus orbiting the sun in meters per second and G is the gravitational constant. If Uranus hasn't slowed down or changed its radius from the sun appreciably by the time the sun loses a significant portion of its mass, you can see that rv² will stay the same but because the mass of the sun has dropped

$m_{sun}<{\frac {r{v}^{2}}{G}}$

This means that Uranus's orbit must change. Either it will settle out in an orbit more distant from the sun or it will escape entirely. For it to escape the gravitational effect of the sun it must have a high enough velocity. The escape velocity is given by:

$v_{e}={\sqrt {\frac {2Gm_{sun}}{r}}}$

which expressed in terms of v_e is

$m_{sun}={\frac {{v_{e}}^{2}}{2Gr}}$

If $m_{sun}<{\frac {{v_{uranus}}^{2}}{2Gr}}$ then Uranus will escape. I leave it as an exercise to the reader to determine if Uranus will escape given the questioner's parameters. (Translation: I feel too lazy to track down the speed of Uranus, the average orbital radius and plug it in myself) 152.16.15.23 (talk) 00:54, 3 February 2009 (UTC)