CaptainPanic

Maybe he means the terminal velocity of a human when he says the speed of gravity? Gravity is not a speed, it's an acceleration, expressed in m/s2.

I agree that a car can (theoretically) become too light... and that it would be a massive problem to keep it on the road. On the bright side, such light materials would finally make it possible to have flying cars! So, that's a good thing But these days, even the lightest cars are often still half a ton (500 kg) at least... and they are often rather low (small profile exposed to any wind). We're quite far from the problem that you describe!

Cyclonebuster, first you ask how the videos can be explained mathematically. Then I answer the question. And then you respond by saying "correct"?? What is this? A test? You switch from being the one who asks the question to the one who approves of the answer... it's a little strange. Anyway, I already have a masters in engineering - I do not need to take tests anymore. Now I'm slightly confused with the topic of the thread. Do you just want to advertise your youtube videos?

Let's assume a few things before answering this question: The container is a theoretical one - which cannot break under any circumstances. The container is filled with any of the gases mentioned (hydrogen, helium or air). The container is not completely filled to the brim with liquid. There is a little space on top for gases. So, we're starting off at extremely low temperatures, where the hydrogen/helium/air is in fact a liquid at atmospheric pressure. If we start to increase the temperature, the liquid itself expands - and that's why we need a little space on top. Otherwise, the pressure would increase extremely rapidly, and we'd be discussing something very different. Now, as we pass the critical point of the gases (helium is already at 5.19 Kelvin!) we no longer speak of a liquid or a gas. We have a supercritical fluid. The thing with a supercritical fluid is that there is NO transition from gas to supercritical to liquid. You don't see a sudden change. So, it's also difficult to say where the transition is exactly (hence the dotted lines in the pictures in the wikipedia links). But I think it's safe to assume that hydrogen, helium and air alike all would remain in their supercritical state up until room temperature. Note: this also means I strongly disagree with J.C.MacSwell's response that there is a 'condensation' at room temperature. That is NOT true. You cannot get any of those gases in a liquid state at room temperature by merely increasing the pressure. You will just get them supercritical, not liquid. The liquid state (and any boiling / condensation effects) can only be achieved at temperatures below the supercritical point. If we assume that air would be pure nitrogen, then the highest of the critical points is 126 K. Above that, and there is NO gas/liquid, only supercritical fluids. Slightly confusing: when the gas is thin enough (although in temperature well above its critical point) we still call it a gas. Therefore, air is a gas, not a supercritical fluid. To make it slightly more complicated: there is still a pressure so high that your gas can turn into a solid. (Transition from either liquid to solid is possible, but also from supercritical to solid). Of course, in a real life test, you're always risk breaking your vessel... and these kinds of tests are typically done in special equipment.

I don't know the answer to your question, but I want to stress that it has not been an objective democratic decision to stick to fossil fuels. It has been a marketing battle between different industries... and the fight isn't over yet. And btw, many sustainable sources of energy show exponential growth curves... how much more can we ask for? Exponential growth is probably as good as it's gonna get. And it's not just Western governments that are subsidizing it. China is rapidly becoming the world's main manufacturer, and will soon have more wind (and solar?) installed than any other country. People are willing to support a lot of stuff, as long as they believe it's the right thing, and as long as their friends and relatives also believe in it. And yes, people will easily go to war if they are fed a bit of propaganda. History has shown that numerous times.

That's a general engineering rule of thumb, and as a thermodynamic argument, I would agree. But you say it's "expensive", and that means you include practical considerations as well. And if you include practical issues, they I think that there is an optimum. If energy gets too concentrated, harvesting it becomes a problem again. But I admit that this is also just a feeling. But actually, you have a very excellent point. If we assume that (1) the world population will continue to grow as it does, and (2) the energy consumption increases per person as well, then indeed, we'll be looking at a 10-20 fold increase of energy consumption (and more). And we can argue that sooner or later, fusion and fission are the only possible sources of energy, because the sun simply doesn't deliver enough anymore. If we agree that this is a realistic scenario, then we must invest heavily in fusion a.s.a.p. However, in today's world, the motivation to get sustainable energy is different. There is no lack of sunlight or wind.

All I try to do is to get you engaged in the discussion. Until now, you posted the idea as a "fire-and-forget" idea... it would be nice if you defend it, and try to find some arguments why it's a good idea.

Yes, so you actually admit that for biofuels, machines (tractors) do all the work. So, you cannot use that as an argument to build a giant space magnifying glass, because that too (just like biofuels) will be mechanized. If you want to compare "how much work" something is, and you want to compare biofuels to a giant space magnifiying glass including something on 2-3 mile high posts, then you need to be a little more detailed. The giant space magnifying glass isn't gonna build itself, and it won't go up into space by itself.

What Fuzzwood meant is: in the bottom reaction, you can remove 6 water from the left, and 6 from the right. Or, if you like you can remove only one H2O. Or two H2Os. Or three, or four, or five... up to a maximum of 6. [ce] 6CO2 + 12H2O -> C6H12O6 + 6O2 + 6H2O [/ce] [ce] 6CO2 + 11H2O -> C6H12O6 + 6O2 + 5H2O [/ce] [ce] 6CO2 + 10H2O -> C6H12O6 + 6O2 + 4H2O [/ce] [ce] 6CO2 + 9H2O -> C6H12O6 + 6O2 + 3H2O [/ce] [ce] 6CO2 + 8H2O -> C6H12O6 + 6O2 + 2H2O [/ce] [ce] 6CO2 + 7H2O -> C6H12O6 + 6O2 + 1H2O [/ce] [ce] 6CO2 + 6H2O -> C6H12O6 + 6O2[/ce] In fact, if you want to add three million water (or any other number) to both sides, it still balances: [ce] 6CO2 + 3000006H2O -> C6H12O6 + 6O2 + 3000000H2O [/ce] The most common way to describe photosynthesis though is this: [ce] 6CO2 + 6H2O -> C6H12O6 + 6O2[/ce] It is the most simple one, because the waters have been reduced to the net change.

The mathematical explanation you're looking for is called "Bernoulli's law" or "Bernoulli's principle".

It's all risk assessment. Unless you do the risk assessment, there is no reason why it would make more sense to worry about Yellowstone than about a 'rebang'. Quoting: Quantitative risk assessment requires calculations of two components of risk: R, the magnitude of the potential loss L, and the probability p, that the loss will occur. Risk R1 = (the entire universe destroyed by another big bang next to your window) * (an absolutely negligible chance it ever happens) Risk R2 = (Yellowstone erupts, North-America vanishes) * (happens only once every 600,000 years) Risk R3 = (You're in a car accident, heavily injured) * (happens perhaps once in a lifetime) Risk R4 = (A bird poos on your head, you need a shower) * (happens quite often) You can do such an assessment for everything... and use that to determine what you should be most worried about. What I mean to say: in this (nearly) infinite universe, we may argue that (nearly) everything is possible... but there's no reason to worry about everything. You must think about the chance that it happens, and how it would affect you and the ones close/dear to you.

I'm not convinved it's necessary... and I'm not convinced that simply throwing (like in the Apollo-program) more money at it will make fusion an economically interesting energy source within a decade. Also, the goal is just to get a secure source of clean energy that strengthens the economies worldwide... it does not necessarily have to be fusion power. Why wouldn't the existing (and working) technologies such as wind and solar be able to do the job? They're clean. They can be built everywhere, enabling a secure supply. And they start to become profitable too... in other words: they tick all the boxes. If you're gonna invest money like we're going to the moon again, why not use those technologies? Don't forget that ITER alone already got 15 billion (billion with a B). It's not like they do not get any money. It may not be the Apollo project, but it's still pretty damn big.

Is the Earth perfect for life? Yes. I think it is safe to say that, indeed, the earth is perfect for earth-based life. Life has adapted to the earth, so it would be more fair to say that earth-based life is perfect for the earth, rather than the other way around.

LOL, KDE vs. Gnome is indeed still relevant in 2011... this is a thread that's worth digging up. Excellent remark by timo (post #11). Anyway, I dislike windows... I never seem to be able to solve any problems in it. And every action you take, and every program/update you install seems to make a Windows computer slower. In my Ubuntu/Gnome computer, that's much easier, and it never gets any slower... ever. Not much experience with KDE - not enough for a comparison. The main downside of Linux is that the Open Office is still not as good as MS Office, and that (I think) is one of the main reasons that Windows sells so well.

I can afford 99 cents for a song I like, and that I might listen often - no problem. I cannot afford 99 cents for a song that isn't really my taste, and that I might listen only a few times in my life. I cannot afford 99 cents for a song that's just funny, but after listening to it once, the joke gets old. And I certainly cannot afford 99 cents for a song that I dislike after listening only half the song. Unfortunately for the music industry, 99 out of 100 songs on my computer fall into the latter 3 categories, and are NOT worth 99 cents. Maybe just 1 cent, not more. Some songs are barely worth their storage space on the harddisk... and if I would run out of space, I would consider deleting those before buying a new harddisk. So, no, as far as I'm concerned, I cannot afford 99 cents for the majority of my music collection. p.s. Downloading is still legal in the Netherlands (uploading isn't though), so don't bother lecturing me about me being a criminal. I know the law.

LOL!!! A huge space magnifying glass, space mirrors and some other contraption on 2-3 mile high posts is easier than growing plants? Could you please explain why it would be so "easy", and include the expected maintenance problems you'll face sooner or later?

All problems regarding fusion are practical, not theoretical / fundamental. Hydrogen bombs and the stars and sun show that fusion works. But the practical problems are absolutely enormous...

Get some new roommates, and tell them that it makes more sense if the new guys take the responsibility of the bills because you'll be there only for a couple of months. If the house is ok, and the location is awesome, then people might just take the room and accept that they must pay the bills for the house. You're the salesman, and you have a good deal for them. In other words: do your best to make it someone else's problem, and make it sound like it's a good deal.

It means we cannot base any policy on it. We cannot make any plans for it. We cannot say: "By 2030 we want to have 5% of our energy from fusion"... because it is not a matter of political will or funding. We just can't seem to get it to work properly.

It is obvious that the OP is withholding some information, deliberately misinforming us about something, or is just joking. We're trying to figure out what's going on. I just posted another suggestion. Maybe he's first dropped a little drop of superglue on the CD box, then placed the object on the glue. Then started spinning, and got dizzy? The OP himself suggested we're dealing with a party trick.

A rough value for commercial water/water heat exchangers is a heat exchange coefficient of 1000 W/m2K. But that's a value you only achieve if both sides of the heat exchanger are flowing (and turbulent). Since your water in the barrel will be practically stagnant, you will be significantly lower. I would guess only 100 W/m2K (note: do not use this to do calculations). If the pipe gets dirty, this number will drop. If the water flowing through the pipe is not turbulent (you cannot check that, but you can calculate the Reynolds number), it will be lower. If you have somehow managed to squeeze 10 meters of piping into the barrel, and that pipe is 2 cm diameter, you have 0.63 m2 of surface area on your pipe. If the pipe is smaller, this number will obviously drop. Let's also assume that you have a 20 degrees temperature difference between fresh water, and water in the barrel. Obviously, if the temperature difference is smaller, the number will be lower. Then the heating power for your water is: P = 100 W/m2K * 0.63 * 20 = 1200 W Showers typically use 10 liter per minute... or 0.17 kg/s of water. So, you can increase the water temperature by about 1200 / (4200*0.17)= 6 degrees, which is considerable. Please note: this is just an estimate, and most likely a high-estimate. I did not look up ANY numbers, especially the heat exchange coefficient can be very different (!), and I don't know how practical it is to put a 10 meter, 2 cm diameter pipe in a barrel.

Again, I agree with your post, and I just wish to comment on one point (which is dangerously off-topic, so I wouldn't mind if mods split this off). This link shows (and I quote) 'the ‘top ten’ wind turbine manufacturers, as measured by global market share in 2007, and some salient features of the technology of some of their flagship designs'. The flagship designs have power ratings of 3, 2.5, 2, 2, 2.1, 3.6, 3, 0.75, 2.5, 1.5 MW. The averave value of that is in fact 2.3 MW (so, quite close to the 2.5 I said). I think it's important to realize the current status of wind energy... that such big turbines are nowadays common. In fact, Vestas is going to have a 7 MW windturbine prototype next year, and they're planning commercial construction of those in 2015! And that's no dream, or plan. That's entered the stage of actually building it. To get this a little bit back on topic... you only would need 143 of these huge wind turbines to replace a complete nuclear power plant.

So, the CD box and the object on top of it are completely horizontal, and at the center of the rotation, and you run around it at arm's length?

Alireza, Both The Thing and Caver451 have not been active on this forum for many years. The last time any of them logged in was in 2007.

Danijel, I agree with almost all you say in your post, but why wouldn't solar or wind energy be green (if applied at a 15 TW scale)? Especially wind energy is just using nature's waste heat. If we would look at a theoretical case where we would supply all the 15 TW by wind energy (no matter how impractical that would be for certain applications), then we'd need 6 million 2.5 MW wind turbines, which are just very ordinary wind turbines. 6 million sounds like a lot, but if you realize that the earth's land surface is 150 million km2, then we would have only 1 wind turbine in every 25 km2 (1 wind turbine in every 5x5 kilometer block). And if we'd build some at sea, or if we would use larger wind turbines (more power), that area would be even larger. Honestly, I don't think that that's such a big impact on the environment.