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Well yes, I do realise that the 2d cross product isn't clearly defined and expected a reply similar to yours for the same reason. None the less, the normal dot product or perpdot product (or whatever you like to call it) is a very useful substitute for the 3d cross product. So far I haven't come across a 3d math/physics problem where the cross product couldn't be replaced by the normal dot product in its 2d version. Keeping the thread on rail, if not by using the above method, how do you then calculate h in a 2d case? Cheers, Mike

In this wikipedia article the orbital element the "specific relative angular momentum vector" h is defined as: h = r cross v where r is the position vector and v is the velocity vector. In two dimensions this is the normal dot product of r and v: h = r.x * v.y - r.y * v.x The trouble is that counterclockwise orbits have negative h values. Is this correct? Cheers, Mike

My primary solution to the Fermi paradox has always been the obvious yet apparently provoking: Noone's out there.

eh... say what?

Yeah, that rocked! Cool vid

That's the spirit! Actually I just dove into SPH recently too. Here are some fairly nice papers and slides: http://www.iro.umontreal.ca/labs/infographie/papers/Clavet-2005-PVFS/ http://www.ifi.uzh.ch/arvo/vmml/solenthalerPublications.php Cheers, Mike

Yeah!

You're welcome Shadow. I did wonder what happened with the program, and I'd still like to see it in action once you've got a working sample up 'n' running If you'd like to take a look at some really simple 2d gravity simulation code samples, I thew some up here: http://www.jernmager.dk/stuff/gravity_code_examples.zip Cheers & gl with your coding, Mike

Hi Shadow, No matter if you're doing this to get an understanding of solar dynamics or if this is "just" for a computer game or a visual demo, you'd probably want to use Smoothed Particle Hydrodynamics, SPH. Basically, this is just a way to represent liquids or gases with a finite, discrete particle system. It is used a lot in game programming, but I think it was originally developed to model huge complex astronomical bodies like star clusters and galaxies.There are a few nice papers and free programs out there. cheers, Mike

Yes, you're both right. Limestone is added to the ore in modern furnaces. The trouble is that if the ore contains Sio2 - and as good as all ores do - then iron oxide FeO combines with it to fayalite Fe2SiO4, which is molten away as slag. Iron bound in fayalite is inacessible because it is chemically very stable. By adding limestone - which turns into CaO in the furnace - you bypass this, since CaO combines With SiO2 and forms calcium silicate. This leaves more FeO to be reduced into metallic iron. I could do this too, but since the ore I use is very clean - +90% iron oxides - I don't need to. Also, we know for sure they didn't do so in the iron age (slag analysis), and since I'm trying to backtrack their process using limestone is out of the question anyway. Cheers, Michael

Hello everyone, In this thread you can read about my favourite hobby, prehistoric iron smelting: http://www.scienceforums.net/forum/showthread.php?t=36160 During the last few years I've been running a series of experimental-archaeological iron smelts in order to find the best possible method for extracting iron from local bog iron ore in a historical context. Among the numerous problems that I've had to deal with, controlling air flow is one of the bigger ones. Here's a schematic showing a common furnace setup: Inner furnace diameter is roughly 30 cm. The front plate is rectangular, roughly 25 by 35 cm. The air inlet is 2-4 cm in diameter. Air supply may be a set of hand-worked bellows or - more commonly - an electric air pump with a watt-meter and vario-trafo attached to it. In order to make the furnace work in the best possible way it is important to heat the furnace uniformly, so that at a given height above the furnace bottom, the entire horizontal section has the same temperature both at the back (away from the air inlet) and the front (above the air inlet). The original excavated furnaces are completely evenly baked and vitrified, as shown in this picture: The trouble is figuring out a way to spray in air in such a way that it distributes itself throughout the furnace bottom and makes the charcoal burn evenly, both at the sides and in the back, away from the air inlet. They knew hot to do this in the iron age, but I still haven't found a good way Normally, I can't make the air reach all the way into the furnace, rendering the back ca. 5 cm of the furnace cold and dead. So, specifically, I'd like to know what is the optimal shape and size of the external tuyere used for spraying in air? If anyone here knows anything about modern furnace construction, ejector / injector engineering, or fluid dynamics in general, please drop a comment on this. If you miss any information, let me know. Cheers, Michael

Hello everyone, Here's a link to my webpage, where you can see some images about smelting iron from local low-grade bog iron ore, just like they did in the iron age 2000 years ago (here in europe). The site is in danish - sorry - but there are plenty of images, and more coming up in this weekend. Usually I spend 20-30 kilos of ore + 30 kilos of charcoal and get a raw iron bloom of 4-6 kilos. http://www.jernmager.dk/projekter.php Unfortunately, I still have some technical issues to deal with. I've posted some in this thread - please take a look at it. http://www.scienceforums.net/forum/showthread.php?t=36161 cheers, Michael

Hello, Could someone please explain how to describe the collision between two spheres (in two dimensions, for simplicity), if friction is to be taken into consideration. Since the collision happens instantaneously, I suppose the most useful / correct way to describe it would be to use impulse rather than force. cheers, Michael

When it comes to choosing an integrator for planetary simulation, Runge-Kutta schemes aren't preferrable, simply because they're not symplectic, ie. they don't conserve energy. RK4 actually looses energy very slowly. I'd reccomend a symplectic 4th, 6th or even 8th order integrator like these (small C++ code sample): http://www.aoc.nrao.edu/~dwhysong/prog/symp.cpp They handle mass-spring-damper systems very poorly, but they are awe-inspiringly accurate and stable when it comes to gravitational simulations. The theory behind them is described in this paper: http://128.125.4.56/education/phys516/Yoshida-symplectic-PLA00.pdf Cheers, Michael

As a big kid some decade and a half ago I stumbled across an old article from the sixties on metallic glass. It explained how tiny pellets of gold and i think zircon were shot onto a very cold surface, thus forming small metallic discs with no crystal structure - in other words a metallic glass. These were apparently much tougher than conventional alloys but still very flexible and without the ability to develop metality, since you can't distort the structure in a material that doesn't have any structure. Cool, I thought, cant wait to get my hands on this stuff! Now, several years later, I'm just wondering what happened to metallic glass as a concept. It apparently still isn't used in any everyday products, nor in the air- or space industry... So, my question is what happened to metallic glass? Does anybody here have any inside knowledge about development or useage of this marvelous stuff? Is it likely that I'll be able to stir my coffee with a metallic glass spoon anytime soon? Does anybody know how one gets his dirty hands on a piece of forementioned material? Cheers, Michael

Pedant! Yes, you figured it out. Congrats! (another time, please "hide" the answer so you won't spoil it for the rest.)

No. No. The only allowed tool is the table top scales. You are allowed to take any number of pills in any desired combination and weigh them once, hereby revealing which bottle contains the harmless pills.

No. That would be considered more than one weighing, which is not allowed.

By specifically using the word "table top scales" I was hoping that it'd be clear that I ment something like this: http://product-image.tradeindia.com/00008118/b/Table-Top-Scales-with-Flat-Bowl.jpg The container at the top may hold an unlimited number of pills.

Yes. Let's just say that each bottle contains one hundred pills. I wasn't quite clear about that, but yes it does. (I will edit the IP accordingly, in order to clarify the puzzle.)

Consider ten identical glass bottles of pills. Each bottle contains one hundred pills. Nine bottles contain poisonous pills. One bottle contains harmless pills. The poisonous and harmless pills are identical in every way except that the harmless ones weigh 1.1 g and the poisonous ones only weigh 1.0 g. Figure out which bottle holds the harmless pills. Your only tool is a table top scales, and you are allowed only one weighing.

Can you tell us anything about the material's chemical composition? It sounds like a glass of sorts. Generally you'd increase the melting point of a glass by adding SiO2, Al2O3 and especially MgO.

Scientsts now clam to be 100% sure they found water on Mars, after the discovery of small white blocks of matter that automatically disappear a days time after beeing dug free by the Phoenix probe. They claim it must be water since it evaporates. But couldn't it just be dry ice (frozen CO2)?

I'd say the sample is still very biased, due to the lack of instrumental sensitivity you mention. The planets found are either very heavy or - as these - very close to the central star, or both.

Are you sure you didn't just make maghemite? http://en.wikipedia.org/wiki/Maghemite