See morphogenesis. Gandalf61 (talk) 14:23, 24 November 2013 (UTC)[reply]

Kirchhoff's first law of spectroscopy says that "A hot solid object produces light with a continuous spectrum." I'm wondering why an hot object wouldn't emit a discrete spectrum. 74.15.137.253 (talk) 03:22, 24 November 2013 (UTC)[reply]

This is basically the behavior of a black body, which is a well-studied subject in physics. Kirchhoff is well known for studying black body radiation. --Jayron32 05:23, 24 November 2013 (UTC)[reply]

Because much more energy states are available. In a hot gas, it's individual atoms (or molecules) emitting, the electrons have a limited number of discrete energy levels they can occupy. In solids there are much more energy states possible. See density of states Ssscienccce (talk) 10:36, 24 November 2013 (UTC)[reply]

Thanks. I don't understand where these extra energy levels are coming from though... 74.15.137.253 (talk) 17:57, 24 November 2013 (UTC)[reply]

In two words:

band theory. --Tardis (talk) 21:45, 24 November 2013 (UTC)[reply

]

See also

Rotational-vibrational spectroscopy Ssscienccce (talk) 21:26, 25 November 2013 (UTC)[reply

]

When you read the specifications of Challenger 2 and M1 Abrams you find that the speed of both vehicles off-road is 40 km/hr although they differ in their engines power , so why ? is it limit for any vehicle which drives off-road ? Tank Designer (talk) 12:32, 24 November 2013 (UTC)[reply]

There is a five ton weight difference between them. Rmhermen (talk) 13:52, 24 November 2013 (UTC)[reply]

But the M1 has higher power to weight ratio, and is faster on the road. Maybe it's the suspension, M1 uses torsion bar, Challenger has hydropneumatic suspension. You don't choose a more complex mechanism unless it offers some advantage, I guess... Ssscienccce (talk) 15:36, 24 November 2013 (UTC)[reply]

With something like a car, top speed is generally limited by air-resistance and small variations in the weight of the car or the size of the engine are less critical than how slippery the design of the body is. In a car, power-to-weight ratio has little to do with top speed - but everything to do with acceleration.

But tanks don't go fast enough for air-resistance to matter much and their complicated drive trains make frictional losses much more critical. Hence, any small variation in the way they are designed (and especially their weight) will have dramatic effects on their top speed for any given engine horsepower. Analyzing the reasons why one goes faster than another is likely to be a complicated business.

SteveBaker (talk) 16:16, 24 November 2013 (UTC)[reply]

Think you'll find this is a safety issue. A WW2 Sherman taking a bend too fast would over-steer and skid off sideways. The low loading per square foot of track gave it little grip. On soft ground the same manoeuvre would lose it one or both of its tracks (too much resistance to side forces). Not a good thing to happen in the middle of a battle. Then there is the issue of pitching up and down over rough terrain. In the heat of battle with the adrenalin coursing through the body, the bod driving, can go too fast and give himself whiplash. Again, this could occur at just the wrong moment in time. --Aspro (talk) 18:37, 24 November 2013 (UTC)[reply]

Also. At 40 km/h diving through rough scrub and into blind ground that one has not seen before and assuming the standard 1.5 second reaction time. Then the tank will have traveled (1.5 X 40 000) / 3600 = 16.6666667 metres be before the driver can even think “Oh Sh-one-T” and try to stop.--Aspro (talk) 18:56, 24 November 2013 (UTC)[reply]

I can't see that the "off road" speed is calculated in any scientific way. Is there a standard "off road" terrain? I can imagine that on some surfaces, either tank could almost reach their top road speed, while on others it would be a lot less than the 40 kph quoted. After all, the Land speed record is set "off road" isn't it? Alansplodge (talk) 20:55, 24 November 2013 (UTC)[reply]

One does not 'need' an "off road" speed calculated in any scientific way (other than injuries received). Jump into a 4x4 and head out off road at 25 mph (40 km/h). Notice, that even with seat belts on, if the ground is rough, you're being bounced up and down quite violently. That's on a modern vehicle with good suspension. Even though the modern tank cost a couple of million, the tank designers don't put a lot of effort into providing a smooth diver experience. So at 40 km/h the human body has a lot of kinetic energy that muscle power can not compensate for. The WP article also states that:The M1 Abrams' powertrain comprises a 1,500 shaft horsepower (1,100 kW) Honeywell AGT 1500 (originally made by Lycoming) multi-fuel gas turbine, and a six speed (four forward, two reverse) Allison X-1100-3B Hydro-Kinetic automatic transmission, giving it a governed top speed of 45 mph (72 km/h) on paved roads, and 30 mph (48 km/h) cross-country. With the engine governor removed, speeds of around 60 mph (97 km/h) are possible on an improved surface; however, damage to the drivetrain (especially to the tracks) and an increased risk of injuries to the crew can occur at speeds above 45 mph (72 km/h). Try driving across

Landrover at just 25 mph and you will see what I mean. So tanks apparently are governed to keep the speeds to with in safe limits in all terrains. In the last Gulf War they may have well have had these limits raised, as the terrain was flat and the TV coverage showed them belting-a-long at fair rate on knots (well, the British built tanks did). But off the production line, they will only go so fast and no more.--Aspro (talk) 23:17, 24 November 2013 (UTC)[reply

]

It's not just a matter of what's safe or 'comfortable' for the crews; the tanks themselves are more fragile than they look. Tanks require a lot of maintenance and repair even under ideal circumstances. There are a lot of heavy moving parts, working under (mechanically) stressful conditions. Consider the M1—there's more than an hour of maintenance required for every hour of operation. A set of tracks are good for 1000 (really) to 2000 (ideally) miles. They suffer a failure that affects combat ability every couple of hundred miles. Remove the governors and start really shaking up that equipment and the enemy won't have to blow up your tanks—you'll do it for him. TenOfAllTrades(talk) 14:55, 25 November 2013 (UTC)[reply]

That would seem to be appallingly bad design. A track life of 2000 miles at an average speed of 20 km/hr is a life of 100 hours. I owned a Caterpillar D4 bulldozer at one time for work on my hobby farm. It was second hand, originally owned by a Shire council for towing road compaction equipment, firefighting, and landfill rubbish work. When I bought it, it had 16,000 hours on the clock, and it still had the original tracks, albiet seriously worn though perfectly usable. Such eqipment is considered obsolete now. Skid-steer machines "Bobcat" style ie machines with wheels and no tracks) have taken over most of the market, but bulldozers are still available, with rubber tracks. It has been found that rubber tracks last even longer and require no maintenance. 121.221.118.84 (talk) 23:43, 25 November 2013 (UTC)[reply]

But you did not have to guarantee combat readiness for your equipment! There is a difference between having a piece of equipment that works, and having a piece of equipment that can be trusted to work when lives depend on it! If you quantitatively study combat engineering, you will find that vast over-expenditure on preventative maintenance is a statistically better choice. Here's another book from the same library: Developing the Armored Force..., which is essentially an interview transcript with Major General Robert Sunell, who oversaw the U.S. Army's armored technologies and operations. He has much to say about maintenance, (and much more to say about tank track design choices). Nimur (talk) 01:12, 26 November 2013 (UTC)[reply]

Valid points in your first para Nimur, but the difference in track life is so extreme (100 hours versus >16,000 hours), and Cat bulldozer tracks so trouble free, that we just have to suspect something wrong with the tank tracks. Last purchase prices nothwithstanding, the military does get junk from time to time. Not all the time, but some of the time. 121.221.118.84 (talk) 02:22, 26 November 2013 (UTC)[reply]

Have you been driving your bulldozer for 16,000 hours through

M60 can only do that for about 250 500 hours (provided it's equipped with steel track). But it's sort of a high-maintenance vehicle. Nimur (talk) 02:38, 26 November 2013 (UTC)[reply

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Furthermore, the Cat D4 weighs about 5 tons - the M60 weighs 50 tons. Right there is a large part of the problem! The Cat's tracks are relatively lightweight and the stress on the links between them is small - but the M60 is an altogether more massive engineering problem. Worse still, the Cat's top speed is nowhere near 40km/hr - and even if it were that high, it doesn't spend much of it's operational life moving anywhere near top speed. The tank, on the other hand drives that fast anytime the terrain allows it. The force on the track links is going to depend critically on weight and speed. Worse still, the Caterpillar is designed to be reliable enough to last for thousands of hours with minimal maintenance - and doubtless there are aspects of it's engineering where compromises were made in other aspects of it's design to get that kind of reliability. But the M60 is designed to get in, do a job and get out again - then get hours of careful maintenance. If they have a trade-off to make between (let's suppose) minefield-survivability and track life - then you can guess which one wins. So it would be no surprise whatever if the lifespan of the tanks tracks were 160 times less than the Caterpillar. SteveBaker (talk) 14:22, 26 November 2013 (UTC)[reply]

Actually, the top speed of my D4 (in top gear) was about 30 km/hr. D4's were often used to tow road compaction devices at up to around 20 km/hr - the same order of magnitude as the tank's average speed. A D4 can weigh a lot more than 5 tons. The basic tractor weight of mine with the particular tracks (D4's were made with 2 optional track types) fitted was about 4.5 tonnes. The blade, blade arms, and hydraulics etc weighed another 3 tonnes, and I had a tree lever on it as well - about another tonne - total 8.5 tonnes. (A tree lever on a little D4 is perhaps not much use - the D4 has only enough traction to push over quite small trees, no more than about 150 mm trunk diameter - but I had the tree lever option anyway.) In any case, if you like, you can compere the tank with a D9 (50 tonne nominal weight typical) - which has at least the same track durability & reliability as the D4. You have a point perhaps about trading off performance against reliability, but hey, the difference is so extreme, you just have to question whether the tank has a deffective track design. 121.221.118.84 (talk) 15:15, 26 November 2013 (UTC)[reply]

SteveBaker is right. Applied engineering is all about choosing the best compromises to suit the intended use. My school mate's elder bother was a tank driver in the early seventies and he reckoned that in a tank in Germany at the start of WW3, his survival time in battle would be about thirty minutes before he was taken out. With aircraft. An engine in piston driven civilian light aircraft last about 1500 hours before an overhaul is required. A WW2 military aircraft need and engine change ever several hundred hours because they were operating on their extreme limits. It took about 160 hours to train a pilot who operational time was about 120 hours. A Private Pilots License can be gain in just about 27 hours. Exalance has a high price. WW2 German tanks were techno-logic engineering marvels compared with the Russian T45 tanks but the latter proved to be more reliable in battle. The engineering compromises of the Russian T45 were better chosen. Do you want your next D4 to have superior work performance and yet be happy that is spends much of its time in the workshop? Or do you want a long lasting bit of capital equipment that has its compromises chosen so that so that it is always earning you money? --Aspro (talk) 20:30, 27 November 2013 (UTC)[reply]

Good points, Aspro, but you have missed a point and made a bad comparison. An aircraft engine, particularly a WW2 piston aircraft engine, is a very complex piece of machinery where weight restrictions dictated severely compromising durability. Bulldozer tracks are extremely simple things - basically heavy plates of very hard steel joined by equally hardened steel pins, passing over specially shaped hard steel rollers running on plain bearings. The very opposite of rocket science and aircraft piston engines. I cannot see why tank tracks cannot also be similarly simple inherently durable engineering. Also, in WW2, production didn't really get underway until the War - and a lot of tanks and aircraft were models that did not exist before the war. WW2 was won and lost on industrial production capacity and went on for 4 years. So it was sensible to engineer them for a life of a few hundred hours (though in fact in non-war situations nearly all turned out to be very long lating). Wars today, and WW3 should it happen, won't happen that way. Equipment used will be what the military on each side has had on hand. Wars now will be won and lost within days on strategy and in-stock might. And when the war is won, the equipment will be returned home and continue as stock in hand and for training. Look at the the wars of USA vs Iraq, or Irag vs Kuwait - all over in days. Afganistan has been going on for years now, but that is a special case due to political mistakes and uses existing on-hand capital equipment anyway. All this means that equipment will not spend 90% of its time in combat like WW2, it now will spend 90% of its time in training and exercises. Since might is determined by $ budgets, the military must now optimise for long trouble free life. 1.122.117.242 (talk) 00:15, 28 November 2013 (UTC)[reply]

The the object of this Reference desk is to answer the OP's question, rather than tutor interjectors to the answers given.--Aspro (talk) 22:32, 28 November 2013 (UTC)[reply]

In the future tanks will be remote controlled drones and one won't have to worry about puny humans inside them. It'll cut down the required armor and ease a lot of problems but even so I'm not sure they'll be able to go much faster over rough ground.

Dmcq (talk) 22:25, 25 November 2013 (UTC)[reply

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Once you have the concept of a drone - the entire concept of a tank may prove unimportant. Without human lives at risk, you can consider a vehicle that is little more than a gun on wheels - if dumping the armor makes it drastically cheaper (which I'm sure it does) then maybe you can have a lot more of them for the same $$$ cost. With more of them, and cheaper - you can more afford to lose them due to enemy action. These kinds of consideration will doubtless produce something that looks nothing like a tank in the end...just as a drone aircraft looks nothing like a conventional manned surveillance plane. Classic surveillance aircraft thinking pinnacled with the SR71 - one of the fastest aircraft in service...modern drone thinking resulted in the Predator - which is probably the slowest! Consequently, I think it's dangerous to speculate about how future drone "tanks" will be...my bet is that there won't be any tanks. SteveBaker (talk) 14:22, 26 November 2013 (UTC)[reply]

Just a question/observation - wouldn't the speed depend a lot on the track and groove depth? Thinkof of a tank like a train the lays down its own track, the potential throw the track off would depend mostly on speed and side stress and the depth of the guard to keep the wheels on the track. I think the speed of the tracked vehicle is determined by the track design and less by power and weight except as it relates to the stress on keeping the wheels on the track. --DHeyward (talk) 04:27, 26 November 2013 (UTC)[reply]

The tracks act as a flywheel - at a constant speed they do not absorb energy above that of ground friction. Cat D4's have two optional track widths - 6U and 7U. 7U's weigh significantly more, cost more, but there is no difference in performance or handling, except that 7U tracks offer greater drawbar pull in soft sand and mud. However I agree with previous respondents that the speed of a tank is most likely limited these days by the shocks and g-forces over rough ground that the humans inside can take. It wasn't always that way - the first tanks in WW1 were only meant to support infantry, so only needed enough power to go at walking speed. But armies soon realised that you could have tank vs tank battles, and high speed tank attacks cf shock and awe as a later American president put it. Then they were limitted by the size of engine that could be fitted within the hull. But engine technolgy has come a long way since, so engine size should not now be a limit to tank speed. I've not driven a tank, but I can tell you what it's like to drive a Cat D4 bulldozer at top speed (about 30 km/hr) on level ground - it's somewhat hairy and scary, as there is no springing in the suspension. As for driving a D4 at top speed or even at a human's running speed, over rough ground - forget it - it will buck like a mad horse, throw you through the air, and maybe kill you. Tanks have sprung suspension, but there is a limit to spring travel and what springs can achieve. Incidentally, a D4, which has a quite small engine, about 35 kW, is just about unstallable. If you hit a large tree, the tree will stop the D4 from moving, but the engine will keep threshing the tracks around, even if set to a low RPM, churning up clouds of dust & rocks and digging holes until the chassis hits the ground. 1.122.117.242 (talk) 03:03, 28 November 2013 (UTC)[reply]

I'm reading On Food and Cooking, and it states that salt increases the elasticity of the gluten. How does this happen at a molecular level? Is it something specific to the chemistry of sodium or chloride ions? Mostly, I'm trying to make puff pastry without adding salt, and I'm trying to understand what I could use in lieu of NaCl to produce the functional effect. I'd like to avoid KCl if used in any significant amount because of taste concerns, but if it's a function of binding available water instead of something specific to the chemistry of the chloride salts, I'm guessing I could try sucrose instead? 2001:558:600A:2F:45D:4092:30A6:49E3 (talk) 21:31, 24 November 2013 (UTC)[reply]

I don't know the answer specifically, but I do know that the elasticity of gluten results from two factors: the tendency of gluten (protein) molecules to curl up, and their tendency to stick to other gluten molecules. Salt solutions can affect both of those things, but not as a function of binding water -- dissolved salt doesn't do that. Instead the ions work their way into the protein structure and alter the force geometries. Sugar would not have the same effect. Also putting any substantial amount of sugar into a dough will radically alter the result you get when you bake it.

When I'm cycling in the winter, my torso usually overheats while my face (and ears if exposed) and feet can get cold. I was wondering whether the body of humans or any other animal ever integrates temperature signals from different areas of the body to do something more sensible like delivering excess heat from the torso to the extremities? — Preceding unsigned comment added by 129.215.47.59 (talk) 22:13, 24 November 2013 (UTC)[reply]

The body does have pretty sophisticated mechanisms for thermoregulation. I would be cautious about "more sensible" - evolution would favor survival over comfort, and it seems likely that heat loss (the major lethal risk) would be greater with diversion of heat to the extremities, especially when the thermoregulation system cannot "know" how soon the body will find warmer shelter. So, I would start by defining "sensible" - or realizing that the body's system is quite good already. -- Scray (talk) 22:24, 24 November 2013 (UTC)[reply]

The body is already acting "sensible" by not moving more heat to an area that is loosing it quickly. Having your nose, fingers, ears or toes freeze is a lot better than having your core temperature drop. Also having those body parts freeze does not always mean you are going to loose them. Over the years I've frozen my nose and my fingers (those several times) and still have them all but my fingers do get cold very easily. See hypothermia and frostbite. CambridgeBayWeather (talk) 00:23, 25 November 2013 (UTC)[reply]

When the core temperature rises, the body will start sweating, hairs on the skin lie flat, preventing heat from being trapped by the layer of still air between the hairs. Arteriolar vasodilation occurs. This redirects blood into the superficial capillaries in the skin increasing heat loss by convection and conduction. But that alone is not enough to keep exposed skin warm, because you can't get enough heat to the skin to compensate the cooling by the cold air. Keep your hand in -20°C

blood viscosity increases at lower temperature, so that would decrease the flow in cold skin. Ssscienccce (talk) 14:39, 25 November 2013 (UTC)[reply

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• I've done quite a bit of winter cycling and experienced all these things. The mechanisms are a bit tricky. To begin with, when your core temperature begins to drop, the body reduces blood flow to the hands, feet, and face -- that's why they get cold. If you exercise enough to bring your core temperature above optimum, the peripheral blood flow will usually come back, especially to the hands. However, the switching is not immediate, and you might have to sweat pretty hard for a while to make it happen. I've done hard riding in 20 degree conditions with no gloves or face mask -- the usual effect is that for the first few minutes I feel a lot of pain, but then the "heat comes on" and my hands warm up and feel fine. For some reason, though, my feet usually get cold, even if I'm wearing good shoes and socks. Looie496 (talk) 17:01, 25 November 2013 (UTC)[reply]

Seems that blood flow in extremities is more sensitive to local temperature than ambient temperature according to this. It also mentions vasodilation when the temperature drops to 4°C (39°F), the Lewis hunting response, in which the alternating vasodilation and vasoconstriction occurs. At 10°C, constant vasoconstriction will occur, with blood flow dropping gradually for ten minutes and then staying constant at that low level (in Eskimos it takes 90 minutes). So your cold feet and warm hands may be because your feet aren't cold enough to initiate the Lewis hunting response. Ssscienccce (talk) 07:38, 26 November 2013 (UTC)[reply]

The article on snake bites includes a section on first aid. Am I correct in thinking this equates to medical advice and that some of the details there should be removed? Bazza (talk) 22:39, 24 November 2013 (UTC)[reply]

Reference desk requests for urgent medical aid are different from discussion of medical matters in articles. Talk and articles are different things. I'm not going to specifics, just saying on a general level - which sort of reflects the underlying difference. 88.112.41.6 (talk) 22:53, 24 November 2013 (UTC)[reply]

It does make an interesting question. Where does one draw the line between medical information or knowledge and medical advise? Anyways, the applicable policy is

it's open to interpretation, it uses the imperative mood, but is it instructing the wikipedia reader or merely illustrating how most first aid guidelines instruct their readers? NOTHOWTO writes: "describing to the reader how other people or things use or do something is encyclopedic", so putting "they" in front of every sentence would fix the problem...

WP:MEDICAL states: "Nothing on Wikipedia.org or included as part of any project of Wikimedia Foundation, Inc., should be construed as an attempt to offer or render a medical opinion or otherwise engage in the practice of medicine." Wile this disclaimer is compatible with providing general information on medical topics, it seems hard to reconcile with answering concrete requests for medical advice on the reference desk. Practicing medicine requires a patient (I think). There's no patient involved when writing an article, there may be when you answer questions from individuals. Ssscienccce (talk) 15:33, 25 November 2013 (UTC)[reply

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Are the rocks of the soda canyon in Colorado mostly permeable? — Preceding unsigned comment added by 99.146.126.108 (talk) 23:59, 24 November 2013 (UTC)[reply]

According to http://pubs.usgs.gov/sim/3224/SIM3224_pamphlet.pdf, the rocks of Soda Canyon are essentially all part of the Goodridge formation, which consists of sandy shale, sandstone, and cherty limestone. All of those should be pretty permeable. Looie496 (talk) 16:33, 25 November 2013 (UTC)[reply]