Wikipedia:Reference desk/Archives/Science/2024 May 3

= May 3 =

Why Bronze Age and not Brass Age?
Why was bronze so much more widely used by the ancients than brass? Was it because zinc was so much harder to come by than tin, or was it because it was so much harder to make high-quality brass than bronze, or was it because bronze had more desirable material properties than brass? 2601:646:8082:BA0:F13B:E84E:494B:CA04 (talk) 04:40, 3 May 2024 (UTC)


 * The principal reason is probably that stated in the lede of Brass,
 * "Brass is not as hard as bronze, and so is not suitable for most weapons and tools [my italics]. Nor is it suitable for marine uses, because the zinc reacts with minerals in salt water, leaving porous copper behind; marine brass, with added tin, avoids this, as does bronze."
 * Zinc seems to have been less familiar (and available?) than tin in the Western ancient world, but more widely used further East. However, its use even there seems to have been more often 'ornamental' that practical, probably because of its poorer mechanical properties.
 * My suspicion is that the fabled metal Orichalcum (Greek oreikhalkos, "mountain copper", but in Latin Aurichalcum, "gold copper") was a form of what we would now classify as brass. {The poster formerly known as 87.81.230.195} 188.220.144.58 (talk) 06:10, 3 May 2024 (UTC)


 * There was no 'Brass Age' because, for many years, it was not easy to make brass. Before the 18th century, zinc metal could not be made since it melts at 420ºC and boils at about 950ºC, below the temperature needed to reduce zinc oxide with charcoal. In the absence of native zinc it was necessary to make brass by mixing ground smithsonite ore (calamine) with copper and heating the mixture in a crucible. The heat was sufficient to reduce the ore to metallic state but not melt the copper. The vapor from the zinc permeated the copper to form brass, which could then be melted to give a uniform alloy.
 * Brief Early History of Brass. Alansplodge (talk) 16:51, 3 May 2024 (UTC)
 * 13 year old me melted a zinc coin in a stainless spoon in a gas flame. I removed the iron the pot or pan lies on, tried to find the flame part of max spoon glow-in-the-dark (spoon incandesced orange) and waited to ensure getting very close to thermal equilibrium. Could I have boiled zinc? The copper electroplate probably didn't melt (post-'82 US pennies are this composition) so I stirred with a straightened paper clip and tried to heat a glob on the clip for a less heat-robbing zinc holder but didn't reach max temp as I didn't want it to drip on the gas holes. A whole penny in the gas ring's bowl didn't seem to boil but did puddle. Sagittarian Milky Way (talk) 22:37, 3 May 2024 (UTC)
 * The issue is not with melting already isolated zinc – the ancients could certainly reach the boiling point of zinc. It is rather that the temperature you need to reduce zinc oxide to zinc metal is higher than the boiling point of zinc metal. As a result, the metal evaporates out if you try to smelt it in the open air like you would tin or lead: either the zinc vapour escapes, or it reacts with CO2 from the furnace and oxidises back again. (Source.) And you can't simply distill the vapours like you could with mercury because zinc vapour is much more reactive than mercury vapour and will oxidise back again in the atmosphere. This is somewhat mitigated if you try to make brass (because it is possible to direct the vapour to mix with the copper before it escapes or oxidises back again), but even then it was pretty difficult. What you need to do to prepare metallic zinc is to trap the vapour until it is cool enough not to react with air – which requires enough trial and error (because, remember, at this point nobody knows what gases are) that it seems that knowledge of how to do this did not spread much outside India (the first to figure it out) in the ancient world.
 * BTW, for early element discoveries, I quite recommend the Sodium Lamp blog. Double sharp (talk) 07:47, 4 May 2024 (UTC)
 * Did they learn boiled water can be trapped and reconstituted before they discovered how to make zinc? Maybe that's how they got the idea to boil zinc ore in an airtight place and let it cool and see if that worked with zinc too. Sagittarian Milky Way (talk) 15:25, 4 May 2024 (UTC)
 * [un-indent] Thanks, everyone! So, it was mostly because bronze had better material properties (at least for the applications which the ancients had for it), plus at least somewhat because of the tricky and non-intuitive method of alloying copper with zinc -- right? 2601:646:8082:BA0:4D5B:DB6C:9EDD:E08F (talk) 11:26, 7 May 2024 (UTC)

Reflex to prevent food falling
Sometimes when a piece of food falls from my hands during eating, I have a sort of rapid reflex to prevent it from falling on floor - either by catching or deflecting it to stay on the table. Is it a sort of catching reflex in physiology? 212.180.235.46 (talk) 18:49, 3 May 2024 (UTC)


 * It's more than just food (as you'll find if you ever find yourself responsible for a small child), though I'm not sure it's a reflex in the technical sense. We have a List of reflexes and the closest thing I can find there is the palmar grasp reflex, which is about how babies and other infant primates grab onto things in an automatic way. This isn't quite the same thing, though. Reflex or not, the human urge to grab things that are falling can be very strong, even when the cause is hopeless or harmful; there's no shortage of videos online of people attempting to right tipping vehicles or catch falling objects they really ought not attempt. I haven't found an appropriate article on this yet, though. Matt Deres (talk) 20:10, 3 May 2024 (UTC)
 * I think that this does not qualify as a physiological reflex, since the pathway is not a reflex arc but involves the whole kaboodle of the visual or tactile perception of a valuable item slipping away, the ensuing cognitive recognition of impending disaster, and finally an appropriate response of fine-tuned firing of motor neurons to (hopefully) save the day, all processed in the neocortex. --Lambiam 20:32, 3 May 2024 (UTC)
 * And what is the mechanism that should switch this "reflex" off if the thing you have dropped is a scalpel? -- Verbarson talkedits 22:04, 4 May 2024 (UTC)


 * I read a long time ago, in a popular rather than a scientific source, an observation that there is a big sex/gender difference in this behaviour. The suggestion was that men tend to try to catch things, even when the chance of doing so is very small, and that women don't, and in fact try to avoid letting the falling object touch them. But having read the above, I would obviously have my doubts about that for a dropped baby. I assume a mother would always try to catch her baby. HiLo48 (talk) 09:57, 5 May 2024 (UTC)
 * In one of Mark Twain's novels (The Adventures of Huckleberry Finn?) a runaway boy is masquerading as a girl, and wearing a dress. Suspecting this, someone tosses an object to him while he is sitting with hands occupied, and he claps his knees together to catch it between his thighs. The tosser (!) then explains that this identifies him as a boy, since a girl would have opened her legs to catch the object in her spread skirt.
 * I myself have occasionally dropped something and moved to catch it (sometimes successfully) much quicker than I can move by conscious, pre-planned volition. {The poster formerly known as 87.81.230.195} 188.220.175.176 (talk) 07:44, 8 May 2024 (UTC)
 * Yes, I think it is Huck. —Tamfang (talk) 18:49, 17 May 2024 (UTC)

takeoff/landing distance graphs
1. I understand that weather stations don't give density as often and runway air's always close to 29g/mol but wouldn't density altitude be more accurate? A drone could fly much easier in a xenon balloon than a hydrogen balloon but a kg/m³ in Earth gravity is a kg/m³ in Earth gravity. So can I just calculate the density altitude from common weather station things like meteorological pressure/altimeter setting and geometric altitude and use that on the ISA+0°C graph? There are formulae and calculators for that online. Maybe they don't want to make the generic jetliner owners manual even longer (they're hundreds of pages) with a graph for every plausible Fahrenheit when it says it's unofficial but I don't know where to read official manual(s). It says each airline gets customized manuals. With the generic manual most temperatures don't have a graph and it doesn't have colder than standard atmosphere or very hot.

2. How do I adjust airport weather station temperature to the temperature that affects runway length used? The thermometers are in a white wood louver a bit lower than large plane wings, they're probably on grass, not significantly heated by jet engines, it should be hotter when it's hot and possibly colder when it's cold. If it's 40 or 50°C in the airport weather station in the local all-time high at 4 meters ground altitude and 40°N and 30°N respectively what's the best estimate and worst case density altitude for concrete runways and asphalt?

3. How do they make these graphs? Do they test fly them, measure the rear gear touch length (plus wheelbase if landing) and add safety factor(s)? How much? Or is it the entire distance from first penetration of a few tens of feet above ground (some precise value) with the lowest part of the plane to foremost front wheel ground touch and rearmost rear wheel ground touch to last penetration of a few tens of feet above ground (another precise value) with the lowest part of the plane or some part of the plane when a certain climb rate or slope is reached if that's later? Sagittarian Milky Way (talk) 22:39, 3 May 2024 (UTC)
 * Runway length corrections specified by ICAO are 7% increase in runway length for every 300m rise in elevation from the mean sea level and 1% increase in runway length for every 1°C rise in airport reference temperature (Tr) above the standard temperature at elevation (STE). Philvoids (talk) 01:00, 4 May 2024 (UTC)
 * So if it's ever -85C=-121F at sea level they could ascend like Jesus or use a runway zero meters long? Sagittarian Milky Way (talk) 19:28, 6 May 2024 (UTC)
 * Also do they ever taxi backwards after aligning the plane so long as they're absolutely sure they won't go too far back? Maybe for that thing where they calculate the least amount of throttle that'll get them safely off and only shove the throttle to TOGA if there's an emergency like someone suddenly driving in their path after V1? Can the back of the wheel be over the blast pad if no part of the plane touches the yellow line? Unless they have a backup camera I don't know if they could get that close in a reasonable amount of time without risking going too far though. Instead of unbraking before leaving idle power are they allowed to release brakes when engines reach spooled up or at some intermediate value if trying to brake till full power would be sufficiently bad? If they ever do that then can they still be over MTOW till brake release or would they have to reach MTOW by the end of idle power or would they even still have to release the brakes before the takeoff throttle push anyway (signifying MTOW and other post-taxi rules activation) then brake again then push throttle? Sagittarian Milky Way (talk) 19:50, 6 May 2024 (UTC)
 * How exactly would a plane 'taxi backwards' rather than being pushed or pulled by a pushback truck or tug (which would not be present at the start of a runway)? Manoeuvering on the ground by Powerback using Thrust reversal is unavailable on many aircraft, and generally frowned upon: ". . . such operation is prohibited or strongly discouraged by aircraft manufacturers as well as airport safety regulations in nations with actual aviation safety regulating agencies." {The poster formerly known as 87.81.230.195} 188.220.175.176 (talk) 08:34, 7 May 2024 (UTC)