Wikipedia:Reference desk/Archives/Science/2024 February 28

= February 28 =

Why are airports never an effing big Greek cross?
Two runways in series instead of in parallel (i.e. 30 Back for landing 30 Front for takeoff 12 Back for landing 12F takeoff if wind azimuth near 120) and the paths never have to cross even in the air. And another two in series at right angles that they switch to if crosswind's too strong. In extreme both engines exploded on fire emergencies they clear all runways much easier landing.

Also why'd it take so long to invent faster runway exits with slight angles? That seems like common sense. Sagittarian Milky Way (talk) 02:28, 28 February 2024 (UTC)


 * Space at existing airports within metropolitan areas is obviously one limiting factor. With completely new builds in rural areas, it could work. Airports are also always looking at ways of minimising noise on the ground. Your proposal would stretch the area affected by aircraft noise to twice the distance. HiLo48 (talk) 02:48, 28 February 2024 (UTC)
 * I assume you mean noise due to the flight paths of the airplanes. But besides noise, different flight paths from different runway directions may also limit construction height near the airport or simply not be possible due to current constructions. See also [//monroeaerospace.com/blog/did-you-know-why-airports-are-oftentimes-located-near-water/] [//www.airport-technology.com/features/airports-close-to-sea-level/] and consider whether this works so well under the proposal. Nil Einne (talk) 09:43, 28 February 2024 (UTC)
 * Well Denver had forced land, forced altitude and enormous property in the middle of nowhere but they made this (one of the most efficient layouts possible). KDEN AirportDiag!.png Sagittarian Milky Way (talk) 04:21, 1 March 2024 (UTC)
 * Even away from metropolitan areas flat land suitable for an airport may be at a premium and the shape of the suitable area limits the possibilities. The angles of runway exits were not much of an issue until the introduction of heavy wide-body aircraft, which take longer to reduce speed and don't turn as readily as lighter aircraft. If the space is available, an L-shaped two-runway pattern offers the same crosswind-avoidance advantage as a cross while having a decreased collision risk and giving an easier task in designing the whole airport plan. --Lambiam 10:03, 28 February 2024 (UTC)
 * Also consider what happens on two inline runways when there's a departure on one and a go-around on the other: the aircraft going around will end up in the wake vortex of the departing aircraft. So the capacity of two in-line runways isn't really much better than that of a single runway. It's better to put them in a staggered configuration.
 * High-speed runway exits appeared in the 1950s, right at the start of the jet age with its high landing speeds and long runways. And at that time, air traffic wasn't so dense that they needed to land 25 planes per hour per runway. PiusImpavidus (talk) 11:38, 28 February 2024 (UTC)
 * Wouldn't some who could afford to fly then like time-obsessed execs like fast trips? Sagittarian Milky Way (talk) 16:31, 28 February 2024 (UTC)
 * Corporate execs want bragging rights: "My bizjet can do Mach 0.92," not "I paid the airport $600k for a high-speed taxiway at exactly the right spot, so now I save 30 seconds each time I visit HQ." PiusImpavidus (talk) 23:02, 28 February 2024 (UTC)
 * Fair enough. Sagittarian Milky Way (talk) 00:14, 29 February 2024 (UTC)
 * And don't non-inline runways in the same direction cause problems unless very separated? Which would increase taxi distance between runway and terminal. Sagittarian Milky Way (talk) 16:35, 28 February 2024 (UTC)
 * Depending on the quality of your surveillance system, and ignoring wakes, you need a horizontal separation of 2.5 to 3NM, or a vertical separation of 300m/1000ft. A modern jet climbs about 1000m/30000ft per minute, i.e. it has the necessary vertical separation in 20 seconds. Takeoff speed is 150knots or higher, so at least 2.5NM per minute, and rapidly increasing. So you would have the required horizontal separation in less than a minute. IIRC, the very best airports manage about one takeoff per minute, so separation is not a problem. --Stephan Schulz (talk) 21:14, 28 February 2024 (UTC)
 * Parallel runways usually have about 500m separation (like 7C/25C and 7R/25L at EDDF). How much you need depends on the operational flexibility you want; parallel departures require less separation than parallel arrivals. 500m additional taxi distance is less than a minute travel time. Or you could put the runways on opposite sides of the terminal (like at EGLL), so there's no additional taxi distance at all. If you have them slightly staggered (like at EDDB) instead of exactly side by side (like at EDDL), you save some taxi distance and you gain some safety during parallel approaches. BTW, LEMD gets close to your solution. It has two parallel departure runways and two parallel arrival runways, pretty much end to end, but with an angle between them. The new terminal 4 is where the runways almost join. PiusImpavidus (talk) 23:40, 28 February 2024 (UTC)
 * So when I see 9,000 feet on Google that won't help too often vs 500 m? We have 300 yards apart in KEWR slightly staggered and 250 yards in KSFO but those are compromises with limited land. I think KSFO might be the record for big planes in my country. LEMD looks pretty cool though I wonder why they didn't join them, Space Shuttle accidents and Gimli Gliders would have an easier land and maybe they could avoid a tire explosion or gear breakdown from trying to stop so hard. Sagittarian Milky Way (talk) 00:55, 29 February 2024 (UTC)
 * I suppose KEWR normally uses one runway for arrivals, one for departures, with no parallel arrivals. KSFO does parallel arrivals, but has some restrictions. The aircraft must fly side by side, so that neither can get into the wake of the other (wakes are angled in a crosswind) and the pilots must see the other plane before aligning with the runway, so they can only do parallel arrivals of planes with the same landing speed in good weather. My local airport (EHAM) has 3 parallel runways with 2.1–2.8km separation and 1.5–3km stagger, allowing uncoordinated parallel arrivals on centre and right (left is departures only, right arrivals only) even in poor weather. They didn't need all that separation for that, but the terminal had to fit in the east gap and the west gap was needed for noise reasons.
 * LEMD was extensively rebuilt about 20 years ago. It used to have 4 crossing runways, 3 of which were closed. The original 18/36 was almost in-line with the new 18L/36R, but was closed. I suppose interference of go-arounds with departures and available space (both land and noise) was the reason. A runway in-line with 14R/32L (the old runway left) to the NW would have led up to La Moraleja, a hill with some expensive housing. PiusImpavidus (talk) 11:41, 29 February 2024 (UTC)
 * KEWR indeed uses one for arrival and one for departure. Sagittarian Milky Way (talk) 04:24, 1 March 2024 (UTC)


 * Does Chicago's Midway Airport fit your description? ←Baseball Bugs What's up, Doc? carrots→ 06:00, 29 February 2024 (UTC)
 * Close but each direction's only one runway in a row not two. If Denver doubled like this the longest strip would be at least 32,000 feet long (16,000 like the current runway for takeoff and a downwind 16,000 for landing). Sagittarian Milky Way (talk) 21:41, 29 February 2024 (UTC)

Does a given body's total rest energy, have any other meaning, besides the body's ability to totally vanish - becoming energetic radiation?
HOTmag (talk) 10:01, 28 February 2024 (UTC)


 * A "kilobar" gold bar, with a rest energy of some 90&thinsp;PJ, has a market value of about €60,000 or $65,000, twice that of a goldbar with half that rest energy. Transformed into "usable energy", the market value of this energy would be much higher, but we do not have the technology for the necessary transformation. The main reason for physicists to use the concept is to avoid conversions in calculating the energy balance of particle interactions, being able to use eV units throughout. --Lambiam 10:31, 28 February 2024 (UTC)
 * By a "meaning", I've meant: a physical meaning (rather than an economic one). HOTmag (talk) 10:47, 28 February 2024 (UTC)
 * The main reason to introduce the concept of rest energy (corresponding to the mass of the body) is that it is necessary to include it in the energy balance to satisfy conservation of energy. This is what enables the conversion into radiation, for example (in practice, other conservation laws may stand in the way of that process). In addition, it determines the inertia and the gravitation of the body (as mass, rather than rest energy). In addition, the internal energy (which is part of the rest energy in the case of composite systems) plays a role in the thermodynamics of the thing (where this is relevant). --Wrongfilter (talk) 10:43, 28 February 2024 (UTC)
 * Note I asked about total rest energy. I can understand how it's helpful in determining the value of energy of the radiation - the body has turned into, whereas the value of the body's mass is already known. However, does the body's total rest energy - also have any meaning - in determining the inertia or the gravitation (or anything else) of the body whose mass is already known?
 * To sum up: besides a Gedanken experiment in which a given body having a given value of mass - totally vanishes - becoming energetic radiation, can you think about any other Gedanken experiment, which involves a body having a given value of mass, and which shows how the body's total rest energy can be helpful? HOTmag (talk) 11:28, 28 February 2024 (UTC)
 * What makes you think that there's a difference between a body's total rest energy and its mass (times c2)? --Wrongfilter (talk) 12:00, 28 February 2024 (UTC)
 * After the formula E=m^2 was discovered, all of us know there's no difference between a total rest energy and a total mass (times c^2). However, before discovering the formula E=m^2, we couldn't predict the value of energy - of the radiation a given body turned into, even when we did know the value of total mass - the body had had - before it became energetic radiation. This fact proves, that the meaning of a given body's total rest energy, lies (for example) in the body's ability to totally vanish - becoming energetic radiation - whereas the value of the body's total mass is already known. That said, I still wonder if this is the only meaning of the total rest energy, whereas the value of the body's total mass is already known. HOTmag (talk) 12:24, 28 February 2024 (UTC)
 * The terms "rest energy" and "rest mass" are practically interchangeable, except that some physicists feel compelled to express the latter in units of MeV/c2. Others have no such compunction and are happy to write, for example, "a K0 particle of a mass of about 650 Mev". --Lambiam 16:05, 28 February 2024 (UTC)
 * Yes, the terms "rest energy" and "rest mass" are practically interchangeable, due to the discovery of the formula E=m^2. However, they are not conceptually interchangeable, as I explained in my previous response you've just responded to. HOTmag (talk) 18:01, 28 February 2024 (UTC)
 * You keep writing $E = m^2$ . The formula is $E = m c^2$ .  The units don't make any sense the other way.
 * As has pointed out, in some contexts, physicists will adopt a system of units where c = 1 (and sometimes h = 1, too), but you still don't square the value of the mass in that case.
 * I would argue that the terms "rest energy" and "rest mass" are wholly synonymous. The former indicates nothing other than, "what is the rest mass of this thing, expressed in units of energy?"
 * PianoDan (talk) 19:31, 28 February 2024 (UTC)
 * Note I'd written to Wrongfilter: "After the formula E=m^2 was discovered, all of us know there's no difference between a total rest energy and a total mass (times c^2)". So, you can figure out that E=m^2 was of course a repeated typo (in which I only kept omitting the "c" before the "2") in my reciting the best known scientific formula, which I was reciting (correctly) by heart - before I knew to count (and before I knew what it meant)...
 * As for your second remark: Well, it seems you didn't read all of my response (mentioned above) to Wrongfilter. If I'm wrong and you did read all of it, so let me put it this way: If Einstein thought like you that those terms were synonymous, he wouldn't feel he had to prove the equivalence between those terms (up to c^2). HOTmag (talk) 23:21, 28 February 2024 (UTC)
 * But he DID prove it, and it's been experimentally verified continuously since then.  We don't have to behave as though that hasn't already been thoroughly established.  PianoDan (talk) 23:26, 28 February 2024 (UTC)
 * Yes, he proved it. But this fact doesn't mean those terms are synonynous. Just as the terms "101+10" and "110+1" are not synonymous, even though it's provable that 101+10 = 110+1. Unless we disagree about what "synonymous" means. It seems that you're coming from a practical standpoint, whereas I'm coming from a conceptual standpoint. HOTmag (talk) 23:34, 28 February 2024 (UTC)
 * If you combine the definition "having the same meaning" of synonymous with the definition "the denotation, referent, or idea connected with a word, expression, or symbol" given for meaning, we can observe that the mathematical expressions "101+10" and "110+1" are two names for the same referent (the integer 111) and so are synonymous in at least some sense. Viewed as strings, formulae, or numerical recipes, they can obviously be distinguished, in which case they have – in another domain than the integers – different referents. --Lambiam 01:07, 1 March 2024 (UTC)
 * Yes, the same argument could be made for the apparent synonymy between tg and sin/cos. Anyway the OP (i.e. me) posted this thread while assuming that tg and sin/cos are not synonymous. And if you wonder now whether, then...no, synonymy is not the shortest word containing at least three Ys. syzygy is shorter :) HOTmag (talk) 09:43, 1 March 2024 (UTC)
 * Yes, the same argument could be made for the apparent synonymy between tg and sin/cos. Anyway the OP (i.e. me) posted this thread while assuming that tg and sin/cos are not synonymous. And if you wonder now whether, then...no, synonymy is not the shortest word containing at least three Ys. syzygy is shorter :) HOTmag (talk) 09:43, 1 March 2024 (UTC)