Wikipedia:Reference desk/Archives/Science/2020 June 28

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June 28

why when I put an ice in a liquid I get 2 layers?

why when I put an ice in a liquid I get 2 layers of liquid? why don't mix by their own? If I mix them they do mix. So they must be soluble. what is the cause of this phenomena and how it can be replicate without melting ices?--Exx8 (talk) 15:48, 28 June 2020 (UTC)

What is the drink? If it's a sugary soft drink, you might get a sucrose gradient (surprisingly we don't have an proper article), What it boils down to is that surgery water (the soft drink) has a higher density than 'normal' water so the latter would sit on top until you equalise the sugar concentration (ie mix). You could replicate this with any two solutions of different concentration, for instance fresh and salt water. Fgf10 (talk) 15:56, 28 June 2020 (UTC)

it's a soy mixture. and it also happens with an orange juice.--Exx8 (talk) 16:04, 28 June 2020 (UTC)

Still the same point, the density of those will be higher than just water. Fgf10 (talk) 16:45, 28 June 2020 (UTC)

They have to actually mix (physically dispersed in one another) in order for the dissolving process to occur at a reasonable rate. Density is one of the key ideas...less-dense things float, more-dense things sink. So if the less-dense liquid (melted ice) is on top of the more dense liquid (salty or sugary water) it mostly stays floating there. Same as when you pour a spoonful of sugar in tea...it mostly sinks to the bottom as a pile even though if you stir it it dissolves to make a sweet liquid. For lots of cool examples, see layered drink. DMacks (talk) 16:27, 28 June 2020 (UTC)

And tea sweetens much more readily if you add the sweetener while the tea is still hot. ←Baseball Bugs ^{What's up, Doc?} carrots→ 16:36, 28 June 2020 (UTC)

• Dissolving isn't relevant here though, as it's water in water. Just different densities of water(-based solutions). Fgf10 (talk) 16:45, 28 June 2020 (UTC)

• Put an ice cube in the top of a soft drink or whatever, let it melt a bit, and then drink off the top. You'll find that it's a bit diluted. Stirring fixes it. ←Baseball Bugs ^{What's up, Doc?} carrots→ 16:55, 28 June 2020 (UTC)

Artificial gravity acceleration

I'm thinking through artificial gravity in space via rotation. Let's say we've got a hollow, kilometer-sized object rotating such that someone (let's call her Spacewoman Stacy) walking on a platform positioned halfway towards the center would feel like it was about Earth normal.

Here's the question I have. If Stacy falls off of the platform (because science fiction settings are notorious for not having protective rails), does her fall accelerate like it would with natural gravity or is it a constant velocity from when she broke contact with the platform? Would the velocity or acceleration gradually increase as a byproduct of being closer to the edge where the artificial gravity is stronger? Does it matter if there's an atmosphere in this hollow space object?

I guess I'm just having trouble understanding what would be transferring the inertial energy from the center to the edge. Anyone got any insights? — Ƶ§œš¹ _{[lɛts b̥iː pʰəˈlaɪˀt]} 15:51, 28 June 2020 (UTC)

If it's been 1g for long enough she will go off on a tangent at 110mph, speed up as much as a wind increasing from c. 0 to 110 mph would do to her in a handful if seconds and probably bounce on the outside at a very damaging speed eventually coming to a stop with low wind and 2g when she's sped up to 220mph. Sagittarian Milky Way (talk) 17:18, 28 June 2020 (UTC)

If she braced on a subway pole or something in advance she could probably hold on with still to spun up times of 5 or less seconds, with a forgiving speed Vs time profile, even fewer seconds if hugging the pole from the right side. 1 second would suck ass. Maybe cause injury or loss of consciousness or flipping around the pole and flying off to the exterior. If it spun up quick after a long time of no g for a time after it reached 1 g the wind would be almost enough to fly her like a flag, if she fell off soon after she would slow down and her speed difference with outside on contact will be even greater than if there was a normal wind gradient. The air wouldn't have had the long time needed to spin up from viscosity and the solid surfaces creating a wind shear gradient yet. Sagittarian Milky Way (talk) 17:50, 28 June 2020 (UTC)

Usually "inertial energy" means "kinetic energy" when that energy is maintained through inertia, as in a flywheel. I assume the whole setup is in free fall (zero gravity). If the kilometer size is the diameter and the radius is half a kilometre, the platform is a half radius from the centre, or r = 500 m 250 m. To obtain 1 g acceleration, the angular velocity ω should be such that ω²r = g, so ω = √(g/r) = √((9.8 m/s²)/(250 m))= 0.20/s. The (tangential) velocity in the inertial frame is then ωr = 0.20×250 m/s = 180 km/h. Stacy is stationary in her rotating frame on the platform by the centripetal force exerted on her by the platform. The moment Stacy falls from the platform, there are no external forces (disregarding possible air resistance), so she will keep coasting in a straight line at 180 km/h unless she hits something. Think of using a sling, swinging it steadily in a circular arc and then releasing the tab to launch a projectile. In this case, poor Stacy is the projectile. There is no energy transfer except that the projectile that has the kinetic energy is moving through space because of its inertia. If there is air, Stacy's speed will change until her speed matches that of the air, which presumably is rotating with the whole object at the same angular velocity. So immediately after her fall that speed will be the same; as she drifts farther away from the centre, the air will move faster and increase her speed.  --Lambiam 00:04, 29 June 2020 (UTC)

You missed a factor of two. —Tamfang (talk) 00:39, 29 June 2020 (UTC)

Thanks, fixed.  --Lambiam 01:22, 29 June 2020 (UTC)

Thanks for breaking it down for me. Your explanation confirms my suspicion that Stacy wouldn't change velocity except from the air once she falls of the platform and that artificial gravity would be have differently than planetary gravity. — Ƶ§œš¹ _{[lɛts b̥iː pʰəˈlaɪˀt]} 16:36, 30 June 2020 (UTC)

There is a science fiction story mybe by Asimov or Leinster about a spaceship shaped as a large rotating cylinder with young people springing from a platform midway from the axis, soaring along a spiral to the ground and braking in the last second (of course) with a pair of plastic wings, exactly as in your example. But another difference to real gravity most people don't consider is the merry-go-round effect due to the centrifugal force: if the cylinder is not very very large the difference in angular velocity between head and feet would make the simple standing up or sitting down a very delicate and critical maneuver. Also would not be possible to stand really straight. 2003:F5:6F12:6900:B815:447B:8293:D661 (talk) 20:35, 1 July 2020 (UTC) Marco PB

Actually, that's not quite true. From a stationary frame of reference, Stacy would be moving in a straight line at constant speed. But, from her own rotating point of reference, while she would experience weightlessness (since she is subject to no forces), she would not be gaining distance from the platform at a constant rate of speed. Assuming for simplicity the radius of platform's sphere is 1 unit, her distance from the platform surface can be described with s(t) = sqrt(1 + (vt)^2) - 1. Here t is time passed, v is the real velocity (from stationary frame of reference), and vt is then real distance travelled on a tangent of the platform sphere, so s is her distance from the center minus the height of the platform (1 unit).

From this we can find her perceived speed as the derivative ds/dt. If the vt factor is very large (if she has fallen many times the platform radius), then ds/dt ~ 1. However while v*t is sufficiently smaller than the radius of the platform's sphere, ds/dt ~ v^2 * t, meaning her perceived speed is proportional to time passed! This means that if the dropoff is sufficiently small compared to the massive size of the platform, Stacy will be observing herself to be in arbitrarily close to linear acceleration, and will not be able to differentiate, based on observation of her surroundings and sense of proper acceleration, between being on the rotating platform and in a gravity well. 93.136.4.100 (talk) 03:56, 4 July 2020 (UTC)

Should the Sahara dust cloud be mentioned?

It seems to be a big deal this year in the Southern United States.— Vchimpanzee • talk • contributions • 21:21, 28 June 2020 (UTC)

The existing article Mineral dust might (or might not) be a good place to store that info. ←Baseball Bugs ^{What's up, Doc?} carrots→ 21:46, 28 June 2020 (UTC)

I'm thinking that's the case. I'll look into it when I have more time.— Vchimpanzee • talk • contributions • 21:48, 28 June 2020 (UTC)

The principle of WP:NOTNEWS suggests that not everything that happens is to be mentioned in an encyclopedia. Is the dust cloud encyclopedic? Dunno. 85.76.77.150 (talk) 22:53, 28 June 2020 (UTC)

Saharan dust already redirects to Mineral dust. The dust drifting all the way across the Atlantic is a recurring phenomenon that is widely discussed in the literature. A commonly used term in the literature is "Saharan plume" or "Saharan dust plume" – technically it is not a cloud.  --Lambiam 23:02, 28 June 2020 (UTC)

Saharan dust plume would be a good article if it gets written up by secondary peer-reviewed sources. If it's just a one-time thing in the news, it might not be, because nobody really noticed it before, for example. I wouldn't create the article until ... on second thought, having checked Google Scholar, there's probably enough to do a decent start-class article which could also include the news mentions. Looks like it has been going on for a long time and ends up lots of places; interesting. But not for me at my current levels of spare time. 98.33.89.17 (talk) 23:29, 28 June 2020 (UTC)

It's also being called "Godzilla" for whatever reason. ←Baseball Bugs ^{What's up, Doc?} carrots→ 04:27, 29 June 2020 (UTC)

And the hashtag #saharageddon is used.  --Lambiam 10:42, 29 June 2020 (UTC)

If anyone is inspired to create Saharan dust plume, here are two recent easy-read RS articles on the topic: [1], [2].  --Lambiam 10:52, 29 June 2020 (UTC)

I didn't mean it was worthy of its own article, but it seems to be a bigger deal this year and may have warranted a sentence in another article.— Vchimpanzee • talk • contributions • 15:43, 29 June 2020 (UTC)

Gawd have you ever seen any worse maps than p. 3 of [3] 98.33.89.17 (talk) 01:51, 30 June 2020 (UTC)

Actual herd immunity COVID-19 strategy versus failing containment

Would the European Union other than Sweden be more at risk allowing travel to and from Sweden or the USA? 2601:647:5E00:C5A0:4578:1E01:D3FD:33D2 (talk) 23:58, 28 June 2020 (UTC)

In no region (including Sweden) is the COVID-19 immunity level anywhere near herd immunity. If the effective reproduction number is less than 1, it is only because of precautionary measures such as social distancing, wearing face masks and frequent disinfecting. Outside Sweden there is no immediate risk in allowing travel to Sweden; the risk is in allowing travellers arriving from other regions. That risk depends on many factors (What is the current case rate in the region of origin? Does the airline do any form of screening? Does it require passengers to wear face masks? Are the planes packed full? Is the culture in the region of origin such that people with symptoms are likely to self-isolate?). That makes the question rather unanswerable.  --Lambiam 00:31, 29 June 2020 (UTC)

To add the the above excellent answer, re: "Is the culture in the region of origin such that people with symptoms are likely to self-isolate?" a related question is whether and how well they are likely to conform to the new culture on arrival. Years ago, long before wearing a face mask in the US was common, I spent a lot of time in Japan. They had an existing culture of wearing a face mask if you were coughing or sneezing. Although the US at the time had no such culture, I immediately started following the Japanese culture and wearing a mask in those situations. --Guy Macon (talk) 01:36, 29 June 2020 (UTC)

Indeed. [4] How do you feel about permitting those who can work and study from home to continue to do so? 2601:647:5E00:C5A0:3942:580A:1BFA:4353 (talk) 16:22, 1 July 2020 (UTC)