Wikipedia:Reference desk/Archives/Science/2021 January 2

= January 2 =

Silvery shiny inside of bags with chips
Bags with chips often have silvery shiny inside. Why is this and how is this done? A thin layer of metal like aluminum? Thank you. Hevesli (talk) 11:14, 2 January 2021 (UTC)


 * This is a metallised film, manufactured by exposing a polymer, such as cellophane, to metal vapor that has been injected into a vacuum chamber. This process is known as physical vapor deposition. Someguy1221 (talk) 11:40, 2 January 2021 (UTC)
 * Thank you. Hevesli (talk) 16:23, 2 January 2021 (UTC)

The loopholes that form the galaxy
According to the hypothesis of the evolution of the universe, when it has not yet evolved, all matter and energy are in balance, and the matter and energy it needs are diffused outward in accordance with the laws of physics (the matter balance so gravity is cancelled), so where is the matter, energy and gravitational to evolve? So it can only be created by God. The process of forming energetic stars in the universe violates thermodynamics. Why is energy transferred from low energy to high energy to form stars that have shapes today? — Preceding unsigned comment added by Fluenceerr11001 (talk • contribs) 13:23, 2 January 2021 (UTC)


 * In mainstream cosmological theories about the evolution of the universe, the early universe was not at all "in balance". What hypothesis are you referring to? Hopefully not one of fringe science. No phenomenon is known to violate the laws of thermodynamics. Every religion has its own evidence-free cosmogonic myth about the origins of the world; there are no a priori arguments for preferring one over the other. --Lambiam 16:21, 2 January 2021 (UTC)

Newtonian Physics, gravity: Do bodies of much larger mass actually fall faster?
The classic example of Galileo dropping 2 objects, (sans wind resistance), reaching the ground at the same time, assumes that the M-earth >> than both m1 and m2.

Thought experiment: If 2 objects of unequal mass are dropped from say 1079 +  1 mile (radius of the moon [to center of gravity] plus 1 mile) above the lunar surface (chosen since there is no atmosphere/wind resistance], we presume that they will incur identical acceleration(lunar) by Newtons law of gravitational force (f=Gm1m2/r2), each object taken independently. We all accept this.

WHAT OF a case whereby a massive object, exactly equal to that of the moon (for simplicity) which, for this example, I'll refer to as Moon(2), were to be 'magically' placed such that the surface of Moon(2) would be positioned to be 1 mile above the surface of Moon(1), (the center of mass of Moon(2) being a radius further from surface of Moon(1)).

FOR an observer standing on the surface of Moon(1): would the observer witness the object Moon(2) to fall at the same rate as say a sufficiently small object such that Newton's Law of Gravitational force exhibits diminished attraction, a "door knob" for example, ( PERFORMED INDEPENDENTLY that the attraction of the door knob to Moon(2) is not involved for that observation)? In other words, do the 2 moons fall faster into each other than a much smaller object would fall?

Does the gravitational attraction exhibited by the massive Moon(2) decrease the time-to-impact?

By Newton,
 * F= G X MASS-Moon(1) X  MASS-door knob / 2(R-moon)^ 2

will be much smaller force than
 * F= G X MASS-Moon(1) X  MASS-Moon(2) / 2(R-moon)^ 2

Would the clock measurement for these falls to the moment of collision be identical?

If so, is this explained away by the sluggish 'inertial mass' of Moon(2), that it is >> than the door knob?

Thank you Mhandley wikip (talk) 15:08, 2 January 2021 (UTC)


 * If air resistance or other effects are negligible the massier object falls faster by causing the Moon to fall to it more but the difference is so slight for any near-future mooncrafts that "they fall the same" is usually good enough. And this is why the astronomical unit (149,597,870,691 meters) is like a few kilometers or few hundred more than Earth's orbit cause the astronomical unit is the orbit of a massless only affected by gravity particle with the year of the massy Earth. Also orbit ellipticity effects I think. Sagittarian Milky Way (talk) 15:35, 2 January 2021 (UTC)


 * Both objects, when in free fall towards each other, fall toward the common centre of gravity (cog) – more precisely, each object's own cog moves toward the common cog – until they collide (unless totally disintegrated by passing the Roche limit). If one has a large mass $M$ and the other a smaller mass $m$, then the acceleration of the less massive object only depends on $M$, and that of the more massive object only on $m$. If $m$ is increased, the less massive object will fall as fast as before, but the common cog has shifted towards it and away from the more massive object, so it has to fall over a shorter distance, thus taking a shorter time. (The more massive object has to fall over a larger distance to reach the common cog, but it will experience a larger acceleration; the combined effect means it takes just as much less time as for the less massive object.) If the less massive object is very small, the common cog is at the cog of the more massive object. If the less massive object grows to have the same mass as the more massive object, the common cog will be at the halfway point. The time to reach that (ignoring the radii of the objects, treating them as point masses) then goes down with a factor of $√2$, like from 14 hours to 10 hours. In general, this factor is $&radic;(1 + m/M)$. --Lambiam 17:09, 2 January 2021 (UTC)
 * Visually, when standing on the Moon or at the object you would judge fall speed relative to the Moon surface, not the mutual center of gravity. Sagittarian Milky Way (talk) 17:38, 2 January 2021 (UTC)
 * The accelerations of the bodies w.r.t. the common cog are proportional to $m$ and $M$, with the same factor except for having opposite signs. So the combined acceleration – and therefore also the velocity with which the distance between the objects decreases, is proportional to $M + m$.  --Lambiam 20:15, 2 January 2021 (UTC)
 * Yes I do not disagree, the physics distance isn't always the layman's distance though. And if the second object is apparently another Moon (!) what would really happen is both Moons get massive tidal forces that would close the mile gap alone if the centers were kept from falling by magic and the Moon had a magic stretching limit of "almost disintegration" (as the Moon is already stretched to that scale by Earth and it's nowhere near the Roche limit). As a mile is way, way, way under the Roche limit the almost weightless rock right under the center will be net pulled from the deeper rock. Halfway down will have about (1 moon gravity divided by 1.5 squared) up minus 1/(2 cubed) down as the gravity of anything higher depth than you but inside your spherical body cancels out which nets to about 0.32 Moon gravities up (stuff falls up now) the Moon centers will have about a Moon gravity divided by (2 squared) pointing up from the far Moon and no gravity (net) from the near Moon so a quarter Moon gravity pointing to the far Moon and the far part will have the normal gravity plus 1 over 3 Moon radii from the center squared this time so a net gravity of 10/9ths normal pointing down. As a cube of pure concrete over c. half kilometer breaks off if you pick it up by the entire roof (Earth gravity) and rocks smaller than Moon flow and crack into balls with 2 or 3 percent of an Earth gravity large chunks of the Moon cannot withstand even these gravity differentials and they will morph into egg-shapes with cracks surrounding some parts entirely probably. The Moon atoms now freed from having to behave like they're connected to the center can now fall however the local gravity wants them to so the middle atoms will fall like they're 1,080 miles up and the near atoms will fall like they're 1 mile up ignoring the center of mass now and both eggs will mash into a single sphere with the far parts falling the furthest - hitting at the speed of tank-piercing (melting too I think) tank rounds or more. Teleporting a cloned Moon 1 mile or even 1 millimeter above the current one will release massive amounts of energy. Sagittarian Milky Way (talk) 23:14, 2 January 2021 (UTC)

Could you, please, explain what the 'center of gravity' is here? Normally we define it for relatively small rigid objects in a given gravitational field as a point, around which an effective torque from gravitational forces vanishes. When calculated for a uniform field, it becomes the center of mass. However, for a system of free bodies like two planets, any force acting on one body has no effect on another body/bodies, hence no torque is tranferred and the whole definition becomes useless. As a result, a 'center of gravity' seems undefined. Do you possibly mean a 'center of mass' instead...? --CiaPan (talk) 20:38, 2 January 2021 (UTC)
 * Obviously, the average location of the two centers of mass. Edit: Don't forget to weight. Or the place they would balance if you made a small scale model of the 2 objects with everything the right density and connected vacuum parts of the line between centers of mass with an infinitely rigid and massless magic rod. Sagittarian Milky Way (talk) 21:19, 2 January 2021 (UTC)
 * I should probably have used the modern term "centre of mass". The centre of mass of a spherical body (assuming it is formed by a succession of homogeneous layers, which is approximately true for planets) is its geometric centre. Given a collection of bodies (not necessarily spherical), each with its own centre of mass, the centre of mass of the whole collection is the weighted average of the individual centres of mass, where the weight of a body is proportional to its mass. So for two bodies that are $400$ units apart, where the more massive body has a weight of $79$ times that of the lesser one, the joint centre of mass is $5$ units away from that of the more massive body and $395$ from that of the lesser body. (Check: $5 + 395 = 400$, and $5 : 395 = 1 : 79$.) --Lambiam 00:16, 3 January 2021 (UTC)
 * Yes weighted, the Earth-me center isn't going to be in the mantle. Average location of the kilos is another way to put it, there's a kilo here, there's one here and one here and continue for the rest of the Moons. Then average them all out. Or count individual milligrams or atomic mass units for small things. Sagittarian Milky Way (talk) 00:27, 3 January 2021 (UTC)
 * Individual milligrams? --Lambiam 09:37, 3 January 2021 (UTC)
 * This discussion makes me wonder if Galileo is turning over in his grave. ←Baseball Bugs What's up, Doc? carrots→ 11:31, 3 January 2021 (UTC)
 * And yet, he moves. --Khajidha (talk) 12:47, 3 January 2021 (UTC)
 * Yeah, assign each point of 3D space inside your cattle or whatever to a single compact milligram with some computer algorithm (you'll have less than 1 milligram left over per cow (if you're lucky?) which shouldn't affect things much) then average the milligrams' locations no weighting needed. Voila! The center of mass of your cattle herd. Sagittarian Milky Way (talk) 13:42, 3 January 2021 (UTC)
 * And the description of that algorithm begins with, "Consider a spherical cow..." --Lambiam 00:04, 4 January 2021 (UTC)

(I'm new to this posting mechanism so pardon if I make a mess). Thank you for the colorful visualization - remind me to not be standing on the surface of the Earth if the moon should ever decide to fall from orbit. Actually, the first line of the first response answers it : yes they fall into each other faster. I'm troubled with the notion that with "F" being much much larger due to mass(es), that the inertial impulse/moment(?) would need be larger to accelerate (each) large mass. Correction - yes I meant "center of mass" and not 'center of gravity', the latter apparently is an attribute of the 'system' (frame?) like in Center-of-momentum frame which looks to address more the GR models.

Still it's odd to me that the total (impulse integral) energy accelerating the 2 large bodies would *exactly* offset the (mutual) gravitational attraction such that a large object appears to fall at the same rate (from whatever given radii) as a small object, (observed from the surface of either body sans wind resistance et al). This is a standard explanation I find in the literature. I guess this contradicts the 'larger objects fall faster' notion. - thanks Mhandley wikip (talk) 13:45, 4 January 2021 (UTC)


 * Well, when two bodies fall at one another, then their relative speed is precisely the same despite the location of the observer (unless one of the bodies is so massive that time at its surface is significantly slower). Whether you are at the Earth or at the Moon, if they approach one another at, say, 10 kilometers per second, you will see the other object falling at you with the same 10 km/s. --CiaPan (talk) 16:59, 4 January 2021 (UTC)


 * ...Nobody has yet mentioned the virial theorem? I know we have a few physicists amongst our contributors - physicists tend to think about this problem a lot, and boy, do they ever have ways to make a confusing mess by adding one little feather to a simple moon/hammer system of equations!
 * I think the original question is specifically trying to set up a scenario where we specifically need certain sophisticated analytical methods that are a standard part of classical mechanics. This isn't an impossible problem to solve: it's just excruciatingly boring and we'll probably need a week or ten to set up the equations, and define our terms, so that we can all be sure we're actually talking about the same thing... which is why this is a standard classroom or homework problem that belongs in, say, the tenth week of an advanced classical mechanics course.
 * For the interested reader, Marion and Thornton set the problem up in Chapter 9; and again in §10.4; but since the general form is an n-body problem, it defies any analytical solution except in special cases. Quoting our esteemed authors, as they proceed toward a solution to this exact problem: "the solutions become formidable."
 * As I frequently remark on this reference desk: during formal physics education at the university level, students with exceptionally mathematical minds dedicate full-time study to this problem, and it usually requires weeks working toward a satisfactory resolution. Though we have many great minds and nigh limitless encyclopedic references at our disposal, it is highly unlikely that we can provide the trifecta summary: concise, complete, correct.  Select but two of these three elements, and we can simplify the problem very nicely.
 * Nimur (talk) 17:28, 5 January 2021 (UTC)
 * Was my solution, which does go does not go beyond standard classical mechanics, found wanting? --Lambiam 20:31, 5 January 2021 (UTC) [Missing word inserted; --L. 04:07, 6 January 2021 (UTC)]
 * I think everything you (Lambiam) wrote was correct; but, just to list out a few items that you didn't consider - from the chapters I was just reading - you did not mention net linear momentum, net angular momentum; energy; collision dynamics (elastic or inelastic); and so, while your answer is great on two points (concise and correct, at least to my reading!) it doesn't completely treat all the other fun physics that appear in a more general (but still entirely classical) treatment: gravitational potential as a scattering field; momentum exchange; all the fun nuances of orbital dynamics, rotations; extensions to n bodies; treatment of the thermodynamics of the system; extension to even larger numbers of particles to derive gas equations from particle kinematics; I mean, I won't pretend that I'm answering any of these things either - the only point I'm making here is that there is a limitless amount of complex system dynamics that can be explored, all the while only considering classically-behaving point-masses under gravity - and it only gets more complex when we add other forces, and other non-idealizations!
 * But I do appreciate that you (Lambiam) did answer with a pretty rigorous mathematical form. You did not link to the reduced mass article, though I think it might be useful to any readers who followed your work and want to see more detail.  Personally I think that your treatment is one of the best ways to introduce the cross-section of computer science with numerical physics, because your method begs for numerical implementation for n particles, using a spatial tree (like an R-tree)... which gives us an opportunity to talk about tree methods in numerical physics: under which conditions do these methods work, and under which conditions they fail catastrophically!
 * Nimur (talk) 20:54, 5 January 2021 (UTC)

Why does dandruff glow in UV (black light)?
I noticed this while in a night club. Is it roughly the same mechanism as with glowing teeth or another one? Thanks. 212.180.235.46 (talk) 20:29, 2 January 2021 (UTC)
 * Dandruff is essentially dead, dry skin. One difference between dead, dry skin and living skin is of course, water.  Water absorbs UV. - Btw, you're not the only one to notice this:    - 2603:6081:1C00:1187:35FD:8CA2:FCB4:6A8B (talk) 22:55, 2 January 2021 (UTC)
 * There are lots of materials that don't contain water and yet don't glow under UV. So perhaps there's some other cause in it. 212.180.235.46 (talk) 09:58, 3 January 2021 (UTC)
 * Such as what? ←Baseball Bugs What's up, Doc? carrots→ 11:28, 3 January 2021 (UTC)
 * Materials that absorb UV won't glow under a blacklight. Water is one of these that absorbs it, but it's not only water that absorbs it. Absorbing UV is not a sign that a material contains water.--Bumptump (talk) 14:21, 3 January 2021 (UTC)
 * Yes, but water (the lack thereof) is relevant to dandruff. 2603:6081:1C00:1187:5D59:514C:3115:6008 (talk) 19:42, 3 January 2021 (UTC)


 * There is an assumption in the question that all dandruff glows in those circumstances. Is that true? Detergents and similar cleaning substances glow under UV light. Perhaps the dandruff in question had come from a head recently washed with a shampoo that glows in UVL. Richard Avery (talk) 11:02, 4 January 2021 (UTC)
 * The Las Vegas Weekly article (linked above) implies that glowing dandruff is common. 107.15.157.44 (talk) 12:52, 4 January 2021 (UTC)