Wikipedia:Reference desk/Archives/Science/2022 August 2

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August 2

Momentum vs. acceleration: baseball situation.

So I will continue this discussion with Nimur in a new question: what hits a baseball harder? A baseball comes at you at 25 m/s (momentum) but you swing a baseball bat at 25 m/s^2 (acceleration) (force), or, a baseball comes at you at 25 m/s^2 (acceleration) (force) and you swing the bat at 25 m/s (momentum)? Or would the ball go the same distance? 67.165.185.178 (talk) 04:48, 2 August 2022 (UTC).

There is not enough information to give a meaningful answer. The bat could have any velocity with respect to the ball. The accelerations are irrelevant.  --Lambiam 08:43, 2 August 2022 (UTC)

Any acceleration that is ongoing at the moment of contact implies an additional contributory force to the elastic impact. A baseball accelerates towards the hitter only while in the hand of the pitcher who is supposed to let go of it. Anything else would dash it all just not be cricket. Philvoids (talk) 10:21, 2 August 2022 (UTC)

It could be due to the force of gravity. Whether within the confines of fair play or not, its value remains irrelevant.  --Lambiam 17:53, 2 August 2022 (UTC)

There are several problems with your question which make the problem unsolvable. Some of them are:

• 25 m/s is a velocity, not a momentum;
• the acceleration of a bat says nothing about its velocity at the moment of collision;
• a ball can't 'come at you with a given acceleration' unless you provide some source of a constant force to accelerate the ball (see Newton's laws).

--CiaPan (talk) 19:52, 2 August 2022 (UTC)

Ah, I changed the word acceleration to force, to stimulate this is momentum vs. force problem. 67.165.185.178 (talk) 22:42, 2 August 2022 (UTC).

That makes it even worse. To analyse the collision between baseball and bat, you need the velocities and masses of both. Alternatively, you could use momentum instead of velocity and/or energy instead of mass, as that gives sufficient information to calculate the others. In SI units, velocity is in m/s, momentum in kg·m/s, mass in kg and energy in J. PiusImpavidus (talk) 08:57, 3 August 2022 (UTC)

I meant this is a "mv" vs "ma" problem than just a "v vs a" problem. Not going to criss-cross (mv vs. a or ma vs. v). Guys, I don't see how this is tough. I once asked a physics professor, what hits a baseball farther: a stationary baseball, or 1 coming towards you? He said 1 coming towards you. 1st, we did it as a velocity/momentum problem, and we got the opposite answer, that is, hitting a stationary baseball hits farther. Then he did the problem again using the acceleration-force model, and got the correct answer. But now, I want to criss-cross, what if we hit the baseball via a velocity-or-momentum to a baseball that is a acceleration-or-force and vice versa. 67.165.185.178 (talk) 23:00, 3 August 2022 (UTC).

Our earlier responders have done a great job expressing the problems - the question isn't stated with enough "formality" to really get a "physics" answer - at least, not the kind you're hoping for, with a clear "Option A hits the ball harder" type of answer. As one example of this - we haven't defined what a "hard hit" is. Let me make a really clear example: what if the ball hits the wooden bat so hard that it completely crushes the wood into a pulpy mess and embeds in the middle of the bat, and does not bounce off at all? We have a formal term for this: it's a perfectly inelastic collision. By one plausible definition, this is the hardest possible hit: the maximum amount of kinetic energy is lost in the collision. But what if the baseball hits ... like "even harder," dude ... so hard that the ball and that the bat dissolve into puffs of powdery wood-pulpy dust? Technically the dust carries away kinetic energy, but tell me honestly - which "collision" was the "harder hit"? We need "formal definitions" for our terms: what does "hardness" mean in physics? (Hint: not what you think!)

This really revolves around something I mentioned earlier: which simplified model works for you?

The thing is, we're not trying to weasel out of the answer with technicalities of definitions (cf. hardness - it's a property of the material in the bat, and definitely not a property of how you swing it - so in context, there's no such thing as a hard hit, only a hard bat!) It's just that - by studying lots of problems, we've sussed out that these are the formalisms that you must adopt if you want meaningful, analytical answers to "real" problems. That's why we have textbook-problems training you in the common methods, preparing your mind using the easy versions of the hard problems. And when we try to translate technobabble back in to plain English - where words are used fast-and-loose - we frequently have to do some wrangling with imprecise language.

Answering the question - what hits the ball farther? "We don't have enough information," but in a realistic simplification, the ball goes farthest if it leaves the bat with maximum velocity. That's almost always going to happen when the ball hits a soft bat - both do a bit of squishing, both spring back to their original shape, and this lets them remain in contact for a long time, so the ball gets pushed along for more microseconds. In the real world, though, the really real most significant factor is how perfectly the batter can place the bat, relative to the pitch - which is actually a question of athletic ability, plus some psychology (guessing what the pitcher will throw, allowing better early preparation), modulated by probabilities that relate not only to each individual (the batter and the pitcher), but also relating to every earlier and later at-bat matchup, and the weather, locale, last week's weather, what they ate last night, ... (all making for one of the most studied issues of applied statistics in sports: baseball statistics).

If we choose to ignore these confounding factors, and fixate only on the interaction of bat-and -ball, we really do ourselves a disservice: we're using a simplified model that throws away the absolutely most relevant information in the problem. Are we really being analytical and rigorous, at that point, or are we just using algebra because that's how to "do it right?"

"But it doesn't work. No airplanes land. So I call these things cargo cult science, because they follow all the apparent precepts and forms of scientific investigation, but they're missing something essential, because the planes don't land."

The Scientific Method, when performed correctly, has predictive power. So - if any one of us has a hypothesis about what method will hit the ball farther, we can test that hypothesis, and I believe we will discover (rather we will confirm what others already know): the player's ability is the dominant factor, and the physical kinetics are actually subservient to that, ranking far lower in the contribution to the distance that the ball travels. But it's all muddied with details. Some players regularly hit the ball farther, but less often, and strike out more often. Other players hit the ball more often, but the mean distance traveled for each hit is shorter. How are we measuring distance in the ensemble?

Nimur (talk) 15:01, 3 August 2022 (UTC)

The size of the Milky Way Galaxy

Okay, so I think this will be a very controversial topic, but I'll try my best to elaborate this problem as clearly as I could.

A little bit of context first. This began when I was verifying the size of the Andromeda Galaxy, where the value of 220,000 light-years for its diameter has become the norm for the past 10 years or so. The reference or this size comes from this paper by Chapman et al from 2006, with the sentence on the section Andromeda Galaxy#Structure (before my edit on this section), has this paragraph in it:

In 2005, astronomers used the Keck telescopes to show that the tenuous sprinkle of stars extending outward from the galaxy is actually part of the main disk itself. This means that the spiral disk of stars in the Andromeda Galaxy is three times larger in diameter than previously estimated. This constitutes evidence that there is a vast, extended stellar disk that makes the galaxy more than 220,000 light-years (67 kiloparsecs) in diameter. Previously, estimates of the Andromeda Galaxy's size ranged from 70,000 to 120,000 light-years (21 to 37 kpc) across.

Except you will never find this in the paper. By examining the abstract and the subsequent press release, it seems that they are even going against this idea. The press release document even says this:

In addition to being metal-poor, the stars of the halo follow random orbits and are not in rotation. By contrast, the stars of Andromeda's visible disk are rotating at speeds upwards of 200 kilometers per second.

This is not exactly suggesting that the diffuse halo of stars is a part of the main disk. More so that does this suggest that the halo defines the extent of Andromeda. This is simply not how you measure a galaxy's size. I first noted this at Talk:Andromeda Galaxy#Let's talk about Andromeda's size for a bit, and changed it primarily because this can be easily corrected, being a misinterpretation of the original reference and what it meant.

The Milky Way, though, is far more trickier than this.

Now, for a deeper context here. I have been working right now on the List of largest galaxies so I am fairly certain that I have some idea about this stuff. A quick search on the astronomical literature tells that galaxies are measured through any of the following methods:

• Isophotal diameter (D₂₅, D_26.5 (Holmberg) or any variation of it, see here)
• Scale lengths (usually for spirals and disk galaxies; logarithmic brightness 2.512 radius)
• Fractional light radius (effective radius r₅₀ (50% of light emitted); r₆₀ all the way up to r₉₀, which is used by the ESO-Uppsala in 1989 in here; see here for context)
• Petrosian radius (used by SDSS, see this, which is very dense)
• Infrared apertures and isophotes (used by 2MASS specifically for infrared, see this paper by Jarett et al, again a very dense paper)

That's all I could find. I might have missed some more, but I'll proceed for the Milky Way galaxy.

The Milky Way as of now has a diameter at 170,000-200,000 light-years. Looking at the reference for this size was this press release in 2015, which centers around the Monoceros Ring, that gave this claim:

Importantly, the findings show that the features previously identified as rings are actually part of the galactic disk, extending the known width of the Milky Way from 100,000 light years across to 150,000 light years, said Yan Xu, a scientist at the National Astronomical Observatories of China (which is part of the Chinese Academy of Science in Beijing), former visiting scientist at Rensselaer, and lead author of the paper.

But here's the thing. This is not how you define the size of a galaxy.

Let's have some context. The Monoceros Ring is a very sparse object, a stellar stream from the galactic thin disk. From this paper from 2018 supports the conclusion that the Monoceros Ring is a structure kicked out from the main stellar disk. Precisely, "M giants in Mon/GASS and A13 have a low velocity dispersion and display a gradient as a function of Galactic longitude (Li et al. 2017), whereas we find the velocity dispersion of the RR Lyrae stars in both Mon/GASS and A13 is much higher and is consistent with halo membership." (text bolded for emphasis)

From what I can understand in the paper, it makes the conclusion that the Ring is not of intergalactic origin (caused by an accreted galaxy), but its origin was from the Milky Way disk. It was not saying that the Ring is still a part of that disk, and certainly nowhere saying that the Ring should be the delimitation of the Milky Way as a whole.

So why am I saying this to you? At this moment the Milky Way has a diameter figure (170,000 to 200,000 light-years) that is larger than Andromeda (which I winded down to 46.56 kiloparsecs (152,000 light-years) per the measurement by RC3 in 1991, using 25 mag/arcsec²; you can also find this figure is you search for Andromeda in the NASA/IPAC Database). We should define the diameters of galaxies using a common surface brightness depth, or at least one that is accepted. The methods I've enumerated above are the ones that large-scale surveys use to define the physical diameters of galaxies for decades, and are used both by NASA/IPAC Database and HyperLEDA. They even gone so far as to emphasize that galaxies should be measured by the isophotal diameter D₂₅. It would just be inconsistent for the Milky Way to have a looser definition while we have a different standard for other galaxies.

So, what is the size of the Milky Way then, using any of those methods above? Well, here is a 1998 paper that answers exactly just that, which gave a D₂₅ isophotal diameter for the Milky Way at 26.8 ± 1.1 kiloparsecs (87,400 ± 3,590 light-years). This is a much more consistent measurement and places it on the rightful comparative size with Andromeda, although it should be noted that this is an old paper that may be using a different cosmological parameter, but still.

So, finally, my question is, should we use this figure as a diameter of the Milky Way, or should it just be added in the infobox and paragraphs of the article, or maybe I am just quite stupid and I lack some context here? Maybe there is a good reason to use the galactic halo and the sparse Monoceros Ring as the definition of the Milky Way's size? It should be noted that I will nt accept the diameter of 220,000 ly for Andromeda, though, as this is clealy a misinterpretive statement. Thoughts? SkyFlubbler (talk) 18:24, 2 August 2022 (UTC)

If the ultimate question is "what should we be using in Wikipedia for these values," I think the answer has to be "what is the most commonly used value?" Granted that "size of a galaxy" isn't necessarily an unambiguous concept, we nonetheless need to steer well clear of original research. That probably means NOT picking and choosing individual articles, but rather looking for something like this survey article in ARAA. Note that I don't actually have ACCESS to that article, so I don't know what values it quotes. PianoDan (talk) 21:05, 2 August 2022 (UTC)

The survey article by van der Kruit and Freeman does not discuss "sizes" of galaxy disks in that terminology, but mentions a concept of "truncation":

"Truncations in stellar disks were first found in edge-on galaxies, where the remarkable feature was noted that the radial extent did not grow with deeper photographic exposures".

It would seem that in laypeople's terms the diameter is twice the radius out from the centre at which truncation occurs. In that case, the following is relevant:

"The truncation in the stellar disk in our Galaxy has also been identified using star counts in a number of surveys (Robin, Crézé & Mohan 1992; Ruphy et al. 1996) to occur at a galactocentric radius of 14 to 15 kpc."

At one spot the article mentions "face-on effective radii of 5.0–7.5 kpc" of galaxies that are "regular and large in the near-IR", which is said to be "comparable to the Milky Way". It is not clear to me how a "face-on effective radius" relates to truncation, and over what range the authors may consider sizes to be "comparable".

 --Lambiam 10:12, 3 August 2022 (UTC)

The effective radius is the half-light radius from de Vaucouleur's law. It's useful for comparing galaxies, but it does not tell you the maximum extent of a galaxy. The truncation radius would do that, if only for the stellar disk. --Wrongfilter (talk) 10:47, 3 August 2022 (UTC)

The concept of the "effective radius" or r₅₀ was used primarily in the context of estimating the stellar mass of a galaxy, but it is rarely used as a physical diameter. It is the way used primarily in ellipticals with a spheroidal profile (spirals and disk galaxies use scale lengths). Physical diameters are mostly measured using the aformentioned D₂₅ isophote. For spirals we could use the truncation radius though I never actually researched this topic before. SkyFlubbler (talk) 16:01, 5 August 2022 (UTC)

• The ultimate problem is that you're asking for the size of a thing that doesn't have clearly definable borders. --Jayron32 11:02, 3 August 2022 (UTC)
• Just a note, I'd urge very strong caution about editing an article to change something cited to a paper if you don't have access to the paper and are only going by the abstract or press releases, media reports etc about it. I strongly suggest you use WP:Resource exchange to obtain access to the paper or tag it as requiring verification or something. (Although this paper is on Arxiv so I'm not sure why you couldn't just read the paper instead of PRs or only the abstract.) Even if you're right about the statement being unsupported, you have to ensure whatever you change it do is supported by the paper or if you remove the ref, you might be removing a ref which is useful but our text simply needs to be changed. Nil Einne (talk) 11:37, 3 August 2022 (UTC)

  It's also worth saying that a single paper does not really mean much, except to say that a single paper has made some statement. Lots more gets published than ends up being useful; if a set of astronomers publishes a paper that defines a certain size for a galaxy, there is nothing that says whatever conventions they used to define that size is widely accepted; it may be idiosyncratic to the specific study in question. --Jayron32 18:43, 3 August 2022 (UTC)

  This applies perhaps less to a "single" paper surveying what is known about a topic, citing 497 other peer-reviewed papers.  --Lambiam 07:02, 4 August 2022 (UTC)

  That's a horse of a different color. --Jayron32 13:13, 4 August 2022 (UTC)

  Yes, I will do it next time, my bad. I tried to check it in the DOI but the only piece in it was the abstract, and did not saw the damned "View article" button. Anyway, I did not remove the reference as it was clearly a high level study by Keck, and a read in the paper still picks up my point and did not compromise the main article. SkyFlubbler (talk) 16:18, 5 August 2022 (UTC)

• @SkyFlubbler: I think that does make sense, as I also watched the recent and more consistent edits regarding diameters for galaxies, which were once pretty very messy, misleading, and inconsistent to being with, and some of them as far as having been made up entirely. The topic of Milky Way disk's size being much larger than previously thought at 150,000 – 180,000 ly across in 2015 was due to the more distant stellar overdensities such as Monoceros Ring, A13, and TriAnd overdensities that might be part of the Milky Way Galaxy, which was kinda ruled out by that said 2018 paper, stating instead that they were kicked out from the galactic disc. Despite that, there's seperate reasons why 2018 press releases then stated that Milky Way might be even larger at 170,000 – 200,000 ly or above, to which it independantly came from a different 2018 paper, which stated that there's disk stars with a very high probability to be present in a distance of 26–31.5 kpc (84,800–103,000 ly) (hence it might represent the radius of the Milky Way as a galactic disc according to some of these websites) or even farther from the Galactic Center, and maybe part of the stellar disk. Surely, they likely didn't know about the methods to properly define the size of a galaxy, and as well takes informations that originated from Wikipedia, with several of them being even incorrect, including the Andromeda Galaxy's diameter being mistated to be 220,000 ly. It was as well stated in many papers that Milky Way's surrounding faint HI disk is well−defined to be as large as 35 kpc (114,000 ly) in radius (much higher than its isophotal radius), though may probably not be considered part of the main galaxy, just like Malin 1, which was once regarded to be the largest spiral galaxy by quoting the HI diameter while neglecting its much smaller isophotal and visible diameter.

As a matter of fact, you can consider me to be another user who independantly found that 1998 paper's 26.8 kpc isophotal diameter for Milky Way, but just in very late July (as I was also supposed to return in July this year), and I was to mention it in the Milky Way page. Even despite being old, there's some recent papers that referenced the 26.8 kpc as the size for Milky Way. Sadly, I cannot find any other and more recent isophotal diameter for Milky Way, or any size based on these alternate methods. However, I found that there's at least one recent paper that gave somehow the size for Milky Way about 32.4 kiloparsecs (106,000 light-years) accross, though I cannot find neither which method this value came from (hence maybe even worse; could be a guesstimate instead of a proper estimate based on reliable calculation), thus I cannot quote it on the Milky Way page whatsover.

Also, the popular illustration of Milky Way that was made in 2010 appeared to extends up to 120,000 ly across, though it is rather now obsolete comparing to newer and more accurate illustrations that were made since 2020, which one made in 2020 (considered to be the most accurate MW visualization by this time) appear slightly smaller at 105,000 ly (Sun's distance to the center being 8.15 kpc, and not sure what happened to the "New Outer Arm"), and another one made later showing the MW being brighter within the 80,000 ly diameter while fainter beyond it. I may also try to upload them on Wikipedia, but need to first avoid copyright violations. Anyway, I decided to change the size of the Milky Way to the 26.8 kpc isopotal diameter until further notice, if I can find any recent galactic (isophotal) diameter (or maybe based on any other reliable methods). ZaperaWiki44^{(✉/Contribs)} 12:44, 9 August 2022 (UTC)

References

Old difficult to pin-point acidosis source

In 2013, at talk:Acidosis, someone complained about a difficulty to find a reference. The reference was added in 2005 as "Needham, A. 2004. Comparative and Environmental Physiology Acidosis and ALkalosis" by a long inactive and not too prolific user, it looks valid and good faith, but poorly formatted and I also didn't manage to track it down. It was cited in a similar form in a couple of later hits in google scholar [1] (other hits can be found, not sure how relevant or promising). The date of addition suggests the date of pubication is legit, "Advances in comparative and environmental physiology" is a quite popular book series that doesn't seeem to have been published in 2004, I considered the possibility of Needham being the place of publication, but that seems unlikely because of the short citation in the same edit. There is at least one Needham, A. and one Needham, A. E. (who may be the same person) active in the field of biology and in 2004 [2]. 109.119.250.133 (talk) 22:26, 2 August 2022 (UTC)

Arthur Edwin Needham, Lecturer in Zoology at Oxford, died 6 November 1993.^[3]  --Lambiam 09:01, 3 August 2022 (UTC)