The h Bar

user116211

11:00 AM

@ACuriousMind, You remember the work function from yesterday's discussion?

user116211

As you know, $V,$ the potential energy was defined to be the negative of $U$ i.e., $V= -~U\,.$

user116211

But it is the case when $U$ doesn't depend explicitly on time $t\,.$

user116211

The relation doesn't hold for time-dependent $U\,.$

user116211

So, how should I define $V$ for $U:= U(q_1, q_2, \ldots, q_n; ~t )\,?$

Secret

ok I am running out of time, thus cross SE analysis stops here for today. But it seems that contrary to what I thought, while the phenomenon is very common across the internet as in there is always at least one example in any media, it seems that for SE, it is only concentrated enough on PSE and MSE to be able to allow me to pick them up via random sampling

user116211

11:04 AM

I dug the book but didn't find any relation between the two in the time-dependent case.

ACuriousMind

@DanielSank cc @Secret I think you massively overanalyzed what happened here. I think the question was just not particularly clearly written. I didn't have to crawl through the chat at all to understand what it really meant to ask, the comment from the OP was more than enough.

user116211

It is only written that $V=- ~U$ is true only for time-independent $U\,.$

ACuriousMind

@MAFIA36790 Who's saying that there is a $V$ for time-dependent $U$?

yuggib

@MAFIA36790 \o

user116211

@ACuriousMind ah!

user116211

11:10 AM

But @ACuriousMind, can $V$ be defined for rheonomic cases?

user116211

He writes:

user116211

> $\delta V = \mathrm dV$ would no longer be true if $V$ were rheonomic depending through $t$ explicitly.

Secret

@ACuriousMind This is one facet of what the meta post is stressing, but there is also another important thing to brought focus on by the meta post is that often (at least in MSE, PSE), when the OP tries to clarify the question, eventually the comment flow stops as the answerers stops following up. The question is then effectively ignored no matter how many edits the OP followed. In a few lucky cases, ~2 months later, there will be a new answerer to answer the question, but often when the question is

user116211

where $\delta$ denotes virtual variation.

Secret

being erroresnously treated, people stopped trying to check how the question is fixed and thus the question is effectively being abandoned

user116211

11:12 AM

I mean he introduced $V$ first through this expression $V= -~U$ without giving separate definition of $V\,.$

user116211

Now, that $U$ is time-dependent and the relation doesn't hold true, how can I then define $V$ for rheonomic/time-dependent cases?

ACuriousMind

@Secret That only happens when the follow-up comment by OP don't actually make it clearer. In this case, OP made a comment to me explaining I misunderstood, but I was asleep at the time and you were acting in the whole chat discussion as if I had ignored that comment when I hadn't even seen it.

Slow down the horses here

@MAFIA36790 Why do you want to define $V$?

user116211

@ACuriousMind Because, as he said in the above quote 'if $V$ were rheonomic', there must be a general definition of $V$ which works for rheonomic cases also.

user116211

@ACuriousMind In fact, can it be defined for rheonomic cases?

Secret

@ACuriousMind No, I never said you ignored the comment, I am saying that the situation of that questions reminds of the more widespread situation I mentioned that I saw across MSE and PSE, even when the OP does end up clarifying the question. As you have said, you closed the question not because of duplicate, but because of being unclear. Daniel later realised that you are not the one who cast the duplicate, but the general point about the phenomenon remains

So our purpose of the metapost is to raise awareness that often questions get erroreously judged because answereers and closers sometimes overlooked the crucial part of the question

and the phenomenon of abandoned questions

ACuriousMind

11:18 AM

@MAFIA36790 I don't know, I don't have these parts of mechanics memorized. But from what you quoted, the sentence "$\delta V = \mathrm{d}V$ would no longer be true if $V$ were rheonomic" could equally well be read as an argument why rheonomic $V$ doesn't make sense. That's why I asked why you want to define a $V$ - what do you need it for?

@Secret I'm not sure how people making mistakes is news, or something that needs particular "awareness".

Users are human, and they are fallible. I think everyone is aware of that. Communicating such that everyone involved understands the same thing is difficult, and often fails. What's the point?

Secret

Well, for MSE and PSE, most questions either get closed or abandoned can be treated back to this, especially when a stream of OP comments followed that apparently get no responses for days or months. (and perhaps we are wrong) but we do noticed that when people answer things in general, they often missed the crucial point in the question that is crucial to kill the question, which is why our idea to raise the awareness of this specific mistake into view.

user116211

@ACuriousMind As why I need for, see, this whole thing started why the law of conservation of energy doesn't hold true for time-dependent $U$ and whether $V=-~U$ holds true for the time-dependent cases. Then digging up the unread chapters of the book, I came across that quote above. the next line confirmed that $V=-~U$ is true only for time-independent $U\,.$

Secret

Perhaps for you it is clear, as evidenced by most of your answers (provided ithe question is clear to you) you often managed to address the crucial point (and hence answer accepted and upvoted as expected), but it seems in general often a couple of answerers failed to do this

ACuriousMind

@Secret I think what I don't get is why you think there's a "specific" mistake here. The mistake is people missing the point of the question, which doesn't seem all that specific to me.

I agree most misunderstandings that happen happen due to this, but that's because it's a really rather broad class of mistakes

user116211

As that of conservation law, I saw that the total energy doesn't remain constant when the Lagrangian $L$ does depend on time. So, that query seems to get answered as well as the second one that $V=-~U$ doesn't hold true for rheonomic cases.

ACuriousMind

11:26 AM

@MAFIA36790 When you say "doesn't hold true", you must have a definition of $V$ other than that. What is it?

Secret

I think the specific mistake is that people often tries to address only the statement that forms the question, but actually to answer the question one need to address the relevant chain of logic that lead to the question, not just the question itself, which is something that most people overlooked

ACuriousMind

I'm not convinced that's something most people overlook.

user116211

@ACuriousMind Yes, that's my question! What is the other definition?

ACuriousMind

@MAFIA36790 Why do you think there is one?

(We're going in circles here!)

user116211

@ACuriousMind yeh; I'm also confused with that ;/

ACuriousMind

11:29 AM

What reason do you have to believe that there must be a meaningful general definition of $V$?

user116211

@ACuriousMind There must be some reasonable point why he said "if $V$ were rheonomic"... What does that mean? What is the definition of $V$ when it is rheonomic? He didn't touch those queries :(

user116211

In fact the whole point of potential energy is for the conservative cases....

ACuriousMind

@MAFIA36790 Without more context I can't really tell, but I already said that that sentence sounds to me like an argument why $V$ doesn't have a good definition. In my interpretation, he's saying that $\delta V = \mathrm{d}V$ is something we want from $V$, but if $V$ (as a function, however it may be defined) would be rheonomic, it can't fulfill that, so it's pointless to try and define $V$.

user116211

@ACuriousMind "... why $V$ doesn't have a good definition."- This answers my query.

ACuriousMind

I already said the same thing here:

15 mins ago, by ACuriousMind

@MAFIA36790 I don't know, I don't have these parts of mechanics memorized. But from what you quoted, the sentence "$\delta V = \mathrm{d}V$ would no longer be true if $V$ were rheonomic" could equally well be read as an argument why rheonomic $V$ doesn't make sense. That's why I asked why you want to define a $V$ - what do you need it for?

user116211

11:34 AM

@ACuriousMind ;P

ACuriousMind

...why are you starring those?

user116211

But he could have written that straight ;(

user116211

@ACuriousMind Sorry, haven't meant the second one.

user116211

Anyways, I would conclude that $V$ is defined only for the conservative time-independent/scleronomic cases and not for the other one as it doesn't make sense to define it for the rheonomic cases.

Secret

@ACuriousMind Well, perhaps "most" is too strong of a claim, thus not necessary convincing, but our experience said we tend to see one to a few cases occasionally. Anyway we thought that the meta post may help address the cases where the aforementioned issues arises. We will let the community to decide whether our aims are actually sound, so we just sit back crossing our fingers for now...

Uh, Danielsank, you sure you got enough sleep, you just went to bed a few hours ago?

but anyway...

ACuriousMind

11:41 AM

@Secret How will it help address those cases?

Secret

When making aware to the answerers that in order to answer those questions with a logic chain structure, one need to address the crucial point instead of the stated question, the answers will become less likely to miss the point hence allowing the OP's questions to be satisfactorily answered, and avoid questions closing as duplicates or other reasons because the related question answered the stated question but not the crucial point that makes them different

It also raised an important message that answerers need to follow up on the answers when OP clarify them unless it deviates to the point of becoming a separate question

ACuriousMind

I don't think the problem is people are not aware that sometimes questions have an underlying issue that's not the literal question itself. It's that they don't always see that issue, and all the "awareness" in the world can't help with that.

@Secret No, they don't. No one is obligated to respond to anything here.

Secret

@ACuriousMind and that issue is often mentioned by the OP in the comments but people are often too tired or (insert reason) to respond to the OP's clarification and result in questions being abandoned

@ACuriousMind This is true...

ACuriousMind

@Secret The idea is that all relevant information is in the question. If I have to read the comments to understand a question, it's a bad question. If there's relevant information in the comments, edit the question. Anyone may do this, but the burden is on the asker.

Besides that, I dispute your claim that this "abandonment" happens all that often. If the follow-up comments are clear, then questions are frequently modified and reopened. However, there are many cases where the follow-up comments are just as unclear as the original post - but in this case you can't blame the commenters/close voters for not responding.

Not every follow-up comment is a clarification.

Secret

The issue is that after that, often for those cases, no one will try to ask what is still unclear about the clarification, thus OPs often just stopped there and waiting for responses

Here's an MSE example:

3

Q: Why is : $ [ \mathbb{R} [x_1 , \dots , x_n ]^{ A_n } : \mathbb{R} [x_1 , \dots , x_n ]^{ S_n } ] = [ \mathfrak{S}_n : \mathfrak{A}_n ] $?

How to show that : $$ [ \mathbb{R} [x_1 , \dots , x_n ]^{ \mathfrak{A}_n } : \mathbb{R} [x_1 , \dots , x_n ]^{ \mathfrak{S}_n } ]= [ \mathfrak{S}_n : \mathfrak{A}_n ] $$ such that : $ \mathfrak{S}_n $ : the symmetric group $ \mathfrak{A}_n $ : the alternating subgroup of $ \mathfrak{S}_n $. ...

But, $ \mathbb{R} [x_1 , \dots , x_n] $ is not a field, so we cannot use : $ [K \ : \ K^{ \Gamma} ] = | \Gamma | $, right ? — YoYo Sep 16 at 18:44

although you are right he/she should include that clarification in the edit and not rely on comments

ACuriousMind

11:52 AM

@Secret That question isn't closed, and has three upvotes. I don't see what it has to do with the discussion.

Secret

The question is 9 days ago, and the answerer Lubin, stopped responsing to Yoyo's clarification since Sep 16. That's what I mean by "abandoned"

The tricky thing is it does make use of your point that "No one is obligated to respond to anything here."

ACuriousMind

@Secret Lubin is not an answerer since that question doesn't have an answer. They presumably wrote only a comment precisely because they didn't want to invest the time to write an actual answer.

Why they didn't want to invest that time is nothing you could know, which is why I don't see what this has to do with the reason of "missing a node" that we were discussing.

Secret

ok, sorry, in that case, Lubin is someone who comment about the question, not an answerer (apologies for terminology). Well, Yoyo need that node he/she said to be addressed in order to consider the question itself addressed, at least that's what I think, perhaps it is wrong to judge what they are thinking based on limited info

However you do raised a point that DavidZ mentioned before regaridng comments and answer:

@ACuriousMind People should avoid answering questions in comments

ACuriousMind

@Secret Sure. But that comment isn't an answer, it's a hint at a general principle that will lead to the answer after applying unknown amounts of work.

If someone posted that comment as an answer, I would downvote it for not actually answering the question.

Secret

yup, in that case, the above MSE might be a poor example to illustrate the phenomenon

ACuriousMind

12:01 PM

Anyway, after all this discussion, I'm still not clear on what exactly your "abandonment" is, and what the meta post is supposed to do about it.

Which might be a sign that either I suck at reading comprehension or you need to make that a lot clearer in the post itself.

Secret

@ACuriousMind Well it seems there are 3 things: 1. People tend to have a habit of using comments as a partial answer to a question, *2. OP is confused and tend to ask these people for clarification, 3. After some time of back and forth commenting these comenters stopped responding and no new comments followed.:: Are we agree on the above obsevation?

and of course there is 3a: OP often fail to include the clarification in the comments in the actual question edit

ACuriousMind

@Secret Yes

I don't see what that has to do with the meta post, but yes.

Secret

Point *2 is where OP often state what the missing node is, and point 3a is a common thing that OP fail to do. Are we agree on these deductions?

ACuriousMind

Those are not deductions, because you haven't deduced them from anything, but yes.

Secret

Ok and finally: Point 3. and 3a. are the reasons why this issue is often left unresolved, and lead to question abandonment for those cases

ACuriousMind

12:09 PM

Also, it's "Do we agree", not "Are we agree".

@Secret Yes

Secret

Hence, the meta post is raising awareness of point 3, in response to point *2

ACuriousMind

I don't see what this has to do with the meta post though, as there your prime example is a question where, as I said, you didn't even give the commenter enough time to respond to even the first clarifying comment from OP

This pattern of commenters getting tired of OPs moving the goalposts in comments is completely disjoint from commenters not understanding the crucial point of a question. In this case, I admit that I simply misunderstood the question as written, but I don't see what this has to do with the partial answer-question-partial answer-question-silence pattern you're now talking about.

Secret

Ok I think it is like this (Sorry I have to make the format into a list) :
1. Question written
Issue 1:
2. Some answers, partial answers given which missed the crucial point
3. (Optional) Question get closed because of unclear, or duplicate (based on the partial answers) and other reasons
4. (Optional) OP edit the question
5. OP clarify question in comments
6. Commenters address the comments
7. (Optional) OP edit the question based on comments
8a. OP clarification is clear, question is saved from closing etc. and answered

So you are right there are actually two separate issues here, in that case, the meta post should be geared towards one of them to be more focused

which one will it be?

By the way, since we have use the term "partial answer" to refer to the commenters, the MSE post is suitable to illustrate Issue 2

In that case, I think 4 days is a unreasonably long time to respond to the clarification

So my point is, while Issue 1 and 2 are technically separated, they tend to occur together cc @DanielSank for more clarification details

@BernardMeurer @0celo7 can you help me test something? Can you screencap my previous message block. I want to check how it looks like on your computer

just screencap everything surrounded by this large grey block is ok

ACuriousMind

12:34 PM

@Secret Okay, so yes, these are two patterns that happen. Why do you think there needs to be something done about them?

0celo7

Why am I doing this

Secret

@ACuriousMind Well, let's focus on PSE (although it is also common in MSE) because quite a lot of "marked as duplicate" is wrongly attributed because of Issue 1 as noted by Danielsank, and the situation is sometimes made worse by Issue 2

@0celo7 I want to check how linebroken messages look like on you guy's comps, in order to test something

ACuriousMind

@Secret You've lost me again, I'm afraid. Let's drop this for now and see if Daniel can explain this to me in another way later on.

Secret

Sure, he can explain it and the meta post better cc @DanielSank

Bernard Meurer

@Secret i.imgur.com/5e4udf4.png

Secret

12:40 PM

@BernardMeurer ok that looks fine, and not some weird jigsaw puzzle like mess, thanks

0celo7

@ACuriousMind As of right now I don't think the proof will happen...there's a line in Petersen that makes zero sense. My advisor asked me "what does that even mean" and I had no answer :/

Bernard Meurer

@Secret No worries :)

0celo7

12:58 PM

@ACuriousMind It says: show that the evaluation map $Iso(M,g)\ni f\mapsto (f(p),df_p)$ is continuous. Note that $df_p:T_pM\to TM$ is linear so that convergence of the values of the evaluation map makes sense.

Problem is, it doesn't make sense.

Maybe if we give $TM^{T_pM}$ the compact-open topology, it does.

@ACuriousMind Any smart ideas?

user228700

1:35 PM

@JohnRennie: Hello again :-) I have a quick question. Are u free?

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