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Today I saw that this question Applying energy and momentum conservation to the problem of pulling a bent carpet at a constant speed about pulling a bent carpet was closed for being a homework question. I think this was an incorrect call that illustrates a flaw with the homework policy and the way people vote to close.

The linked question is a subtle mechanics question, where the answer depends on the precise way one models a continuously deforming object. Naive application of conservation laws will give contradictory results. The question is elegant and the setup is nice. Despite this, the question is closed with 4 downvotes.

Now, I agree it's important to protect the site from the constant deluge of low-quality questions copy-pasted from introductory textbooks, whose solutions amount to just plugging numbers into standard formulas. But this question isn't remotely one of them! There is real conceptual content here, more than in most questions about quantum field theory. The question text explicitly asks about this conceptual content; the OP has already done the boring calculations for us.

Based on this, I have no idea why this question should have been closed. I don't want to be too rude, but I suspect that experienced close voters are letting themselves slip into an autopilot mode based on cheap heuristics. They see that the question is about mechanics, and that common mechanics equations are involved, and so in less than 5 seconds conclude the question must be trivial without really understanding it. I would daresay that the majority of the close voters would be unable to get the correct answer for this problem on a first try.

Does anybody stand by the closure of this question? If you do think this question should be closed, why not automatically close every mechanics problem containing an equation? Is that really what the homework policy is meant to achieve?

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    $\begingroup$ To add to the reasons why this question fails the 'autopiot test', the first title was just the name of the book the question's from, and there are abundant screenshots of the question. And there were the sour posts by the OP on meta and in the H bar. (Of course, all this has nothing to do with whether the question is actually on-topic or not) $\endgroup$
    – user191954
    Feb 19, 2019 at 16:07
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    $\begingroup$ @Chair But the sour posts are an effect, not a cause. I certainly think the OP is entitled to be sour! $\endgroup$
    – knzhou
    Feb 19, 2019 at 16:08
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    $\begingroup$ @Chair A screenshot in itself isn't a bad thing. The majority of the question, at the time it was closed, is text written by OP describing their own approach. A screenshot is a heuristic for detecting low effort, but the question was clearly not low effort. $\endgroup$
    – knzhou
    Feb 19, 2019 at 16:09
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    $\begingroup$ Well, while the OP did a lot of work, the core of the question is still 'am I right?', and further the OP's replies to the several answers is to argue that they are incorrect. I'm not convinced that it isn't covered by our homework policy. You disagree, so perhaps we can agree it is at least in a grey area. $\endgroup$
    – Jon Custer
    Feb 19, 2019 at 17:34
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    $\begingroup$ @JonCuster Is that not grounds to close any mechanics question where the OP makes an effort? Surely, if the OP has made an attempt, a completely natural question for them to ask would be whether it is correct. That's what I do whenever I ask a question. The question is a conceptual one, about whether their solution or the official one has modeled the system incorrectly. $\endgroup$
    – knzhou
    Feb 19, 2019 at 17:51
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    $\begingroup$ @JonCuster What I'm seeing by browsing the tag is that the only questions that aren't closed are the ones that contain no equations, because the OP is too confused or too lazy to write any. These are precisely the worst questions of the bunch, but they don't trip the "mechanics + equations = homework" heuristic, so they survive. It's just backwards. $\endgroup$
    – knzhou
    Feb 19, 2019 at 19:03
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    $\begingroup$ It's literally a check my work question, which was rejected by the community long time ago, cf. q/6093. $\endgroup$
    – Kyle Kanos
    Feb 19, 2019 at 20:57
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    $\begingroup$ @KyleKanos Under that criterion, can you give an example of a recent mechanics problem with equations in it you don't think should be closed? $\endgroup$
    – knzhou
    Feb 19, 2019 at 20:59
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    $\begingroup$ @KyleKanos We all know this the policy stated in that question is not actually enforced. QFT questions of that exact type are asked every day, and I've never seen any of them closed. $\endgroup$
    – knzhou
    Feb 19, 2019 at 21:00
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    $\begingroup$ @knzhou no I cannot bc I am at work and don't have the time to scroll through my close history--it is public, so feel free to scroll through it though. I generally don't vote of QFT because I never studied it, but if they are of the type "did I do this right" it fails the above policy on check my work and should be closed. $\endgroup$
    – Kyle Kanos
    Feb 19, 2019 at 21:05
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    $\begingroup$ @knzhou For another point of anecdotal data to go along with Kyle's, I also generally don't vote on QFT unless it is obvious, because I know little about it. That extends to a lot of non-newtonian physics questions for me. I can generally only vote on things if I understand the question, so it takes quite blatant off-topicness for me to VTC any of those questions. $\endgroup$
    – JMac
    Feb 19, 2019 at 21:24
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    $\begingroup$ @JMac I do the exact same thing: I only vote on a question if I fully understand it, which means in practice that I skip over 70% of the questions in the close queue. In fact, I skip over Newtonian mechanics questions all the time. Just because we understand Newtonian mechanics does not mean we automatically understand all questions about it -- there is a lot of subtlety there, enough to have kept classical physicists busy for centuries. I'm just saying that a subtle question deserves a more careful review. OP explicitly argued that their question was subtle. They did their job, we do ours. $\endgroup$
    – knzhou
    Feb 20, 2019 at 2:11
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    $\begingroup$ @NorbertSchuch But what is the line between a problem and a question? I would say a problem is just a specifically formulated question. Good "tricky" Olympiad problems are tricky precisely because they have something new and conceptual at their core. $\endgroup$
    – knzhou
    Feb 22, 2019 at 12:13
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    $\begingroup$ @knzhou I agree this is tricky, but it is not clear how to ask a good conceptual question here, except for asking the problem, or posing the problem and asking "what is the conceptual difficulty here". This is not a conceptual question. Or one poses problem and answer in one package. As I said, I wouldn't mind at all having such tricky problems here, but I'm not sure how to clearly separate them from homework problems -- in the end, they are much like them, just that it is considerably trickier to find the right trick. $\endgroup$
    – Norbert Schuch
    Feb 22, 2019 at 15:38
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    $\begingroup$ @alephzero No, it really is more subtle than that. The answer that is correct depends on how the carpet is modelled microscopically. This is explained very nicely in section 5.8 of Morin's Classical Mechanics, which devotes a whole five pages to this issue. But even if you still think it's trivial and obvious: note that I and a few other high-reps users who posted on the main question are disagreeing with you. Doesn't the very existence of such disagreement imply the question's subtle? $\endgroup$
    – knzhou
    Feb 22, 2019 at 21:06

2 Answers 2

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The first problem I can see is the presentation of this question.

It could be far better organized to get across what the question itself is trying to address. At first, after reading most of the question, it appears as though OP is just trying to work us through what they believe to be a good solution to a common problem. It's only at the end that they show another example from a textbook which they are actually running into the problem with. That could definitely be made more explicit upfront.

Also, this question is providing a lot of information; but it isn't clear on the initial read what the important parts of the question are. I think this is it's primary flaw, and honestly, I can't say I would personally vote to reopen in it's current state (though I likely wouldn't close it either). To quote from our page regarding check my work questions:

The only kind of "check my work" I think we should allow is the one where a derivation is presented, leading to a wrong result, and the question is "It seems as if step X is wrong? But it should be right because of Y, so why is this not the case?". There must be a reasonable explanation (by established physics, of course) of why the derivation is expected to work in the eye of the asker, and then the answer pointing out the flaw in the reasoning can actually be useful, since the question is then essentially "Why is the physical principle Y not applicable here?" The question should also be edited to reflect that.

I believe that what OP is trying to ask could fundamentally fit into this point for one of the few types of acceptable "check my work" questions. That said, in it's current state, it doesn't seem to be focused on any conceptual aspect of the problem, but is instead presenting several entire solutions and looking for us to contrast them in their entirety. It's not really focused on any conceptual question.

If OP wishes to organize his thoughts a bit better and maybe point out where in these derivations we should be comparing, and the theoretical problems with the other answer, it would be perfectly on topic. As is, I would personally say it could use a bit of work before reopening. The question could definitely be improved to fit within current policies before being reopened; but the core of the question could still be salvaged (and some people may find it good enough already).

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    $\begingroup$ I thought the OP went out of their way to be clear about the issue: the official solution models the bending of the carpet as sharp, while they model it as smooth. And this leads to different results with respect to conservation of energy and momentum. (Note that the organization was actually better before the question was closed. They made it less explicit because they were told "problem is from a book = homework = closed" in comments and chat.) $\endgroup$
    – knzhou
    Feb 19, 2019 at 19:24
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    $\begingroup$ I am also rather bewildered about the "check my work" policy. It sounds extremely vague. Under that reasoning, the majority of my questions should be closed, because I do present work, but they are not. $\endgroup$
    – knzhou
    Feb 19, 2019 at 19:25
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    $\begingroup$ @knzhou That part was quite unclear to me, because that specific detail stands out no more than all the other details they included in the lengthy question. It's really not clear to me what the conceptual implications of that one factor are; which would make the question far more inline with current policies. Your "check my work" questions clearly show the applicable concepts and the reasoning for what you believe the issue to be, and it's presented in a very comprehensible order. The problem here isn't necessarily what is being asked; but how. $\endgroup$
    – JMac
    Feb 19, 2019 at 19:40
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    $\begingroup$ @knzhou If the question asked "why are they applying this concept in this way compared to this other method that makes sense to me" is a lot easier to sort through than "Here is a solution along with it's logic. Here is another solution along with it's logic. What is wrong with one of them?" even if the logic presented in the question does in some way show which concepts are different $\endgroup$
    – JMac
    Feb 19, 2019 at 19:43
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    $\begingroup$ I would argue that such changes to the question probably would make it on topic, but most certainly should only be done by OP because it is a rather significant change to the post. $\endgroup$
    – Kyle Kanos
    Feb 19, 2019 at 20:59
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    $\begingroup$ @KyleKanos I completely agree. I don't think we should be guessing at the intended conceptual question, but I do think OP could find a good solution to his problem if he framed it as a conceptual question. $\endgroup$
    – JMac
    Feb 19, 2019 at 21:03
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    $\begingroup$ +1. The problem is not with the contents with IMO primarily with the presentation. The question would benefit from serious editing. Agree with @knzhou that it’s not completely run-of-the-mill; if properly edited would benefit the site. $\endgroup$
    – ZeroTheHero
    Feb 20, 2019 at 0:05
  • $\begingroup$ "and maybe point out where in these derivations we should be comparing" I did point out, when I wrote "How does energy and momentum conservation happen in this specific system?". $\endgroup$
    – Hans
    Feb 22, 2019 at 17:51
  • $\begingroup$ "The only kind of "check my work" I think we should allow is the one where a derivation is presented, leading to a wrong result" - that's exactly what I did, but additionally it turns out that the solution in the book is wrong. So my question asks where did the authors went wrong. $\endgroup$
    – Hans
    Feb 22, 2019 at 17:54
  • $\begingroup$ The usual presentation of this type of question is a coiled string uncoiling and falling through a hole, contrasted with a straight piece of string falling over the edge of a frictionless table. There are other scenarios, such as material falling onto a moving conveyor belt. None of this is particularly original - there are examples in most textbooks at the appropriate level. $\endgroup$
    – alephzero
    Feb 22, 2019 at 21:07
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    $\begingroup$ @Hans Why would a question a la "This is my solution, and the book says otherwise, which is right" be better than a question "This is my solution, is it right?" $\endgroup$
    – Norbert Schuch
    Feb 23, 2019 at 7:12
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Our "no homework" policy and our "we're not going to check your work" policy are contradictory, we must get rid of one or the other. I prefer to get rid of the "no homework" policy as that will lead to far more well written problems to be addressed here. Generically formulated problems that can be addressed without having to address any mistakes the OP made, are more attractive to people searching this site for questions and answers. Also closing questions by referring to previously asked questions will work a lot better if the questions are addressed in a generic way.

The problem of students not doing their homework themselves is an issue that's very easily addressed by their own universities, it's not something we should be concerned about. We should not let the quality of our questions and answers be compromised due to being overly concerned about such a non-issue.

What should worry us is that there is now a de-facto censoring of good questions that people with a basic understanding of physics could benefit a lot from. If we only have good questions suitable for people who are already into physics, we're not really advertising the subject well to people who aren't already into it. The Mathematics Stack exchange does far better job in that respect.

Questions like this one:

Question 15.3

A broomstick rests on two wine glasses as shown in the figure. Will the wine spill out after a strong downward blow to the center of the broomstick with an iron rod?

are currently not allowed but would certainly make this site a lot more popular!

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    $\begingroup$ "We should not let the quality of our questions and answers be compromised due to being overly concerned about such a non-issue." I'm pretty sure the current policy on homework questions was due to the quality issues with those questions and how it negatively impacted the site. There can be very interesting questions based on homework; but it shouldn't be up to the ones answering to figure out what the interesting or confusing concept is. The question should at least highlight what concept is giving the asker trouble (i.e. not just be "check all this work and tell me what is wrong"). $\endgroup$
    – JMac
    Feb 25, 2019 at 12:46

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