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See this Physics SE post. I maintain that it doesn't violate the homework policy for someone to provide a complete answer because (1) the OP attempted the question and got it wrong (2) the OP has the answer key and posted the correct answer. They are missing something conceptually and trying to understand why they got the wrong answer.

I don't see any reason why a complete answer would be conducive to cheating. In my opinion, the moderators applied the HW policy without consideration of these details, simply because a complete answer was provided to a "homework" question.

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    $\begingroup$ Check my work problems are off-topic. $\endgroup$ – Kyle Kanos Sep 14 '14 at 0:36
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    $\begingroup$ @KyleKanos It's not a check my work problem. It's a fundamental "I don't understand why..." They did it, got it wrong, and don't understand what they're missing. That doesn't fall into this category. If you read my response, you'll see why the OP would have good reason to misunderstand because the question asked of the OP is ill-posed in the first place. $\endgroup$ – user3814483 Sep 14 '14 at 3:12
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    $\begingroup$ It's phrased as "Can someone explain to me how to get 40s instead of 20s that I got?" I do not see any other interpretation than "check my work." $\endgroup$ – Kyle Kanos Sep 14 '14 at 11:19
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    $\begingroup$ @KyleKanos "Check my work" to me is "Hey, I got 1 + 1 = 2" is that right? Why would anyone ask you to check their work after they've submitted it, gotten it wrong, and have the answer key in their hand? There's more to this. I disagree. $\endgroup$ – user3814483 Sep 14 '14 at 15:10
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    $\begingroup$ @user3814483 well, check-my-work questions are also those where someone presents a detailed derivation and asks us to check it, not just where they ask whether they have the right answer. (Not sure if that's what you meant) I don't think of this as a check-my-work question, rather as a no-effort homework question, but the treatment is the same. $\endgroup$ – David Z Sep 14 '14 at 19:27
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Let me expand upon the argument that I first put forth in the comments on that question.

Why the homework policy applies

The first sentence of the homework policy specifies what sorts of questions it applies to:

A "homework question" is any question whose value lies in helping you understand the method by which the question can be solved, rather than getting the answer itself.

This is intentionally somewhat vague; it speaks to the intent behind the policy (see below for more on this intent) rather than giving an objective procedure for determining whether a question is homework-like, because frankly, we haven't been able to come up with such a procedure. In practice we rely on our high-reputation members' intuition for how homework questions are written to determine which questions it should apply to.

However, the relevant part of the question we're discussing reads

An electric kettle rated 220V, 2000W needed 10 minutes to boil water when it is half filled with water in Singapore where the output voltage is 220V. Estimate the amount of time needed to do the same task if the kettle was brought to the USA where the output voltage is 110V.

...

Can anyone explain how 40 minutes was derived in the question above?

I hope we all agree that this question does satisfy the criterion presented in the first sentence of the policy, and thus that the homework policy is applicable.

Why the question was closed

The relevant part of the homework policy for this point is this (emphasis added):

We expect you to narrow down the problem to the particular concept that's giving you trouble and ask about that specifically. That produces a question that is more relevant to others who might be having the same problem,

And let me quote again the core question from the post being discussed:

Can anyone explain how 40 minutes was derived in the question above?

I think it's quite clear that this core question, considered in isolation, is not about a conceptual issue that was encountered while trying to solve the problem, it's asking for a complete explanation of the solution. But of course there was more in the post than just that core question and the quotation of the homework problem. Between them is the following:

My answer to this question is Option 3 (20 minutes) because I thought that since output voltage is the USA is 2 times less than that of Singapore, it would take twice as long to do the same task of boiling water half filled. However, the answer key was Option 4 (40 minutes).

However this part does not actually ask anything, nor does it modify what the core question is about. Accordingly it doesn't change the fact that the question is asking for a complete solution.

There is also this piece:

P.S. I don't quite understand what it is meant by " An electric kettle rated 220V, 2000W" ??? I get that 2000W means the kettle uses 2000J/s and V means work done by kettle/charge. But what is the difference between the 2 (power and charge of the kettle)? And how does the V of kettle differ from output voltage?

which I don't believe is relevant for two reasons: first of all, it shouldn't be possible to sneak a request for a complete solution through by also asking something else in the same question. To implement that, posts which primarily ask for a complete solution but include some other supplementary questions should still be closed. Besides, "P.S." explicitly indicates that the following material is not the main focus of the question.

Regarding these two points,

(1) the OP attempted the question and got it wrong (2) the OP has the answer key and posted the correct answer.

I would say again that the homework policy requires an asker to narrow down their question to the specific conceptual issue that is giving them trouble. Both points (1) and (2) are true but they do not change the fact that the OP in this case is not asking about a specific conceptual issue.

Why the answer was deleted

Simply, this excerpt from the homework policy covers it:

If someone posts an answer to a homework-type question that gives away a complete or near-complete solution, in most cases it will be temporarily deleted.

I maintain that the answer which I deleted gives away a near-complete solution, in that it explains how one would derive the given answer of 40 seconds. Yes, it does identify that the question is a poor one, but that doesn't change the fact that the answer gives away a near-complete solution. (For now I'm refraining from posting that near-complete solution here, but if it becomes important I'll edit it in.)

What is the intent of the homework policy?

All the above discussion has addressed why (I think) the question should be closed and the answer deleted under the letter of the homework policy, but not under its spirit. In particular, let's consider the argument that, regardless of what the HW policy actually says, its intent is to prevent cheating and a complete answer isn't conducive to cheating in this case. I don't agree, for two reasons. First, because the value of the problem lies in the OP working through it step by step, doing as much of it as possible, so as to learn the method. By giving away a complete explanation of how the answer is achieved, we rob the OP of that value. And second, because any time we provide a complete or near-complete answer to a homework-like question, it pushes our site's reputation a tiny bit toward that of a place where people can come and get their homework problems explained to them. We don't want that. This is supposed to be a site primarily geared toward physics research and applications, not a homework help site. And we already get too many homework-like questions despite not having the reputation of a site that welcomes them.

This is expressed in other words in the following quote from a Math Meta post, which is also quoted in our homework policy:

Providing an answer that doesn't help a student learn is not in the student's own best interest, and if a solution complete enough to be copied verbatim and handed in is given immediately, it will encourage more people to use the site as a free homework service. In the spirit of creating a lasting resource of mathematical knowledge, you may come back after a suitable amount of time and edit your response to include a more complete answer. Or even better, the student can post his own correct answer!

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  • $\begingroup$ What if the OP simply asked, "Why would it take a kettle four times as long to boil water in US vs Singapore?" You wouldn't close the question, people would answer and while it wouldn't be a "homework question" per our rules here it wouldn't change a darn thing from the OP's standpoint or other reader's standpoint. $\endgroup$ – user3814483 Sep 14 '14 at 15:17
  • $\begingroup$ It might still be a no-effort homework-like question. Asking about a kettle raises some flags. (Not literal flags, I mean it stands out as an odd thing to ask.) $\endgroup$ – David Z Sep 14 '14 at 19:32
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I don't speak for anyone else here, but my reason for wanting to discouraging homework-like questions is two-fold:

  • Firstly because if we don't that will be all the site does
  • Secondly because people don't learn from being told. They learn from figuring things out. If you must answer, then provide them with a leading answer so that they still have to have the realization themselves: that is where the learning happens.

But frankly, if left to my own preferences, we just wouldn't entertain homework-like questions at all.

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I can see your point, that the question is about what related power to voltage, but as it stands the question is a check my work question and ripe for closing.

I have attempted to rework the question into a conceptual one that stands a chance of being reopened. I note there is a deleted answer that addresses my reworked version of the question. If the consensus is that the question is now OK maybe the answer could be undeleted.

As a general comment: if you feel strongly that a question should be reopened then editing it and voting to reopen is the best way forward. All users with enough rep will see the reopen request appear in the review queue, and if your edit is persuasive enough they will vote to reopen the question.

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  • $\begingroup$ Thanks for that. $\endgroup$ – user3814483 Sep 14 '14 at 15:16
  • $\begingroup$ I was reluctant to make this edit myself because removing the core of the question seemed like it should be reserved for the OP to do, if they want, but I guess it's actually not such a drastic change. In any case I agree that the question after editing does not need to be closed. $\endgroup$ – David Z Sep 14 '14 at 19:29
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    $\begingroup$ @DavidZ: since the question is open again maybe you could undelete user3814483's answer. $\endgroup$ – John Rennie Sep 15 '14 at 5:54
  • $\begingroup$ @JohnRennie I still think the answer should remain deleted, regardless of whether the question is open or closed. $\endgroup$ – David Z Sep 15 '14 at 6:05
  • $\begingroup$ @DavidZ: OK. I guess it's incumbent on me to provide an answer since I've been making a fuss. So here goes. $\endgroup$ – John Rennie Sep 15 '14 at 6:21

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