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This question was put on hold recently (and answered in the comments).

I do not question the decision but would appreciate further guidance about the particular reason so I can adjust future questions accordingly. Also I am not requesting the question be re-opened.

The question is not homework (I cannot tell whether it is "homework-like"). It is a simple calculation-oriented question so maybe that qualifies. It is certainly about physics and requests a definite answer.

The comments were helpful and the point seemed to be that I must come to grips with prerequisites before dealing with a topic that depends on them. Point taken. But that doesn't really apply to the question as much as it does to me. The question was well edited by Qmechanic (who actually went to the book and added the original equation numbers) and it seemed to me that a straightforward answer might be helpful to others who jumped into Feynman's book even if they did have the background.

Parenthetically, I did not show my own work because I didn't think it would help anyone to do so. My hope was that by seeing the answer I could make some necessary inferences about the topic (variational calculus and Lagrangians) and proceed further. Of course Feynman gives many hints in his text to allow this to some extent anyway but an authoritative answer can lend one confidence.

Also parenthetically, this doesn't mean I didn't do work before posting the question. I am not working in an academic setting so I don't have professors or fellow students to consult.

Any further guidance about the specific deficiencies of the question would be appreciated. The question was answered generously in the comments so it's more or less moot but I would like to have a clear sense of what is not acceptable so I can steer clear of it.

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    $\begingroup$ Props for working on improving your question and seeking opinions on how to do it. It's refreshing to see somebody who doesn't just rage about how unfair we are and how we are trying to censor true genius by closing their question! $\endgroup$
    – tpg2114
    Commented Jul 8, 2014 at 1:33

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The problem with that question is that it really didn't have anything to do with Feynman's book or quantum mechanics (as indicated by Qmechanic's tag edits). And I agree that someone not being "classically educated" in physics cannot necessarily see this, but the question is really about variational calculus as it is applied everywhere in physics from the first introduction of the Lagrangian formulation.

So the deficiency of the question is that it does not ask about the right thing - you ask for an explicit derivation because you do not know what deeper principle is behind it. Essentially, you ask "How does variational calculus work in the Lagrangian formulation?", which is something most classical textbooks are happy to answer (and which every quantum mechanics textbook presupposes you know). Admittedly, if you had asked that thing, it would almost surely have been a duplicate or been also answered by "Did you read any introduction to Lagrangian formalism?".

The question shows that you did not learn the prerequisites (as commonly understood by the physics community) for the given topic, and as such, it is off-topic. This does not invalidate your desire to learn about physics, it does only invalidate the angle from which you tried it. Ask about things of which you are sure that they are not commonplace anywhere where you could have found it with decent effort.

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  • $\begingroup$ "the question is really about variational calculus as it is applied everywhere in and out of physics" FTFY. To whatever extent it is a questions about the calculus of variation (something I haven't taken the time to form an opinion on) it is a pure math question. $\endgroup$
    – dmckee --- ex-moderator kitten Mod
    Commented Jul 7, 2014 at 23:17
  • $\begingroup$ @dmckee: I agree. But then, aren't many more theoretical questions pure math? What about this one? True, it talks about Hamiltionians and somesuch, but Hamiltonian vector fields on a symplectic manifold are also purely mathy, and purely treatable by the techniques of symplectic geometry and PDEs, as Qmechanic shows there. Many of our "physical" concepts have been "hijacked" by the mathematicians who cast them into forms which aren't connected anymore to physics. $\endgroup$
    – ACuriousMind Mod
    Commented Jul 7, 2014 at 23:24
  • $\begingroup$ @ACuriousMind That's technically against the HW policy, but it's from a time when things weren't closed so efficiently. However, we still are lax on theoretical mathy questions. Ones just asking for a derivation are still prone to be closed, but it really depends. If it's physics-y math "hijacked physics", we usually leave it alone. (On the other hand, I've seen theoretical questions that really just need help with a normal integral without any physical techniques, those should be closed) $\endgroup$
    – Manishearth Mod
    Commented Jul 7, 2014 at 23:34
  • $\begingroup$ This answer is very helpful. I do see that the question is essentially a math question not directly related to F's book. And that the state of my knowledge is not unrelated. I accept this and will keep it in mind, thanks. $\endgroup$
    – daniel
    Commented Jul 7, 2014 at 23:46
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You're one of the few people to handle a homework closing the right way (more or less). Kudos.

The question is not homework (I cannot tell whether it is "homework-like"). It is a simple calculation-oriented question so maybe that qualifies. It is certainly about physics and requests a definite answer.

The homework-like bit comes from the fact that essentially you want us to work out some steps and show them to you. ("I think if I saw it done once methodically it would clarify a lot.")

If you can convert the question into something ansking for physical concepts, that would fix it. Something that asks the question this comment begins to answer would be good. (I'd give a sample of a conceptual question like that that you can adapt, but I've got to go and it's best if it comes from you since you know your own needs)

The comments were helpful and the point seemed to be that I must come to grips with prerequisites before dealing with a topic that depends on them. Point taken. But that doesn't really apply to the question as much as it does to me

Well, to give a very extreme example, someone with no understanding of subtraction might ask a mathematical question on ... say .. special relativity. It is possible to give an answer to it (in this extreme case let's ignore the length of the answer). But that answer won't really be useful to anyone, people requiring conceptual help in special relativity. It's localized.

That's part of the reason why your knowledge is necessary to be known -- we should have to spend less time explaining prereqs, and the answer should be useful to others as well.

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  • $\begingroup$ Your answer and ACuriousMind's answer are responsive and helpful. I think it's enough to allow me to make a better call next time, thanks. $\endgroup$
    – daniel
    Commented Jul 7, 2014 at 23:47
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    $\begingroup$ @daniel no problem :) Also, Physics Chat may be helpful for you when people are around, this is advanced enough so people won't mind helping :) $\endgroup$
    – Manishearth Mod
    Commented Jul 7, 2014 at 23:55
  • $\begingroup$ giulio's comment, which you linked, just answers the question: "How do we calculate the variation of the action with respect to a path?" (which is indeed the question I think the OP wanted to have answered). This is, by the variational calculus argument of dmckee under my post, a pure math question and thus off-topic. Or so I would conclude from your comments. Am I wrong? $\endgroup$
    – ACuriousMind Mod
    Commented Jul 7, 2014 at 23:56
  • $\begingroup$ @ACuriousMind This is sort of an open area of debate actually. Pure math that arises from a physics question/concept... we haven't really decided very clearly if it is on topic or off. See for example this question and this question $\endgroup$
    – tpg2114
    Commented Jul 8, 2014 at 1:23
  • $\begingroup$ @tpg2114: Huh, not what I expected. Thanks for the links, however. I will have to dwell on this for a while. $\endgroup$
    – ACuriousMind Mod
    Commented Jul 8, 2014 at 1:26
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    $\begingroup$ @ACuriousMind Feel free to contribute to the discussion or start a new one (if you want to take a different approach to it -- like "How to handle questions that don't ask about pure math but whose answers are pure math" or something). The discussions sometimes end up being unresolved just because it's the same groups of people always responding. $\endgroup$
    – tpg2114
    Commented Jul 8, 2014 at 1:32
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    $\begingroup$ @tpg2114 (3 comments up) actually we did resolve that issue fairly recently - the help center has been updated to say that questions about pure math in a physical context are on topic. $\endgroup$
    – David Z Mod
    Commented Jul 8, 2014 at 21:47

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