I posted this question (previously called "Lagrangian of two pendulums tied together") five days ago, whereafter it was soon closed as off-topic. The reason for this was understandable: it looked as if I was asking for the solution of some exercise.

As I realized my mistake, I radically rewrote my question to embody the real, broader question I wanted to see answered and to be more relevant for others. My question became significantly different: I actually completely rewrote the title and body and changed some tags, so I thought it would be better to post it as a new question, as the comments and closure reason of the old post became completely irrelevant for the new question. However, one minute after my new post, it was marked as a duplicate of my old question and I was told to move it to my old post, so I did.

This brings me to my questions:

  • Why is my question still not reopened? What can I do to improve the chances of it getting reopened?
  • Why wasn't I allowed to post it as a new question, as my question essentially differs from the original one?


Thanks to Emilio Pisanty, I was able to describe the core of the problem, so I've rewritten the post for a second time. I kindly ask those who have the necessary privilege to give my question a second chance. In my eyes, the close reason has become completely obsolete.

Update: the post was reopened! Hooray!

  • $\begingroup$ The question as it's currently posed has a lot of cruft (all of the detailed geometry) that detracts from the general message you're trying to ask about. If your main point is that for rigid, inextensible connectors, there are two valid $\alpha_2$s for each possible $\alpha_1$, then you can just say that, justify it with an image, quote the final result, and link to the revision history for the detailed derivation. If you want to ask about general methods to deal with that kind of situation, then don't bog yourself in detailed geometry. $\endgroup$ Commented Jan 12, 2018 at 11:27
  • $\begingroup$ I voted to close and keep the question closed because you're simply asking us how to solve a particular problem you're struggling with. $\endgroup$
    – Kyle Kanos
    Commented Jan 12, 2018 at 12:10
  • $\begingroup$ @EmilioPisanty Okay, I will try to compress the example. It is indeed maybe too long to make my point clear. $\endgroup$
    – Safron
    Commented Jan 12, 2018 at 12:46
  • $\begingroup$ @KyleKanos You were right to close my original question, but in my new post I'm not asking for someone to solve some particular problem for me, I'm asking for a general method of reducing dependent coordinates when coordinates cannot be written as a function of the others. This is actually relevant for a wide variety of problems. What did I miss? $\endgroup$
    – Safron
    Commented Jan 12, 2018 at 12:54
  • $\begingroup$ @KyleKanos I hope my second edit can now persuade you to vote for the reopening. If there is still something I can improve, please say so, because I'm eager to know the answer, and this closed status isn't really improving the chances of finding one. $\endgroup$
    – Safron
    Commented Jan 13, 2018 at 19:55

1 Answer 1


Just to close this off - the question is now much improved and has been reopened. Thank you for your work in improving the question and getting it to a shape where it becomes a valuable resource for future visitors. The thread definitely makes this list in my books.


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