I do not understand why this elementary question was closed. The official reason calls it a "homework-like question" and asks the poster to "show some effort".

One could argue about whether this is homework-like (though it seems clear to me that this is not an assigned problem, but rather something the poster stumbled upon while, quite admirably, attempting to invent and work out an illustrative example). But surely it shows an appropriate level of effort. The OP used the relativistic momentum formula (not the Newtonian formula as alleged in a comment) to compute momentum in an unambiguously defined frame before and after an explosion, came up with different numbers, is disturbed that momentum appears not to be conserved, and wants to know what's wrong here.

Given that the OP did all the relevant calculations, what additional effort was expected of him?

  • $\begingroup$ It was closed on an earlier text and wasn't re-opened because the poster posted a improved version before any one got around to re-examining the original version. I suppose I could have re-opened the original and closed the second one, but that seemed to be unnecessarily pedantic. $\endgroup$ – dmckee Jul 3 '16 at 15:41

This was going to be a comment, but it got a bit long winded (as usual for me :-).

The initial version of the question was rubbish and was rightly closed in accordance with our policy. It has since been edited and now I think it's a lot better. Indeed when it's reopened (as it surely will be) I'll be interested in answering (assuming someone doesn't beat me to it). I think it will be a nice demonstration of why we use four-vectors in SR.

I assume the mods are going to let us do the voting to reopen, but as it's already got two reopen votes the (re)opening ceremony shouldn't be far off.


I can't say why it was closed since I'm not the one who closed it.

But... as I understand it, the argument you make here is that (1) the homework policy perhaps shouldn't apply to the question in the first place, and (2) the question did show enough effort not to be closed. That's true, but it's not a valid argument against closing the question:

  1. The homework policy applies to all questions whose value lies in understanding the method by which the problem is done, not in getting the answer itself. Not only to assigned questions. In fact this is the first sentence of the full homework policy.
  2. There are two main points to the (summary of the) homework policy: (1) show effort and (2) ask a specific conceptual question. This question shows effort, yes, but you also have to consider whether it's asking a conceptual question (as opposed to how to do the problem, what's the next step, or similar).

That being said, you could argue that this is a conceptual question: asking why the Newtonian formula for momentum doesn't work. I could see that being a valid argument for reopening the question. It's still not a particularly well written question, and could do with some editing before getting reopened, but I think there is an argument to be made that it's fine; just not the argument you happened to put forth in this meta post.

So to directly answer your question,

Given that the OP did all the relevant calculations, what additional effort was expected of him?

None. But that's not the whole story.

  • $\begingroup$ I think I misunderstood what the question was actually getting at when I closed it, but by the time the OP had clearly explained he had posted a much better second version. $\endgroup$ – dmckee Jul 3 '16 at 15:49
  • $\begingroup$ Mea culpa for not looking at the original of the question before posting this. But for the record: Now that I have looked at the original, I still think it's a perfectly clear conceptual question and I still think it should not have been closed in the first place. $\endgroup$ – WillO Jul 3 '16 at 16:22

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