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This question. Seeing it closed has shaken my faith in this community, so I am looking for an explanation.

  1. A bullet is the most trivial engineering construction I can think of.

  2. Open any physics problem book and there will be found any number of engineering constructions, easily as complex as a laser. Insofar as the particulars of a construction are not the focus of a question, I do not see how it excludes the question from the field of physics.

  3. This is a kind of a question an answer to which can glorify physics. It is captivating and educational.

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    $\begingroup$ I disagree that point 3 is valid. I don’t think the question in question is engineering, but i don’t think it’s strictly physics (or interesting, captivating, etc). $\endgroup$ – Kyle Kanos Dec 1 '19 at 20:16
  • $\begingroup$ @KyleKanos So what is it then? $\endgroup$ – Ignat Insarov Dec 2 '19 at 7:06
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    $\begingroup$ There are elements of physics in the question, just as there are elements of material science & engineering, so it’s a multi-domain question. I also do not believe it is on topic, even with covering some physics, due to rather broad scope, as dmckee pointed out. $\endgroup$ – Kyle Kanos Dec 2 '19 at 12:13
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    $\begingroup$ Seeing it closed has shaken my faith in this community I think you are taking Physics SE way too seriously. $\endgroup$ – BioPhysicist Dec 2 '19 at 12:40
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Engineering? I disagree.

There simply isn’t a question about designing or building things there. The question is about the variation of a physical outcome as a function of a (or several) physical parameters.

In particular, engineering shouldn’t mean “this issue is complicated for practical reasons” as that applies to experimental questions.

Now, the thing that might be a problem is the shear scope of the issue. As one of the existing answers points out this is a significant field of study in its own right (and to steal a line it has a “long and glorious history”).

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  • $\begingroup$ So should it be re-opened? $\endgroup$ – Ignat Insarov Dec 1 '19 at 19:06
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    $\begingroup$ We don’t generally re-open just to change the close reason (to too broad in this case), and meta is as much about learning what consensus may exist as it is about working out what consensus applies. As you see JMac’s take differs a bit from mine. $\endgroup$ – dmckee --- ex-moderator kitten Dec 1 '19 at 20:04
  • $\begingroup$ So the real issue is that the space of speeds and materials is too wide? There are 2 speeds and 2 materials being asked about, giving 4 cases — is this the real problem? $\endgroup$ – Ignat Insarov Dec 1 '19 at 20:17
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To me, this is definitely a complicated question that depends a lot on the engineering specifics. If it had stuck to the question in the title, and asked primarily about that in relation to faster bullets, other things equal, I could maybe see the case that it's on topic as conceptual.

I'll address your points as best I can.

  1. A bullet penetrating a plate is absolutely not a "trivial engineering construction". Impact mechanics are fairly complicated, let alone the mechanics of a fast moving object shearing through plates. It's not always trivial; depending on the properties of the materials involved the problem may change quite a bit.

  2. This question focused on a series of particulars, such as changing the material, increasing the speed, and greatly increasing the speed. The particulars are the focus of the question, which makes it quite broad. I may have voted to close it as too broad myself.

  3. In my opinion, due to how many factors the question asked about; this question can give the wrong idea in several different ways, instead of glorifying anything. If people answer it without addressing the broad range of different situations, it may give people the false impression the situation is more simple than it actually is. If people actually try to address everything, it will give the impression that this site allows questions that require entire chapters from engineering books to properly answer.

Coming from experience, this is absolutely the type of thing you would find explained in detail in an engineering textbook; in terms of the details OP seems to be asking about. I can't see any physics textbook digging into all the different conceptual details to answer this. It really doesn't seem on-topic here unless it can be revised to focus on a specific physics concept; instead of the broad application of many.

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  • $\begingroup$ To 1: what if we look at the distinction between a difficult physical phenomenon and a complex engineering construction? For example, star life cycle is a difficult physical phenomenon, but it is not an engineering construction at all. Similarly, you are making a case that this is a difficult physical phenomenon, but, as I have shown by example, this alone does not make it a non-trivial engineering problem. Can you agree with that? $\endgroup$ – Ignat Insarov Dec 1 '19 at 19:14
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    $\begingroup$ @IgnatInsarov I don't really understand your comment. I disagreed with the notion that a bullet was a "trivial engineering construction". High velocity projectiles are far from trivial from an engineering perspective. $\endgroup$ – JMac Dec 1 '19 at 19:42
  • $\begingroup$ What you actually explained was that it is a difficult physical phenomenon: impact mechanics, shearing... I showed by example how a difficult physical phenomenon might not involve an engineering construction at all, let alone trivial or complex. So, from the question presenting a difficult physical phenomenon we cannot conclude that the props used to demonstrate it are complex engineering constructions. As they say, non sequitur, that is to say, you have not really given any justification for a bullet being a non-trivial engineering construction. $\endgroup$ – Ignat Insarov Dec 1 '19 at 19:54
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    $\begingroup$ @IgnatInsarov As an engineering construction, I can tell you it would be non-trivial, that is what I am saying by that. The engineering considerations for determining what occurs on an impact would depend on a wide scale of parameters and considerations, so I consider that non-trivial. I wouldn't have closed the question as an engineering question; but it is far too broad to be on topic regardless right now. It's not asking a clear conceptual question. It's trying to explore way too many different aspects of the situation in the same question when they aren't tied to the same concepts. $\endgroup$ – JMac Dec 1 '19 at 20:18
  • $\begingroup$ So the real problem with this question is that too many ideas from across the field of physics are required for an answer? Would any question discussing high speed collision of a bullet and a target be unacceptable by the same consideration? $\endgroup$ – Ignat Insarov Dec 2 '19 at 7:11
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    $\begingroup$ @IgnatInsarov Would any question discussing high speed collision of a bullet and a target be unacceptable by the same consideration? If it was a well-focused and well-written question about a physics concept, then no. $\endgroup$ – BioPhysicist Dec 2 '19 at 12:42
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I do not see how it is possible to answer this using physics principle. I suppose all engineering is physics (or stamp collecting) but one would need tables of various properties of materials. This is IMO mainstream engineering.

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