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Derivations in textbooks

  1. Aug 12, 2018 #1

    Would anybody else agree that derivations of key results in physics texts - school or college level - proceed in a manner that betrays no compunctions in the mind of the author about fore-knowledge of the pre-known form of the final expression?

    I have a strong conviction that the great ones of yore, those responsible for determining the underpinnings of modern science, didn't proceed by thinking in such terms as, for example..."Next, dividing everything by j*w, I get:..."

    Would appreciate a discussion.

    Best regards,
  2. jcsd
  3. Aug 12, 2018 #2
    I think it doesn't really matter how you prove something unless your proof is wrong.

    For example:

    Let's say I know A for some reason.
    From A I show we can derive B, and from B I discover C, D and F.
    Now let's say I find a way to prove A given D and F.

    Ok, now imagine I forget about A. What am I going to do? Well, I could use D and F to get A, but - according to your reasoning - this would be cheating because I used A to derive D and F :wideeyed:. If you say so then you are implying that A is "more important", "more fundamental" than B, C, D and F, but I don't think this would be a valid assumption - I mean, it is totally arbitrary.

    Hope I explained myself clear enough

    PS: if you find a very nice and smooth proof why would you ignore it just because who solved the problem first used a different approach? It doesn't make much sense. Also if you find an other proof for an argument maybe you will see it under a different point of view and you will be able to discover new things, even if the proof seems artificial.
  4. Aug 12, 2018 #3

    What gives you such a strong conviction? Take the Pythagorean Theorem. There are many ways to prove the result. Have you seen Euclid's proof (proposition 1.47 of the Elements)? Why do you think he chose to prove it that way? Hint: read 'alternate methods of proof' in the comments below the proof on the linked page.

    Have you read Newton's Principia? What do you think of his proof of the shell theorem (propositions 30 and 31 in the link). Do you think that Newton had already had an inkling of how gravity behaved in (and near) spherical bodies before he worked this out?

    Newton still worked in the spirit of axiomatic geometry. Have a look at Lagrange's Analytic Mechanics and you'll find classic results (once stated geometrically) re-stated (and derived) with the new tool of analysis.

    I'm not sure I understand the issue.
  5. Aug 13, 2018 #4
    I used to feel this way, like it was cheating to always know the answer that you were working to derive. But after practicing physics a lot more, I realize that this is kind of how it works. Much of the time you know or have a feeling of what the answer should look like, or how it should behave. So proceeding with a derivation has a lot to do with your intuition about what form the answer should take.
  6. Sep 2, 2018 #5
    Appreciate the response. You over-estimate me much.

    I do not look to prove anyting. My disappointment stems from the fact that the proofs I am learning in college all appear to be taught in a manner that makes them conveniently straight-forward. A little too straight-forward.

    My apologies if I wasn't clear. But someone did get me (see below).

    Greatly appreciate the answer, plasmon_shmasmon.

    I hope you are right about what you say. I really do.

    What makes the process of going through a derivation very uninspiring, and in the education system I have bequeathed you are expected to rote learn & reproduce proofs in exams, is that they are perhaps a liitle to convinient; it's like the author is guiding you towards the form of the answer that (s)he expects to arrive at. They seem to lack Soul.

    But, again, I hope you are right.
    I really do.

    P.S. To all the responders above - I realised while answering to plasmon_shmasmon that the situation changes a little if we restrict our posts to "derivations" as opposed to "proofs". I don't know how "proofs" came up in the first place; my initial post is, in fact, restricted to "derivations".
  7. Sep 2, 2018 #6


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    There's a big difference between doing research and presenting the results of the research. The former is a messy process littered with false starts and dead ends. The latter, in contrast, involves trying to convince others you are right using clear, logical arguments. Textbooks, by their nature, focus on the latter. Of course, the content will seem contrived, but it reflect the improved understanding of the topic developed over the years.

    You seem to be conflating the presentation in the textbook with the assessment methods of your instructors. I can understand your frustration if all your instructors expect of you is to regurgitate what was in the textbook on the test. That just requires memorization; it doesn't really require a significant level of understanding. I'd lay the blame on your instructors, not the textbook.
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