I always found this principle a bit "strange". Is it about physics or about math? It looks to me like a funny way to write equations in, say, another form, very useful, but I am not sure about its "fundamentality" from the physics point of view.
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Replying to @rayohauno
i would argue that there's no more fundamental principle. it's one of those things that has remained true from classical mechanics all the way down to quantum field theory
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Replying to @InertialObservr @rayohauno
it's a principle because there's no (as we now know) any a priori reason why Nature chooses the path that makes the integral over kinetic *minus* potential stationary
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Replying to @InertialObservr @rayohauno
We do though, although it hasn't been made very pedagogical. It's basically because we want to make this exact differential: dL = p dv + F dx, and also have the convention that F = - dU/dx https://physics.stackexchange.com/questions/86008/motivation-for-form-of-lagrangian … and https://web2.ph.utexas.edu/~mwguthrie/t.lagrangian.pdf …
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Replying to @RyanDavidReece @rayohauno
i wouldn't call this an a priori reason though. it just seems to say that it's because it returns the right EOM, but what the 'right' EOM are is an empirical fact
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Replying to @InertialObservr @rayohauno
I actually think it is a kinda of a priori reason in a way because I think this really is implicitly defining what we mean by "force".
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Replying to @RyanDavidReece @rayohauno
hmm i think i disagree with you on this one about this being a purely mathematical statement.
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Replying to @InertialObservr @rayohauno
It is a consequence of the convention F = - dU/dx. That minus sign is what caries through to make the minus sign in L = T-U.
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Replying to @RyanDavidReece @rayohauno
right but the *relative* minus sign, no matter where it goes, is physical. That's the empirical fact i'm talking about. The fact that nature wants to "go downhill", as it were.
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Replying to @InertialObservr @rayohauno
That is our convention of what "down" means.
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yes, but it's the fact that nature chooses one thing over the other no matter what we call it, mathematically represented as a minus sign
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i.e. define "down" as the thing nature chooses , call it plus or minus or what you want it doesn't matter. But we need to distinguish between them somehow mathematically. But this i argue is physical content.
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Replying to @InertialObservr @rayohauno
F = ± dU/dx would cause L = T ± U. Is convention and math. What forces are needed to explain any given observation is physics.
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