15 March 2018 23 9K Report

In teaching, or as a student in physics, oftentimes a difficulty becomes a motivation for new understanding. In this context, what difficulty do you see in using Lagrangian or Hamiltonian methods in physics, also thinking of avoiding difficulties ahead, for example, in teaching or learning Quantum Mechanics?

As a reference, please read the following. "Consider the system of a mass on the end of a spring. We can analyze this, of course, by using F=ma to write down mx'' = −kx. The solutions to this equation are sinusoidal functions, as we well know. We can, however,  figure things out by using another method, which doesn’t explicitly use F=ma. In many (in fact, probably most) physical situations, this new [150 years old] method is far superior to using F=ma. You will soon discover this for yourself when you tackle the problems and exercises for this chapter [see instructions below, or search in Google]. We will present our new [150 years old] method by  rst stating its rules (without any justi cation) and showing that they somehow end up magically giving the correct answer. We will then give the method proper justification.", in Chapter 6, The Lagrangian Method, Copyright 2007 by David Morin, Harvard University.

Morin continues, "At this point it seems to be personal preference, and all academic, whether you use the Lagrangian method or the F = ma method. The two methods produce the same equations.However, in problems involving more than one variable, it usually turns out to be much easier to write down T and V , as opposed to writing down all the forces. This is because T and V are nice and simple scalars. The forces, on the other hand, are vectors, and it is easy to get confused if they point in various directions. The Lagrangian method has the advantage that once you’ve written down L ≡ T − V , you don’t have to think anymore."

instructions: search in Google, or please write requesting the link.

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