# Euler Lagrange derivation in book

• SUDOnym
In summary, the book Classical Mechanics by Herbert Goldstein is a good resource for learning the derivation of the Euler-Lagrange equation.

#### SUDOnym

Hello

Can any1 recommend a book that will show the derivation of the Euler-Lagrange equation.
(I am learning in the context of cosmology ie. to extremise the interval).
Ideally the derivation would be as simple/fundamental as possible - my maths is not up to scratch!

The first chapter of Classical Mechanics by Herbert Goldstein has an excellent and very succinct derivation starting from D'Alembert's Principle. (By succinct, I mean that the information density is high and it takes a while to wade through it! However, all the steps are clearly laid out.)

The second chapter uses variational calculus to derive the equation from Hamilton's Principle. Again, the information density is high, but the treatment is excellent and complete.

That the book has been constantly in print for 60 years speaks well of its merit.

I think maybe I'll take the opportunity to give my intuitive derivation (very non-rigorous and objectionable, but cute), which I don't think you will see anywhere else (at least not in this form), since I came up with it. To save time, I'll be fairly sloppy.

I'll just do the case of one particle confined to a move in a straight line. You want to choose a path q(t) that is extremal for the functional

$\int L(q,\dot{q},t) dt$

Now, you want to perturb the path by a little bit and the integral doesn't change (to a first order approximation). You can imagine that the path is formed by a bunch of vectors, dq_i, going tail to tip, each one being traversed in time dt. So, imagine you make one of the vectors longer, starting at time, t, so that q gets bigger (make the next vector have the same tip). This makes the integral go up by $\frac{\partial L}{\partial q}$.

But, you have also changed $\dot{q}$ because the velocity vector is longer. So you add $\frac{\partial L}{\partial \dot{q}}$ at time t and you subtract it at time t + dt.

So you have three terms that are changing the integral, but you can see that the latter two are just $-\frac{d}{dt} \frac{\partial L}{\partial \dot{q}}$ when dt is small.

So, there you have it: $\frac{\partial L}{\partial q}-\frac{d}{dt} \frac{\partial L}{\partial \dot{q}} = 0$

Quite a beautiful and transparent (perhaps, after some thought) argument, I find, if you are well-prepared enough for it. The case of more particles and dimensions is not significantly different.

Ah, the wonders of not being rigidly formal all the time...

## What is the Euler Lagrange derivation in book?

The Euler-Lagrange derivation in book is a mathematical method used to find the equations of motion for a given system. It is based on the principle of least action, which states that the path taken by a system between two points in time will minimize the action (a measure of energy) along that path.

## Why is the Euler Lagrange derivation important?

The Euler-Lagrange derivation is important because it provides a systematic and elegant way to derive the equations of motion for a wide range of physical systems. It is widely used in fields such as classical mechanics, quantum mechanics, and field theory.

## What is the difference between Euler Lagrange and Lagrange multiplier?

The Euler-Lagrange derivation is a mathematical technique used to find the equations of motion for a system, while the Lagrange multiplier method is a tool used to solve constrained optimization problems in mathematics. Although they share the same name, they are distinct methods with different applications.

## How is the Euler Lagrange derivation related to Hamilton's principle?

The Euler-Lagrange derivation is closely related to Hamilton's principle, which states that the path taken by a system between two points in time is the one that minimizes the action integral. In fact, the Euler-Lagrange equations can be derived from Hamilton's principle.

## What are some real-world applications of the Euler Lagrange derivation?

The Euler-Lagrange derivation has numerous applications in physics and engineering, including celestial mechanics, electromagnetism, and fluid dynamics. It is also used in fields such as economics and control theory to model and analyze systems with complex dynamics.