# Lagrangian equation for motion

1. Aug 25, 2013

### Avichal

A few doubts regarding lagrnagian method to deal with motion of particles:

1) It seems like a heuristic method of solving for motion of a particle. In newtonian mechanics, you carefully consider all the forces and find out the particle's motion. In this, based on intuition you guess the paticle will move in such and such trajectory, decide the generalised coordinates and solve it.
Am I right?

2) I was studying its proof when it said that ∂T / ∂x = 0. I don't understand this.
T is kinetic energy which of course is in terms of velocity. Velocity is dx / dt. Then how come it is zero?

2. Aug 25, 2013

### ehild

The Lagrangian is function of the general coordinates an momentum components. If the expression of the kinetic energy contains the momentum components only, its partial derivative with respect any coordinate is zero.

As an example, consider the vertical motion of a ball thrown up. Its coordinate is x, the momentum is p=mdx/dt, the potential energy is V=mgx, kinetic energy is T=p2/(2m). The partial derivative ∂T/∂x is clearly zero, as T does not depend explicitly on x. In case of functions of two variables p and x, and p also function of x, then the total derivative of a function f(p,x) is df/dx=∂f/∂x+∂f/∂dp dp/dx. If f does not depend explicitely on x, ∂f/∂x=0, but the total derivative is different from zero.

ehild

Last edited: Aug 25, 2013
3. Aug 25, 2013

### WannabeNewton

You don't guess the trajectory. The trajectory is exactly what you are solving for. What you do is write down the lagrangian in some generalized coordinates, vary the action looking for a stationary point (i.e. $\frac{\delta S}{\delta q} = 0$) which then results in the equations of motion (Lagrange's equations). Of course in practice you would just write down the lagrangian in some generalized coordinates and then plug it straight into Lagrange's equations.

Last edited: Aug 25, 2013
4. Aug 25, 2013

### voko

What you mean by "guessing" is taking into account the constraints of the system. For example, a mass swinging on a light rod is constrained to move in a circle, so its has only one degree of freedom, so you need only one coordinate to describe its motion. Up to this point, there is no guessing involved. Then you can be creative, because the coordinate can be anything that describes the motion unambiguously, the angle with the vertical being a particularly convenient choice.

You need to show the proof to discuss this meaningfully.

5. Aug 25, 2013

### voko

That's not the only way to obtain Lagrange's equations. In fact, Lagrange himself, who was a pioneer of the calculus of variations, did not use this approach in his Mécanique analytique to obtain the equations. He did use variations, though, in ways that will make a contemporary mathematician cringe, to obtain them from an equivalent of Newton's second law. That method, however, can be carried out successfully using modern conventions.

6. Aug 25, 2013

### WannabeNewton

That's quite interesting. I have to say I have never seen Lagrange's approach myself but I will certainly search for translations of his original manuscript on google, thanks voko!

7. Aug 25, 2013

### voko

What he does is briefly this:

1. He obtains what we now call D'Alembert's principle (which he calls the general formula of dynamics): $\sum_i (m\vec{a}_i - \vec{F}_i) \cdot \delta \vec {r}_i = 0$ (he did not use vectors, of course). He does that by accepting that a constant force changes velocity linearly with time.

2. He does some highly formal symbolic manipulation on $\sum_i m\vec{a}_i \cdot \delta \vec {r}_i$ and shows that it is equal to $\sum_i (\frac {d} {dt} \frac {\partial T} {\partial \dot{q_i} } - \frac {\partial T} {\partial q_i }) \delta q_i$.

3. He observes that all forces in nature are such that $\sum_i \vec{F}_i \cdot \delta \vec {r}_i$ is integrable, thus there is some $V$, the partial derivatives of which give the (minus) forces.

4. So he ends up with $\sum_i (\frac {d} {dt} \frac {\partial T} {\partial \dot{q_i} } - \frac {\partial T} {\partial q_i } + \frac {\partial V} {\partial q_i }) \delta q_i = 0$ and observes that if the coordinates are chosen so that $\delta q_i$ are independent, the equation yields a system of $\frac {d} {dt} \frac {\partial T} {\partial \dot{q_i} } - \frac {\partial T} {\partial q_i } + \frac {\partial V} {\partial q_i } = 0$.

5. If the variations are not independent, he suggests the method of multipliers could be used (which he introduced earlier, in Statics).

8. Aug 26, 2013

### Avichal

Okay, I got it wrong. But there is obviously a change in approach which I am unable to express it clearly.
Can someone explain what is the change in method for solving the motion of a body compared to Newtonian mechanics?

9. Aug 26, 2013

### vanhees71

The Hamilton principle is more general than just Newtonian mechanics. Its the basis of all fundamental physics in the sense that all fundamental natural laws are formulated in terms of the Hamilton principle of least action.

Applied to Newtonian mechanics it's of course equivalent to Newton's postulates. In more complicated applications usually the Hamilton principle is an elegant tool to derive the equations of motion, to analyze their symmetry properties, finding conservation laws and then finally to solve the equations of motion or at least get a qualitative understanding of the motion.

10. Aug 26, 2013

### voko

There are two major differences. First, you don't care about forces, you use energies. Second, you especially don't care about the normal forces due to constraints - instead, you choose coordinates that eliminate motions impossible due to constraints.

This latter bit is probably what you confuse with "guessing".

11. Aug 26, 2013

### Avichal

Thanks!
I am still learning lagrangian mechanics and so far some of the problems seem to be easily solved using this. Why isn't this taught in high school? Does it have some deeper concept hidden behind it that I am not getting, because this can be easily taught at high school level.

12. Aug 26, 2013

### voko

Well, I am not sure about you, but my physical education started when I had zero knowledge of calculus, and I do not think I ever came across partial derivatives in high school. Lagrangian mechanics makes heavy use of these, so covering it in high school would seem quite challenging to me. Grasping how Lagrangian mechanics emerges from Newtonian mechanics, or some variational principle requires quite a bit of skill. Applying it to problems is much easier, yet a firm technique in calculus is still requisite.

13. Aug 29, 2013

### Avichal

How did this idea of lagrangian equation come up?
If you don't want to worry about forces and solve directly you rely on the idea that energy remains constant. So the obvious equation that you come up with is dT/dt = 0 where T is the total energy.
So is the lagrangian equation just an advanced version of conservation of energy?

14. Aug 29, 2013

### voko

Conservation of energy gives you only one equation. If the number of degrees of freedom is one, that is enough to describe its motion. For example, a mass on a spring: total energy is $m\dot{x}^2/2 + kx^2/2 = E_0$. Using the Lagrangian approach, we would get $m\ddot{x} + kx = 0$. It can be seen that the latter is simply the former differentiated with respect to time, so they both yield the same solution.

If you have more degrees of freedom, then conservation of energy alone is not enough. Conservation of energy gives just one equation, but there are more than one unknown.

15. Aug 29, 2013

### Avichal

So, can I view Lagrangian equation has some advanced equation for conservation of energy?
Basically I'm looking for an intuition behind the Lagrangian equation - how it is true and why it works? Also the proof does not help. It just looks like some algebraic manipulation of newton's laws to get to this equation.

16. Aug 29, 2013

### voko

No, that would not be correct.

Fundamentally, Lagrange's equations are equivalent to Newton's laws, so you could equally ask "why it works" about the latter. It works because we know it does. There are a few equivalent formulations of mechanics and you cannot really say that one is more fundamental than some other. The conservation laws are even more fundamental, and we expect that any given formulation must have them, but they alone do not give a full description (except in very simple cases). You need something else, and that something else is given in Newton's laws, Lagrange's equations, the principle of least action, etc.

17. Aug 29, 2013

### Avichal

I'm not asking why it works (perhaps it seemed so from my previous post).
For me Newton's laws seem very intuitive (maybe because I have been learning about it). I can directly relate it to the experiments performed by Galileo and other people.
But Lagrangian equation seems to come out of nowhere. How do I get an intuitive understanding of it?

18. Aug 30, 2013

### vanhees71

For me, it's the opposite. I find Newton's equations hard to use for deriving equations for a concrete application. The Hamilton principle is much simpler, because I can choose the approrpiate coordinates right away and analyze the symmetries of the problem, simplifying the solution significantly. The only way you get used to that is to just use it in solving problems!

19. Aug 30, 2013

### voko

Lagrange's equations are not essentially different from Newton's laws. If one uses Cartesian coordinates, then one obtains Newton's laws as Lagrange's equations. The distinctive general form of Lagrange's equations is due to the use of generalized coordinates and generalized forces (or potential energy).

For any generalized coordinates, one can obtain equations similar to Lagrange's equations from Newton's laws. But this is a "less automatic" process.

20. Aug 30, 2013

### zerokool

Avichal,

I think the derivation you are talking about in your second question is related to the Lagrangian of a free particle. What the math is saying is the kinetic energy does not change with or is independent of position. The interpretation of this is the kinetic energy, i.e. velocity, of a particle in empty space does not change with position, i. e. a particle in motion will remain in motion unless acted upon by an external force. The example you bring up is a really beautiful way to prove Newton's first law. What I've written here is from Landau's Mechanics chapter 1 section 3.