Finding the Lagrangian for an elastic collision

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Homework Help Overview

The discussion revolves around finding the Lagrangian for an elastic collision between two particles and proving the conservation of linear momentum. The problem is situated within the context of classical mechanics, specifically focusing on Lagrangian mechanics and the implications of elastic collisions.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Problem interpretation

Approaches and Questions Raised

  • The original poster attempts to establish the Lagrangian as \( L = m(\dot{x_1}^2 + \dot{x_2}^2)/2 \) but expresses uncertainty about demonstrating momentum preservation. They also consider new coordinates related to the center of mass but struggle with formulation.
  • Some participants question the terminology used in the problem, particularly regarding the term "preserved," suggesting it refers to the conservation of total linear momentum. They emphasize the need for an interaction term to model the collision and inquire about the nature of elastic collisions and the corresponding potential.
  • Others suggest that the momentum conjugate to a cyclical coordinate may not be the same as the linear momentum, indicating a need for clarity in definitions.

Discussion Status

The discussion is ongoing, with participants exploring various interpretations of the problem and offering insights into the nature of elastic collisions and the formulation of the Lagrangian. Some guidance has been provided regarding the need for an interaction term and the implications of conjugate variables, but no consensus has been reached on the specific approach to take.

Contextual Notes

There are indications that the problem may involve assumptions about the nature of the collision and the potential involved. The original poster's attempts at formulation suggest a need for further clarification on the definitions and relationships between variables in different coordinate systems.

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Homework Statement


a. Suppose two particles with mass $m$ and coordinates $x_1$, $x_2$ collides elastically, find the lagrangian and prove that the linear momentum is preserved.
b. Find new coordiantes (and lagrangian) s.t. the linear momentum is conjugate to the cyclical coordinate.

Homework Equations

The Attempt at a Solution


For (a) I thought that $L=m(\dot{x_1}^2+\dot{x_2}^2)/2$ but I don't know how to show the preservating of momentum.
For (b) I thought taking the new coordnates to be the center of mass and the distance of each particle from it but somehow I can't formulate it.

Can anybody assist with proving the momentum preservation and its conjugation in (b) for cyclical coordinates?
 
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The question uses the word "preserved" in an odd context. It seems to mean that total linear momentum is conserved.

To include the collision you will need an interaction of some kind. I am hoping that the context of this question will provide you some guidance as to what is expected. An elastic collision means a conservative potential of some kind. This is "advanced physics homework" so it is probably not too much to expect you to know what that means with regard to a potential. So, what does it mean for a collision to be elastic, and what does a potential have to be such that it will produce an elastic collision?

Once you have that you show that total linear momentum is conserved by showing it is an invariant. And you do that by showing its bracket with the Hamiltonian is zero.

For part b) you need to remember what it means for the momentum to be conjugate. The question seems to be worded in slightly sloppy terms, possibly deliberately so. I think they are trying to get you to note that the conjugate variable and the momentum are not necessarily the same thing in different coordinate systems.
 
May can use: ## \int L\,dt = 0 ##
 
theodoros.mihos said:
May can use: ## \int L\,dt = 0 ##

Except that you forgot a bit. You forgot a ## \delta ## in front of that. Then you get ## \delta \int L\,dt = 0 ## which just gets you the Lagrange equation of motion. The integral you wrote is not usually zero.
 
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I would consider the two masses to be connected by a spring of infinite spring constant at the point of collision.
 

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