The pendulum using Lagrange in cartisian

In summary, the conversation discusses the process of obtaining the equation of motion for a simple pendulum using Lagrange formalism in both spherical and Cartesian coordinate systems. The speaker suggests that the failure to include a constraint in the Cartesian derivation is the issue, and explains how to solve the problem using the constraint x^2+y^2=r^2=const. The conversation also mentions that this constraint is unnecessary in the polar coordinate system.
  • #1
Ananthan9470
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I am newly learning Lagrange formalism and I learned how to get the equation of motion for a simple pendulum using Lagrange in the spherical coordinate system. But I am unable to derive the same using the Cartesian system. If someone can please tell me what is wrong with the following derivation, that would be great.
Sorry for the sloppy paint work.
 

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  • #2
From what I can tell, you forgot to add a constraint to the problem in the beginning, which in your case would be of the form [itex]x^2+y^2=r^2=const.[/itex] since the point mass is assumed to be at a constant distance from the center. From there you can express one of the coordinates as a function of the other one, and solve the equation of motion (not sure it'll be pretty though).
 
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  • #3
kontejnjer said:
From what I can tell, you forgot to add a constraint to the problem in the beginning, which in your case would be of the form [itex]x^2+y^2=r^2=const.[/itex] since the point mass is assumed to be at a constant distance from the center. From there you can express one of the coordinates as a function of the other one, and solve the equation of motion (not sure it'll be pretty though).

OK. Thanks a lot. And in polar, you don't have to worry about that because you are keeping your r coordinate a constant to be equal to the length of the pendulum so the constraint is automatically taken care of? Thanks.
 

Related to The pendulum using Lagrange in cartisian

1. What is a pendulum?

A pendulum is a weight suspended from a fixed point so that it can swing freely back and forth under the influence of gravity.

2. What is the Lagrangian formulation?

The Lagrangian formulation is a mathematical approach used in physics to describe the motion of a system in terms of its energy rather than forces. It is based on the principle of least action, which states that the path taken by a system between two points is the one that minimizes its total energy.

3. How is Lagrange's equation used in the analysis of a pendulum?

Lagrange's equation is used to find the equations of motion of a pendulum using the Lagrangian formulation. This involves finding the kinetic and potential energies of the pendulum and then using the Lagrangian to derive the equations of motion for the system.

4. What is the difference between a Cartesian and a polar coordinate system?

A Cartesian coordinate system uses two perpendicular axes (x and y) to describe the position of a point in two-dimensional space, while a polar coordinate system uses a radius and an angle to describe the position of a point in two-dimensional space. In the context of a pendulum, a Cartesian coordinate system is used to describe the position of the pendulum on a horizontal plane, while a polar coordinate system is used to describe its position in three-dimensional space.

5. Why is the Lagrangian formulation useful in the analysis of a pendulum?

The Lagrangian formulation is useful in the analysis of a pendulum because it provides a more elegant and concise way to describe the motion of a system compared to traditional Newtonian mechanics. It also allows for a deeper understanding of the underlying physics and can be applied to more complex systems with multiple degrees of freedom.

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