Undergrad Playing with Lagrangian and I screw up

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The discussion centers on the correct application of the Lagrangian approach in physics, specifically regarding kinetic energy and the formulation of equations of motion. It emphasizes that the Lagrangian should be treated as an abstract function of independent variables rather than jumping ahead to specific solutions. The transition from abstract equations to specific particle trajectories occurs only when solving the Euler-Lagrange equations. Additionally, there is a noted inconsistency in how textbooks present this distinction. Understanding this process is crucial for accurately applying the Lagrangian method.
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Lagrangian, Kinetic Energy (where am I going wrong)
I am sorry for all these questions this morning.

Could someone read the attached and tell me where I am going wrong?
 

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Trying2Learn said:
Summary:: Lagrangian, Kinetic Energy (where am I going wrong)

I am sorry for all these questions this morning.

Could someone read the attached and tell me where I am going wrong?
The Lagrangian approach requires that you treat the Lagrangian as an abstract function of independent variables. You can't jump ahead, solve the equations of motion, and plug these back into the Lagrangian and start again.

It's only when you are solving the Euler-Lagrange equations that you transition from the equations in their abstract form, to the equations that represent a specific solution in terms of a particle trajectory etc.

Text books tend to vary to the extent that they make this distinction clear.
 
Ah ha!

Thank you!
 

Thread 'Reference frames, center of rotation, etc'

Topic about reference frames, center of rotation, postion of origin etc Comoving ref. frame is frame that is attached to moving object, does that mean, in that frame translation and rotation of object is zero, because origin and axes(x,y,z) are fixed to object? Is it same if you place origin of frame at object center of mass or at object tail? What type of comoving frame exist? What is lab frame? If we talk about center of rotation do we always need to specified from what frame we observe?
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