Playing with Lagrangian and I screw up

[Trying2Learn]

TL;DR

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?

 

[PeroK]

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.

 

[Trying2Learn]

Ah ha!

Thank you!