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### Is Lagrangian Mechanics important for Physics GRE

Posted: **Wed Jul 27, 2016 7:12 am**

by **Rianowl2016**

I am preparing for Physics GRE in October. However, my undergraduate course only covered Lagrangian Mechanics very briefly . Do I need to put much effort in this part for GRE physics ?

### Re: Is Lagrangian Mechanics important for Physics GRE

Posted: **Wed Jul 27, 2016 11:26 am**

by **TakeruK**

The tests change from year to year, but I would say there are probably 2-3 questions that might directly ask about the Lagrangian formalism of classical mechanics.

https://www.ets.org/gre/subject/about/content/physics says that classical mechanics is 20% of the exam (i.e. ~20 questions) but this is a giant topic in which Lagrangian mechanics is just a small part. My advice is that it would be useful to know the following:

1. How to write the Lagrangian (practice on a few basic systems like a pendulum)

2. How to derive the equations of motions from the Lagrangian

3. What "action" means

4. Noether's theorem (i.e. symmetries in a system leads to conserved quantities)

I think just these basic things that you might have already covered is about the depth you'd need for most Lagrangian questions. Sure, there may be an occasional advanced question in some tests, but it's generally not worth it, in my opinion, to prepare for every advanced question possible. I would spend the additional hours making sure the other 90% of the test is well known and take the gamble that the 10% "advanced questions" would be in things I have more experience in. But that's just me.

### Re: Is Lagrangian Mechanics important for Physics GRE

Posted: **Mon Nov 28, 2016 7:58 pm**

by **throwawayhehexD**

If given L = some junk, you should be able to know how to answer questions like "what is the conjugate momentum corresponding to theta?" or stuff like. I don't think I've seen any question (in practice problems or that actual exam I took) that relies on more than very basic knowledge of what the Lagrangian is and how to solve for either the equation of motion using the Euler-Lagrange equation or solving for one of the conjugate momentum, or some basic question.

Or maybe some basic result like the partial time derivative or the Lagrangian and its relation to the time derivative of the Hamiltonian or stuff like that.