# Lagrangian: How did they get from this step to the other?

1. Feb 24, 2013

### unscientific

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution
I don't understand where the third term from the first equation of 5.192 come about.. as clearly L doesn't depend on x at all, so ∂L/∂x should be zero.

Last edited: Feb 24, 2013
2. Feb 24, 2013

### Chopin

The second and third terms both come from differentiating the $m\ell\dot{\theta}\cos{\theta}$ term...you get two terms because there are two $\theta$'s in it.

Last edited: Feb 24, 2013
3. Feb 24, 2013

### unscientific

ah that makes sense! thanks alot!