# Zwiebach page 148

1. Oct 21, 2007

### ehrenfest

1. The problem statement, all variables and given/known data
Here is Zwiebach's solution to Problem 8.6.

Can someone explain the sentence "we do not use the equations of motion".

I used the equations of motion and I got a completely different answer!

2. Relevant equations

3. The attempt at a solution

2. Oct 22, 2007

### Jimmy Snyder

Show what you have done.

3. Oct 22, 2007

### ehrenfest

Sorry. I stupidly forgot to upload the attachment. Anyway above the parenthetical that says note that we do not use the equation of motion, I used the equation of motion to get

$$\delta L = \epsilon \frac{d}{dt}(\frac{\partial L}{\partial \dot{q}} \dot{q} )$$

And the rest is totally different for me.

So, why can I not use the equation of motion here?

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