Zwiebach Problem 8.6: Explaining "We Do Not Use the Equations of Motion

  • Thread starter Thread starter ehrenfest
  • Start date Start date
ehrenfest
Messages
2,001
Reaction score
1

Homework Statement


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!


Homework Equations





The Attempt at a Solution

 
Physics news on Phys.org
Show what you have done.
 
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?
 

Attachments

  • zwiebach sol 86.jpg
    zwiebach sol 86.jpg
    28.3 KB · Views: 390

Thread 'Initial conditions for orbits around a wormhole'

Hi, I had an exam and I completely messed up a problem. Especially one part which was necessary for the rest of the problem. Basically, I have a wormhole metric: $$(ds)^2 = -(dt)^2 + (dr)^2 + (r^2 + b^2)( (d\theta)^2 + sin^2 \theta (d\phi)^2 )$$ Where ##b=1## with an orbit only in the equatorial plane. We also know from the question that the orbit must satisfy this relationship: $$\varepsilon = \frac{1}{2} (\frac{dr}{d\tau})^2 + V_{eff}(r)$$ Ultimately, I was tasked to find the initial...
View full post »

Thread 'Help to convert units of a simple formula'

The value of H equals ## 10^{3}## in natural units, According to : https://en.wikipedia.org/wiki/Natural_units, ## t \sim 10^{-21} sec = 10^{21} Hz ##, and since ## \text{GeV} \sim 10^{24} \text{Hz } ##, ## GeV \sim 10^{24} \times 10^{-21} = 10^3 ## in natural units. So is this conversion correct? Also in the above formula, can I convert H to that natural units , since it’s a constant, while keeping k in Hz ?
View full post »

Similar threads

Can Lagrangian Method be Applied to Solve Rocket Motion Equations?
Replies
3
Views
2K
What are the approximations used in these equations of motion for two coupled oscillators?
Replies
5
Views
1K
Embarrassing Mistake in Arithmetic? Check Here for Help!
Replies
2
Views
976
Understanding solution to equations of motion of n coupled oscillators
Replies
4
Views
2K
Solving a recursive equation of motion
Replies
16
Views
2K
Period of Motion from Equations of Motion
Replies
10
Views
2K
Finding the equation of motion for Born-Infeld lagrangian
Replies
1
Views
2K
Decide if the energy surfaces in phase space are bounded
Replies
1
Views
1K
Deriving equations of motion of abelian gauge field coupled to scalar
Replies
10
Views
2K
Using Lagrangian to show a particle has a circular orbit
Replies
4
Views
3K

Hot Threads

Recent Insights

Back
Top