Recent content by JPaquim

  1. J

    Adding a total derivative to the Lagrangian

    I'm sorry, but I'm just not finding it trivial. Could you write it out for me, please?
  2. J

    Adding a total derivative to the Lagrangian

    I recently posted another thread on the General Physics sub forum, but didn't get as much feedback as I was hoping for, regarding this issue. Let's say I have two Lagrangians: $$ \mathcal{L}_1 = -\frac{1}{2}(\partial_\mu A_\nu)(\partial^\mu A^\nu) + \frac{1}{2}(\partial_\mu A^\mu)^2 $$ $$...
  3. J

    Calculating derivatives of a Lagrangian density

    Should I reask this question in the Relativity sub-forum? I don't seem to be having much success here...
  4. J

    Calculating derivatives of a Lagrangian density

    Ok, thank you for the feedback. BTW, the following Lagrangian gives rise to the same equations of the motion as the above one: $$ \mathcal{L} = -\frac{1}{2}(\partial_\mu A_\nu)(\partial^\mu A^\nu) + \frac{1}{2}(\partial_\nu A_\mu)(\partial^\mu A^\nu) $$ So they should differ only by a total...
  5. J

    Electrostatic E Fields - Possible/Impossible

    @ Okefenokee: The E field is the gradient of the electric potential field, not the charge distribution. 2. Yes, but only if the curl has a singularity at a point. For instance the field (-y/r^2, x/r^2) (the so-called irrotational vortex) has zero curl everywhere, except at the center, where...
  6. J

    Why does an object tip over instead of sliding

    Yes, I generally agree with you that statics can only get you so far. You'll eventually have to include horizontal and angular acceleration to figure out how it actually moves. However, the problem is first easier to visualize in terms of equilibrium of forces and torques, and I find the picture...
  7. J

    Why does an object tip over instead of sliding

    Hmm, you need to know to some rigid body statics to understand this. The tipping over isn't just a matter of how much force you apply, but a conjugation of that and the point of application of the force. Let's say you have a large rectangular box, upright, and you apply some horizontal force F...
  8. J

    Time for an object to fall to a larger one

    I've added some stuff to my answer, maybe you'd like to check it out :)
  9. J

    Time for an object to fall to a larger one

    I'm assuming you're talking about a body whose mass is much smaller than the Earth's, dropped with zero velocity from an arbitrary distance from the Earth. If so, please check out this article or this one. Basically the body follows a degenerate ellipse trajectory, similar to the trajectory of...
  10. J

    Calculating derivatives of a Lagrangian density

    Hey everyone, I wasn't really sure where to post this, since it's kind of classical, kind of relativistic and kind of quantum field theoretical, but essentially mathematical. I'm reading and watching the lectures on Quantum Field Theory by Cambridge's David Tong (which you can find here), and...
  11. J

    Figuring symmetries of a differential operator from its eigenfunctions

    Ok, I agree with you. How can I figure out the "finite" version of the transformation from its infinitesimal counterpart?
  12. J

    Dx in polar?

    You can figure out the differentials dx and dy from the general formula for a multivariable differential: $$ x = x(r, \theta) = r \cos \theta \Leftrightarrow dx = \frac{\partial x}{\partial r}dr+\frac{\partial x}{\partial \theta}d\theta = \cos \theta\,dr - r \sin \theta\,d\theta $$ $$ y = y(r...
  13. J

    Figuring symmetries of a differential operator from its eigenfunctions

    So, I understand that the derivative operator, D=\frac{d}{dx} has translational invariance, that is: x \rightarrow x - x_0, and its eigenfunctions are e^{\lambda t}. Analogously, the theta operator \theta=x\frac{d}{dx} is invariant under scalings, that is x \rightarrow \alpha x, and its...
  14. J

    Representing a factorial through its pseudo Z transform

    Ok, so I was playing around with some Z transforms. I'm sorry about the long derivation, but I'm a bit unsure of the mathematical rigor, and want to make sure every step is clear. I started with the recurrence relation defining the factorial: $$n!: u_{n+1}=(n+1)u_n=u_n+nu_n $$ $$ u_0 = 1 $$...
  15. J

    Creating intuition about Laplace & Fourier transforms

    I had thought about how the fact that the norm of the basis elements isn't really defined since the integral doesn't converge, but didn't really reach any conclusion... How can you justify integrating over a finite interval? You mention the Hilbert basis won't guarantee pointwise convergence...
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