Talk:Pendulum (derivations)

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I think it'd be good to add a way to derive the Pendulum using Lagrangian formulation.

Using Lagrangian mechanics the pendulum can be derived through the use of the Euler-Lagrange equation of motion. Defining Lagrangian as the difference between kinetic and potential energies,

$L=T-V\,$

where T is kinetic energy and V is potential.

$T={\frac {m}{2}}\left({\dot {x}}^{2}+{\dot {y}}^{2}\right)$

$V=mgh=mgl(1-\cos \theta )\,$

$L={\frac {m}{2}}\left({\dot {x}}^{2}+{\dot {y}}^{2}\right)-mgl(1-\cos \theta )$

$x=l\sin \theta$

where x is the displacement in the horizontal direction

$y=l\cos \theta$

y is the displacement in the vertical direction

${\dot {x}}^{2}=l^{2}{\dot {\theta }}^{2}\cos ^{2}\theta$

${\dot {y}}^{2}=l^{2}{\dot {\theta }}^{2}\sin ^{2}\theta$

$L={\frac {m}{2}}\left(l^{2}{\dot {\theta }}^{2}\cos ^{2}\theta +l^{2}{\dot {\theta }}^{2}\sin ^{2}\theta \right)-mgl(1-\cos \theta )$

Using Lagrange's Equation

${\frac {d}{dt}}{\frac {\partial L}{\partial {\dot {\theta }}}}={\frac {\partial L}{\partial \theta }}$

we get

${\frac {d}{dt}}{\frac {\partial L}{\partial {\dot {\theta }}}}=m\left(l^{2}{\ddot {\theta }}^{2}\cos ^{2}\theta +l^{2}{\ddot {\theta }}^{2}\sin ^{2}\theta \right)=ml^{2}{\ddot {\theta }}^{2}$

${\frac {\partial L}{\partial {\theta }}}=-mgl\sin \theta$

$ml^{2}{\ddot {\theta }}=-mgl\sin \theta$

${\ddot {\theta }}=-{\frac {g}{l}}\sin \theta$

--Nefreat

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Wrong negative signal in some formulas of the derivations

The explanation for the negative signal in equations:

$F=-mg\sin \theta \,$

$a=-g\sin \theta \,$

namely, that $g\,$ is negative because it is pointing downward, is simply not true.
If the formulas refer to absolute values, then there should be no minus signal.

The reason behind these negative signs is the authors wish to obtain the correct signal in equation:

$\ell {d^{2}\theta \over dt^{2}}=-g\sin \theta \,$

My argument for this minus signal is as follows. The starting equation is

$a_{t}=g\sin \theta \,$

where $a_{t}\,$ is the tangencial acceleration.
Now, in Figure 2, let us suppose that the bob is going UP. In this case we should write:

$a_{t}=-{dv \over dt}$

because the tangencial acceleration and the velocity are in opposite directions. And,

$v={ds \over dt}$,

because velocity is in the direction of the growing of the arc $s\,$. This gives:

$a_{t}=-{dv \over dt}=-{d^{2}s \over dt^{2}}$

and since $s=\ell \theta \,$, we finally have

$a_{t}=-\ell {d^{2}s \over dt^{2}}$

The same argument goes when the bob is going DOWN. In this case we have:

$a_{t}={dv \over dt},$ and $v=-{ds \over dt}$

But the final result is, as before,

$a_{t}=-\ell {d^{2}s \over dt^{2}}$

Rui Ferreira 18:05, 12 April 2007 (UTC)

Symbols used

I am a person who is just beginning to understand the concepts described in this article. I find the information in the article to be very valuable. However, one minor point was a bit confusing to me. The "L" symbol used for the length between the pivot point and the center of mass in the diagram and the one used in the equations are different. It took me a bit to grasp that the two different symbols meant the same thing. It's a minor point, I know, but other burgeoning physicists like myself might find this a bit confusing. Lxman (talk) 21:27, 23 December 2010 (UTC)

Deletion

Looks like theres no actual discussion about deletion. Some guy just decided to put up this page for deletion. Well no go sir, please at least attempt to discuss it before putting a deletion ultimatum on the page. This page should be *merged*. Since it hasn't yet, it shouldn't be deleted. Theres already a merge suggestion from 2010. Lets take that suggestion and merge it. Fresheneesz (talk) 19:45, 9 June 2011 (UTC)

If you look at the pendulum_(mathematics) page, you will see that all the information that is on pendulum_(derivations) is already integrated into the body of pendulum_(mathematics). The discussion about the merge/deletion started on Talk:Pendulum_(mathematics)#Merge_Pendulum_(derivations)_in. It should have been carried out here but I was just going with what was already started.

To explicitly state my reasons for the deletion nomination:

• Two derivations about the motion of pendulums do not deserve a separate page when the pendulum_(mathematics) page has other derivations on it.
• The information was absorbed into pendulum_(mathematics) so there was not reason to link to another page.

Would it be preferable to redirect? Since the only page that linked to pendulum_(derivations) was pendulum_(mathematics) and they are now merged, it would be better to remove the page altogether.

I added a link to the discussion on Talk:Pendulum_(mathematics)#Merge_Pendulum_(derivations)_in asking people to come here.

Phancy Physicist (talk) 03:48, 10 June 2011 (UTC)

I support redirecting, if all the worthwhile material has been merged to Pendulum (mathematics). One good argument for redirection rather than deletion is simply to preserve the edit history here, which is substantial.--ShelfSkewed Talk 04:31, 10 June 2011 (UTC)

Since it has been about a month I'm going to change this to redirect to Pendulum (mathematics).

Phancy Physicist (talk) 22:36, 5 July 2011 (UTC)