Re: forced vibration question
From: Bjoern Feuerbacher (feuerbac_at_thphys.uni-heidelberg.de)
Date: 09/13/04
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Date: Mon, 13 Sep 2004 17:27:29 +0200
Mike wrote:
> Bjoern Feuerbacher <feuerbac@thphys.uni-heidelberg.de> wrote in message news:<ci3n8g$98i$1@news.urz.uni-heidelberg.de>...
>
> [snip]
>
>>
>>>Bjoern was nor right that the gravitation force
>>>on a pendulum is an extrnal force
>>
>>Nonsense. It *is* an external force.
>>
>
>
> In a literal sense it is. But in the standard terminology used in
> Mechanics it is not, since it is a force derived from a potential
> function -V(r). Usually you break down the forces into potential and
> nonpotential. The later includes non-conservative forces. Forces
> derived from a potential function independent of t (in general) are
> called conservative.
Agreed. So what??? In what way does this change the fact that it is
a force external to the system of the pendulum???
>>>it is not 'external' because if not the gravitation
>>>*the pendulum will not work as an harmonic system-
>>
>>Absolute utter nonsense. It will work just as well without gravitation.
>
>
> I think he is right although he does not use the standard terminology.
> If there is not a force derived from a potential function, like a
> spring force or gravity force, there can be no harmonic oscillation.
We considered here a pendulum consisting of a spring with a weight
attached. Such a pendulum will work without any problems even in the
absence of gravity.
>>>to the gravitation is *part of the orriginal system*
>>>external or not it is aprt and parcel of the orriginal and thats is the main point
>>>a real 'external force is a force that *was not existing* while the
>>>orriginal system was created!!
>>
>>Absolute utter nonsense. An external force is simply a force which comes
>>from outside the considered system. And gravitation does do that indeed.
>
>
> Gravitation is part of the system in classical mechanics. It is a
> force derived from a potential function -V(r).
How do you get "gravitation is part of the system" from "it is a
force derived from a potential"? Looks like a non sequitur to me.
[snip]
Bye,
Bjoern
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