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Re: [Phys-l] internal/external conservative/nonconservative forces !?!?



OK, I'l bite :-).

John D. pointed out some ideas presented in a site:

<http://www.physicsclassroom.com/class/energy/u5l2a.cfm>

First, John did not cite the site correctly when stating that

"the internal forces are «sometimes referred to as nonconservative forces» while the others are «sometimes referred to as conservative forces».

The site stated these other way round which might make a bit more sense. However, this is not John's point in his criticism. It seems to me that the site is not careful at all in defining what "system" means as without a well-defined system it is quite hard to see what forces are internal and what are external. For instance, a system could be a falling ball. In that case the gravitational force is an "external" force. On the other hand, if the system includes both the ball and the Earth then the gravitational force is internal to the system (this is ssumed in the example given by the site).

Another point to be made concerns the usage of conservative and non-conservative forces. I don't think that it is a good idea to equate an "internal" force and conservative force. However, I can see the point the site is aiming at. A conservative force in physics refers to "a force with the property that the work done in moving a particle between two points is independent of the path taken" as Wikipedia puts it (<http://en.wikipedia.org/wiki/Conservative_force>). In addition to this Wikipedia makes a similar point to the site: "Informally, a conservative force can be thought of as a force that conserves mechanical energy." And this is the idea the site *attempts* to convey if I understand it correctly. And I could add that this informal statement is about all you need to know about conservative force in a usual HS mechanics course addressing energy.

So...to answer John's question: "Is there any possibility that this is not as bad as it looks?" I would say that the approach taken by the site *could* probably work in a HS level IF the notions of system and conservative/non-conservative forces are developed carefully enough.

Best wishes,

Antti Savinainen
Finland


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