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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"
(<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 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.
And this is the idea the site *attempts* to
convey if I understand it correctly.
From this we conclude that the scheme at classroom.com forclassifying forces is wrong coming and going. The things it
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.