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Re: Newton's 3rd law? was Re: inertial forces (definition)



Ed Schweber (edschweb@ix.netcom.com)
Physics Teacher at The Solomon Schechter Day School, West Orange, NJ

Brian McInnes writes:

> Ah, but in the model I use (as does Arons and as
> does Knight) frictional forces do not do work.
>They are localized and do not move with the body
> and so the line integral of the frictional force is
> zero. The frictional forces are the means whereby
> kinetic energy is reduced and thermal energy increased.

I seem to recall that Arons was more concerned about friction doing no
work in case like walking where there is no displacement of the foot along
the surface and Fd must be zero because d is zero. When a box slides across
a rough surface there is a displacement.

But what of the walker? I understand Arons' conceptual difficulty. But
Arons approach presents other problems The walker's center of mass is
accelerating and we teach that only external forces, here friction, can
cause this to happen. Are we now to say that while only external forces can
cause a system to accelerate they don't always do the work? Maybe it is
better to say that while it is always the external forces that do work and
change KE the energy exchange can involve macroscopic or microscopic forms
of energy.

But I have another pedagogic problem with this. While Arons has inspired
much of what I do, and a often refer to him in explaining what I am trying
to accomplish to students and parents, I wonder if at this point he isn't
trying to make too fine a distinction.

In our efforts to be precise do we sometimes overload our students with
details that they are not ready to process? I tell my students that they can
never understand everything on the first pass through and that it is
frequently the better students who do more than just memorize who have the
most uncertainties. However, one of the things they must learn besides the
physics itself is to distinguish between not understanding a subtlety and
being completely lost and then how to deal with the frustration of not
getting an immediate understanding. And that physicists purposely look for
the flaws in their understanding and slowly learn to change the frustration
into the pleasure of pursuit.

Enough pontification for this morning!

Ed Schweber