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

[Brian McInnes <bmcinnes@PNC.COM.AU>]

Ed,
Let's start with your important pedagogical point:
>   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?

Is the problem with the student or with OUR understanding and fixed
ideas?  Are we set in our ways of building up a model for mechanics
because that is the way we have always thought of it and that is the
way that so many of the traditional texts treat it?  Do we think it
will be too subtle for our students because, temporally at least, it
is too subtle for us?

The point as issue here is whether the notion that a non-zero net
external force causes a particle to accelerate and also does work on
the particle can be automatically translated to a non-rigid extended
body.  I would suggest that the acceleration occurs but work need not
be done.
Many traditional text books tell us that work must be done, although
some are resiling a little from that statement in their latest
editions (e.g .Halliday, Resnick and Walker).

I agree completely with Leigh when he suggests making energy the
cornerstone of teaching here and dealing with it before work.
We don't need to calculate the net work done by all the external
forces acting on a particle or an extended non-rigid body to find the
change in kinetic energy.  What we need is something which for the
extended body is different to the work: the integral of the net force
acting on the object over  the displacement of the centre of mass: an
application arising from Newton's second law. Because of our
backgrounds and the texts we have been using we call this quantity
"work" and dismiss arguments to the contrary as subtle or wrong.

As Leigh wrote: "is the question of what to name the agent of change
operating in the system really physical?"
He also wrote: "No, I don't advocate abandoning the concept of work."
I agree. The concept is needed when we put the first law of
thermodynamics together.  Work will be done on a system when its
boundaries are deformed and, when that occurs (e.g. the piston on a
cylinder of gas being moved, a wire being stretched or the area of a
surface of liquid being expanded or shrunk) we need to be able to
calculate it to fill in the LHS of the equation representing that law.

Brian McInnes