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

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

On Mon, 25 Oct 1999, Ed Schweber wrote:

>     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.

In cases like the walker viewed from the frame of the ground, external
forces can provide 100% of the impulse (and, therefore, 100% of the change
of momentum)  without mediating *any* transfer of energy to or from the
system.  Do they do work?  Well *that* depends on your definition of work
(and there are *many*) and it depends on your reference frame.

As you mention later on, it is far from clear that we want to burden
introductory students with all this.  Nevertheless, I think one can at
least do a better job than is done by most textbooks (with notable
exceptions like Randy Knight's, Tom Moore's, Chabay and Sherwood's) of
distinguishing between 1) the simple integrated form of Newton's Second
Law which establishes a clean and useful relationship between the net
force, the motion of the center of mass, and the change in center of mass
kinetic energy (and which has often been called the pseudowork-kinetic
energy theorem) and 2) what you get when you integrate each external force
individually over the motion of its "point of application" and add up all
of the contributions thereby accounting for the net mechanical transfer of
energy to or from the system.

John Mallinckrodt      mailto:ajm@csupomona.edu
Cal Poly Pomona          http://www.csupomona.edu/~ajm