Re: PHYS-L Digest - 13 Sep 2002 - Special issue (#2002-406)

["Carl E. Mungan" <mungan@USNA.EDU>]

>  > Step 1. LOOK IN MECHANICS CHAPTER
>  > Apply work-kinetic-energy theorem to block AS A WHOLE => -umgx = -m*v0^2/2.
>
> The conclusion is false.

Free-body diagram. The net force on the block is -umg. This equals
ma. Thus a = -ug which is a constant. Thus we can use the equation of
kinematics, v^2 = v0^2 + 2ax. But v = 0.

The conclusion follows and is therefore valid.

> There is no work/kinetic energy theorem for the block as
> a whole.

Correction: There is no such theorem in your *interpretation* of
mechanics. You prefer to look at W-K only from a particle view.
That's fine and I have no problems with your personal preferences.
"De gustibus non disputatum."

But many other people (myself included) find it convenient to take a
different tack. We insist there is a logically sound theorem
applicable to the block as a whole. We can agree there is a problem
of what to *name* various items. I prefer to stick as closely to
conventional textbook names as possible. I admit there is some risk
of confusion. I accept responsibility as a teacher to try to clarify
this for my students. Other people prefer to rename everything.
Recent articles in TPT and elsewhere, for example, have called for
banning "work" and "heat" and other such terms. They are free to do
so in their classrooms. But as long as the texts I have to use employ
them, I will too. I think the risk of confusion is *greater* if we
try to rename things. Namely contact with the existing literature and
with conventions are lost. This works only if you can assume the
students will never encounter these concepts again in their lives. At
least here at the Academy, I cannot assume that. Carl
--
Carl E. Mungan, Asst. Prof. of Physics  410-293-6680 (O) -3729 (F)
      U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5026
mungan@usna.edu    http://physics.usna.edu/physics/faculty/mungan/