Re: Swartz letter in AJP (work-energy theorem)

[Hugh Logan <hloganPHY@CFL.RR.COM>]

Jim Green wrote:

> At 09:29 30 08 2004 , the following was  received:
>
> > This is mostly a heads-up regarding a letter to the editor from Clifford
> > Swartz regarding the Work-energy theorem in the new issue of AJP (Sept.
> > '04).
>
>
> I was a little disappointed by the remarks in response to this post.
> They
> seemed to say something like -- These are new thoughts, I have never
> heard
> them before, I never learned anything like this in college..... I suppose
> that Galileo received the a similar response -- no he was thought to be
> evil, wicked, bad, and nasty.  Swartz is spoken of here as "silly."

I looked through the responses to this thread, but could not find Swartz
described as "silly." Someone wrote that he was trying to be "cute" in
the AJP letter. I used the word "silly" in connection with Swartz, but
not about Swartz, on a PHYSLRNR post that was not related to this
thread.  I quoted Swartz's description of  a text  by someone else, in
which he said that there was something wrong or  "silly" on almost every
page.

> Then he attacks friction.  Let's take a side bar: consider a cold widget
> which is sliding on ice,  the widget slows.  therefore its level of
> energy
> is decreased.  The _only_ way to change the level of energy of a
> system is
> to do work on the system.  What then does this work?  Obviously it is the
> ice.  But how?  By exerting a frictional force.  Hence, friction does
> work.  And again he does not understand the work/energy theorem.

I am a little puzzled as to why freshmen cannot calculate negative work,
while it is
useful at the junior level "to keep track of which system is doing
what." I am reminded
of a somewhat similar confusing item in the high school text,
_Foundations of Physics_,
2nd ed, by Lehrman and Swartz. On p.45 of the _Teacher's Guide_ for that
text, one finds
"The cavalier way in which we treat the difference in definition of
speed and velocity is
very deliberate. It is a reaction to certain situations in our own
experience where knowledge
of the defined difference became  the essential feature of motion study
to be tested with
true-false questions."  On p.61 of the text, They states that the words
"velocity" and "speed"
are usually used interchangeably in everday life. They acknowledge  the
difference, defining
velocity as the time rate of position, and "speed" as the magnitude of
velocity. Later, on. p. 62, They
write, "When there is no reason to make the distinction, the words are
often used interchangeably."
In the _Teacher's Guide_, they state that the [average] velocity is
defined as  (delta  d)/(delta t),
where d is presumably position. In the text (p.62) average speed is
defined as v-bar=(delta  d)/(delta t).
When I used this text for the first time in 1970, I found all this very
maddening after the clean presentation in PSSC. I am presenting this in
the hope that it might help in the interpretation of Clifford Swartz's
letter. Sometimes, his homey style seems like he is thinking, "You know
what I mean" or, perhaps, "You should know what I mean." His letter
would be clearer if he elaborated on what he meant. I have learned more
from _Foundations of Physics_ than from most books, but it was a little
muddy at times.

I would have forgotten this, except for the fact that, back in 1970, I
quoted L&S's description of their treatment of velocity and speed as
"cavalier." One of the students, a grandson of a famous person that
everyone on this list would recognize, asked what "cavalier" meant. That
caused me more difficulty than the physics, although I knew what was
meant from the context.

> Isn't it worth the effort to go through the thought experiment.
> Should one
> call him silly and dismiss him out of hand just because these thoughts
> have
> not been heard before.

It is worth the effort. I wasn't too attentive to some previous
discussions about work and energy with all the fine distinctions, so I
will leave it to others to start  going through the thought experiment.

Hugh Logan
Retired physics teacher