Re: Work done by Friction

[Sam Held <held@utkux.utcc.utk.edu>]

Tim,
	Wow, this topic will get a lot of discussion.  In fact, I am going
to start with a lot of brain stroming and babbling.  Here we go.  
	If you go to http://xxx.lanl.gov/find and look for the word
friction in titles of papers in all years in all the groupings, you will
find 47 papers.  These deal with friction in many different situations but
there are at least 10 good papers I spotted on "dry friction" that may
help you understand the complexity of friction.  
	When I was taught in high school and when I teach discussion
sections for the introductary classes, I use the phrase "work done by
friction".  Why?  Because in an algebra based course, it is the only way
to explain it without going into a 30 minute digression into friction
(just like htis one).  When I taught the physics majors, I used the
expression but gave a disclaimer (10 minutes) about how mathematically
complex it really is and how one should really integrate due to its random
nature.  Fricion Force = mu * Normal Force is only a approximation, so
does that really solve the problem or add to it.  That is not even
mentioning the difficulty of the velocity dependence, which they will not
see until their sophomore or junior year.  Friction is an empirical thing
and no theory can correctly predict it.  (OK, enough of that babbling)  
	Friction is a non conservative force so mechanical energy is not
conserved.  Friction releases converts mechanical energy into heat energy
(and maybe some sound energy) so total energy is conserved.  In my
opinion, to be consisent with the work-energy theorem (W=E) is the only
reason that the phrase "work done by friction" is used.  This term
simplifies things for introductory physics students because they don't
have enough of the basics in physics and especially math to understand the
complete problem.  
	So after all that, the bottom line is I think it is OK to use but
if you have advanced students or an extra class period, cover it in depth
to show students that there is still a lot of unknown out there.  


							Sam Held


sheld@utk.edu


On Tue, 21 Jul 1998, Tim Burgess wrote:

> Hi!
> 	I'm with a group of high school physics teachers. We are in disarray.
>
> 	Textbooks that we use commonly refer to the "work done by friction"
> in  slowing the speed of objects.  There are some who think this approach
> is flawed.  Others indicate that the right answer is commonly obtained
> by treating such friction as work.
>
> 	Some would just ignore the textbook treatment and indicate that
> "work done by friction" is not the way to address the energy "dissipated".
>
> 	There are strong opinions present here.  What is the thoughts of 
> those of you on this list?
>
> Tim