Re: Work done by Friction

["Dr. William Newbolt" <NewboltW@madison.acad.wlu.edu>]

> Date:          Tue, 21 Jul 1998 07:33:03 -0500 (CDT)
> Reply-to:      phys-l@mailer.uwf.edu
> From:          Tim Burgess <tburgess@jaguar1.usouthal.edu>
> To:            phys-l@atlantis.uwf.edu
> Cc:            phys-l@atlantis.uwf.edu
> Subject:       Work done by Friction

> 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
Wouldn't it be nice if we could handle all of these cases 
with a simple "work done against friction" argument!   I 
generally think about these problems as follows:

	I.  If you have a system with no internal degrees of 
	    freedom.  One block (wood or metal) sliding on an 
             inclined plane you can generally talk about 
             the work done by the frictional force and 
             equate it to mechanical energy dissapated.

        II.  If your system has internal degrees of freedom 
            the situation gets much more complicated.  With 
            a locomotive or a sports car it is much more 
            difficult to determine or even define the "work 
	   done against friction"  Analyzing a complicated system 
            this way is not very helpful and can be very 
            confusing -- as you have, no doubt, discovered. 
                                                            
                       WBN
Barlow Newbolt
Department of Physics and Engineering
Washington and Lee University
Lexington, VA 24450
Telephone and Phone Mail:  540-463-8881
Fax: 540-463-8884
e-mail: NewboltW@madison.acad.wlu.edu

"Prediction is very difficult, especially about the future."
                                          
                                          Neils Bohr