Re: Friction

["Daniel L. MacIsaac" <danmac@physics.purdue.edu>]

> > If I read the above paragraph correctly, am I to conclude that slipping
> > friction (kinetic) is always greater than static friction?  Balderdash.
> Always under the conditions of the tests!  Clean dry surfaces, etc.
> >
I have to express disbelief as well.  I don't think I paid enough for my 
tires to have them designed to smoothly tear away in sheets of rubber when I
drive.  Perhaps some exotic racing compounds with extremely high wear rates 
may behave in this fashion, but my tires (el-cheapos, admittedly) weren't
designed to be driven in skids, and may even be hard enough to chatter as much
as strip away smoothly.  Most likely they would simply disintegrate under
race conditions; they would be pretty undrivable at such speeds.

How far do professional racers get on a set of professional racing tires, 
anyway?  I suspect the design of those tires is such that they wear VERY 
quickly, and extrapolating the wear behaviours/characteristics of those 
tires to everyday driving situations might be inappropriate.  How many sets
of tires are used by one car in the Indy 500?  They certainly seem to change
them a lot more often than I :^).

Driving on non-ideal surfaces (snow, ice, gravel) and expecting better 
acceleration during skids strikes me as a negative survival situation, as
well as one fraught with legal overtones of accountability in the classroom.
Do you teach driver's ed?  Do you have lawsuit protection?  Maybe we should
not teach about kinetic friction at all to drivers under 25 yrs old :^)

Dan M

PS -- sorry about the hiatus (coping w/2 isummer courses, homework & exams),
and yes I will post more on the tides in a bit; also some comments from
interviews with 6 HS physics teachers here at PU for an AP minicourse.

Dan MacIsaac, Assistant Professor of Physics and Astronomy, Northern AZ Univ
Visiting Asst Prof, Purdue Univ;  Adjunct Faculty, Indiana Univ at Kokomo
NEW NET ADDRESSES:  danmac@nau.edu  http://www.phy.nau.edu/~danmac