Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Favorite Test Questions



Date: Wed, 11 Mar 1998 19:20:37 -0500
Reply-to: phys-l@mailer.uwf.edu
From: Jerome Epstein <jerepst@worldnet.att.net>
To: phys-l@atlantis.uwf.edu
Subject: Re: Favorite Test Questions

Like most of your students, I am also having difficulty
answering the above question. According to my dictionary
and most physics textbooks, static refers to forces
exerted on bodies at rest. As the racers leave the
starting line, both the wheels and the rest of the
racing cars are in motion. thus, static friction would
not apply at all to the situation as described.

The point of contact between tire and road is static
unless the wheels are slipping. The fact that the car is
moving is irrelevant. The surfaces in contact are
momentarily at rest with respect to each other. Static
friction applies (you hope -- else you are out of
control). J. Epstein


The spinning wheels AT THE STARTING LINE undergo kinetic
friction from the time that they start spinning until
that race car comes to a full stop at the end of the
drag race.

I'm afraid this is just not true.

J. Epstein
There is a lot of good physics here. If nothing else, this
problem can give us an appreciation for the limitations of
the empirical law of friction. An example: The wide
tread used on dragsters gives me the feeling that they
don't really believe that the force of friction
is independent of the area of contact.

In my opinion we need to point out that the laws of
friction are empirical laws which are frequently useful for
modeling a particular mechanical system. Just as
frequently, however, they are not useful. Another good
example is Ohms law. For light bulbs and conductors made
from the frequently used conducting materials it is very
useful, but for diodes it is not so useful. Of course, it
is hopeless for superconductors. 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