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: Car acceleration



Phys-L List members

John Denker's analysis of static friction, below,
is an excellent, well worded, non-flaming response
that should be upheld as an example of truly professional
physics teaching, not only to all of those on this
list-serve effort..... but to all in the physics-teaching
world beyond.

His analysis is well-worded, easy to understand, and supplies
interesting information that is not usually found in most text books.

Herb Gottlieb from New York City
(Where we really need more physics teachers like John Denker and John
Hubisz))


On Mon, 04 Feb 2002 09:39:59 -0500 "John S. Denker" <jsd@MONMOUTH.COM>
writes:
Herbert H Gottlieb wrote:

I think that you are confusing STATIC friction with
MOVING friction here.

We can fruitfully identify _three_ cases:
-- fully dynamic sliding friction
-- quasi-static rolling friction
-- fully static friction

A car tire in contact with the road (not grossly skidding)
is a fine example of quasi-static rolling friction. It
has little in common with fully-dynamic sliding friction,
and much in common with fully-static friction, and so most
people lump the two together as "static friction".

The car as a whole is moving relative to the road, but
the tire, in the one region that really matters, is
virtually not moving relative to the road.

STATIC friction occurs only when the
car's velocity is zero.

At the proverbial place where the rubber meets the road,
the relative velocity of rubber and road is locally zero.

In such conditions the friction
force direction cannot always be determined until after
a horizontal force is applied to the car. Static friction
is a kinda hindsight calculation that can be fully measured
only when it is no longer being applied.

This is one of the properties that quasi-static rolling
friction has in common with fully-static friction (and
sets them both apart from fully-dynamic sliding friction).

More generally, a "force of constraint" is likely to have
this hindsight character.

most of our physics textbooks ignore static friction

Really? Do they also ignore forces of constraint?



Herb Gottlieb from New York City
(Where it's nice to live but I wouldn't
want to be a tourist here)
herbgottlieb@juno.com