Re: Car acceleration

["John S. Denker" <jsd@MONMOUTH.COM>]

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?