Re: Friction

[Herb Schulz <herbs@interaccess.com>]

> 2.  Max acc is always achieved with some slippage of the tires.  Therefore,
> slipping friction must be greater than static friction.  Drag racers do not
> spin their tires because they are out of control.  They do it because it gives
> the max acc.  Likewise a race car in a turn is deliberately set up to slide.
> Sliding is faster.  To stop the slide you have to slow down!

Howdy yet again,

Race car drivers don't 'slip' because the kinetic friction forces are
larger than the static friction forces; it's because the tires can no
longer exert the strong sideways forces (provided by the static friction
force; i.e., the needed fs would be greater than the fsmax) needed to keep
the car body moving tangentially around the turn.  Drag racers know that
spinning the tires is a sure way to LOSE!  They initially spin the tires to
warm up the rubber to make it softer and more adhesive.

I see no problem with talking about static and kinetic friction, etc. as
EXAMPLES of different kinds of forces.  The misconception in the use of
F=ma is that most instructors say that F is THE force acting on the object
while they should emphasize that the F is the NET force acting ON the
object (the VECTOR sum of all the individual forces).  That emphasis on the
word ON is because students also seem to have great problems with
understanding Newton's Third Law.
But that's another story...

Good Luck,

Herb Schulz
(herbs@interaccess.com)