Friction

["Eugene P. Mosca" <mosca@nadn.navy.mil>]

> Since when is the dynamical effect the same?  Static 
> friction is a force of constraint that prevents relative 
> motion parallel to two contacting surfaces and which 
> dissipates no energy.  Various kinetic frictions (i.e. 
> rolling, sliding, etc.) provide no constraint and *do* 
> dissipate energy.  This is a *significant* difference--even 
> for high school students.

I agree, even in the case of a car that is gaining speed when 
the torque delivered by the engine causes the drive wheels to 
skid.  If skidding occurs mechanical energy, but not 
necessarily kinetic energy, is necessarily dissipated.

A skidding wheel's motion is not constrained and the 
direction of the kinetic-frictional force will be opposite to 
the direction of motion.  A rolling wheel's motion is 
constrained to move in a direction perpendicular to the 
wheel's axis and the direction of the static-frictional force 
is directed so as to oppose any tendency to skid.  As long as 
the rolling wheel's angular acceleration is small--motor is 
off, no brakes are applied, etc.--the static-frictional force 
component perpendicular to the axle will be small.  Never-
the-less, the component parallel to the axle can be quite 
large.

I seem to recall reading an analysis of the skidding of a car 
when the front wheels are locked versus when the rear wheels.  
I anyone able to give the source of such a reference?

I recall that when radial tires were introduced into the U.S. 
they came with a warning that they should be installed in 
pairs and that if only one pair was installed that they be 
installed on the rear axle.  Perhaps radial tires provide 
greater traction so this warning was intended to prevent any 
lock-up of the rear wheels during heavy braking, an 
occurrence which would likely result in a spinout.

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Eugene (Gene) P. Mosca           Phone 410-293-6659 (Fax 3729)
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