Re: Friction

[Lowell Herr <herr@catseq.catlin.edu>]

> I happened to mention to my son how hard headed you guys are and he told me
> about a Ferrari braking test he read about in a magazine some time ago
> (probably road & track or car & driver) At any rate they tried all kinds of
> braking techniques and got the best results by just jumping on the brake
> as hard as they could and locking up all 4 wheels.

As the "hard headed" soul who started this friction discussion, may I 
suggest the following experiment.  Connect a Vernier Student Force Probe 
to the MPLI and fire up MPLI for Windows.  Pull a mass across a 
horizontal table surface with constant speed.  Observe the force on the 
CTR.  Then stop pulling for a moment and repeat the above pull.  It is 
very easy to observe the difference between static and kinetic friction.  
The racing car example is different than the one I describe as the tires 
get hot when they slide and that changes the "stickiness" of the tire 
with respect to the road, whereas the more controlled lab experiment 
eliminates this variable.  Try it.  You will like it.

The "real world" racing examples throw in variable difficult to control.  
Last summer in our Project PHYSLab, one teacher actually made some 
pavement and took a portion of a auto tire and used a force probe to pull 
a portion of the tire across the surface of the homemade road.  As 
expected, static friction was indeed greater than the kinetic friction.  
Of course there was not the usual heating of the tire and road as occurs 
when a car goes into a slide.  Conclusion - Static friction is greater 
than kinetic friction when the variables are carefully controlled and 
this is certainly an important aspect of science.

Lowell Herr
The Catlin Gabel School