Re: Favorite Test Questions

[herbgottlieb@juno.com (Herbert H. Gottlieb)]

On Wed, 11 Mar 1998 10:05:23 -0500 Tim Folkerts <Tim.Folkerts@valpo.edu>
writes:


> "To achieve maximum acceleration, drag racers spin their tires as they 
> leave the starting line.  How does this compare with your 
> understanding of static and kinetic friction?"

Like most of your students, I am also having difficulty answering 
the above question. According to my dictionary and most physics
textbooks, static refers to forces exerted on bodies at rest.
As the racers leave the starting line, both the wheels and the 
rest of the racing cars are in motion. thus, static friction would 
not apply at all to the situation as described.

The spinning wheels AT THE STARTING LINE undergo kinetic 
friction from the time that they start spinning until that race car 
comes to a full stop at the end of the drag race. 

At the starting line, spinning wheels provide kinetic sliding friction
until the brakes are released and the race car starts to race forward.
Thereafter, the wheels undergo ROLLING FRICTION .

Perhaps the question should have asked the student to express
her understanding in terms of kinetic sliding friction, and 
rolling friction.

Herb Gottlieb from New York City
(Where spinning wheels burn rubber unnecessarily)

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