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Re: Friction



From the computer of Roger Pruitt

To Dave Dockstader:

It is my understanding that drag racing tires are made of the same rubber that
the sad ball in the happy/sad ball set is made of. This is a rather "sticky"
rubber.

A student into drag racing once told me that the reason for spinning the tires
is to heat up the rubber so that it begins to melt and thus "flow" into the
surface of the pavement. This increases the frictional force between tire and
pavement.

What is happening is that by heating the tire by causing it to spin on the
pavement, the driver is changing the material and hence the coeficient of
friction between the tire and the road. The hot tires behave differently than
cold tire rubber. In Dr. Shapiro's example, and that is true for the rest of
us, we start to skid because the tire is not rolling, and the coeficient of
friction in the skid is less than if we allow the tire to roll. Passenger car
tires are of a different rubber than racing tires. Antilocking brakes are
based on the idea that we maintain the greatest coeficient of friction between
tire and road by allowing the tire to roll--NOT slide.

As has been pointed out in many previous notes, there is a world of difference
between situations in the real world and the laboratory. However, the
laboratory can be a great place to examine the real world under controlled
conditions. We just have to be careful to keep the conditions the same.

Roger

Roger A. Pruitt
Physics Department--Fort Hays State University
600 Park Street e-mail: phrp@fhsuvm.fhsu.edu
Hays, KS 67601 v-mail: 913-628-5357