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From: mwg@petrus.UUCP (Mark Garrett)
Newsgroups: net.physics
Subject: Re: Friction
Message-ID: <655@petrus.UUCP>
Date: Wed, 23-Oct-85 09:16:47 EDT
Article-I.D.: petrus.655
Posted: Wed Oct 23 09:16:47 1985
Date-Received: Thu, 24-Oct-85 07:45:53 EDT
References: <956@decwrl.UUCP>
Organization: Bell Communications Research, Inc
Lines: 25

++
> Here's another car/tire question:
> It is very much easier to turn the wheel (using the steering wheel) of 
> a moving car than that of a stationary car.  The surface area of tire 
> on the road does not change as it begins to rotate.  The difference in 
> ease of turning, then, is the difference between friction on 
> stationary object and friction on a moving object.  What is the 
> mathematical relation, in this case, between the two frictions?  Can 
> this relation be generalized for other objects?
> -R. Shuster

Actually, when the car is stationary, the friction is dynamic, and when
it is moving the friction is static!  The real issue is the friction
involved in the stress of the tire.  Suppose the car is stopped.  When
you turn the wheel slightly, you distort the tire somewhat.  The
footprint stays in the same place but the rest of the tire is moved.
As the car rolls forward, the tire comes back into shape, and the contact
patch re-orients itself to the direction of the rest of the tire.  If you
force the wheel when stationary, then you drag the contact patch across
the pavement (very difficult); if you are rolling, then the only friction
is that of the sheer forces inside the (distorted) rubber.  The faster
you are moving, the less the tire is distorted before being able to restore
its shape. Therefore, the steering resistance decreses with increasing
speed, although this function is very non-linear.
-MWG