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From: tino@hou2f.UUCP (A.TINO)
Newsgroups: net.physics
Subject: Re: Friction
Message-ID: <547@hou2f.UUCP>
Date: Wed, 23-Oct-85 15:24:33 EDT
Article-I.D.: hou2f.547
Posted: Wed Oct 23 15:24:33 1985
Date-Received: Thu, 24-Oct-85 08:06:58 EDT
Organization: AT&T Bell Labs, Holmdel NJ
Lines: 20

>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?
___________________________________________________
Not knowing anything about cars I will assume that the linkage
connecting the steering wheel to the car wheels works the same
whether the car is moving or at rest.

When you turn the wheel of a stationary car the tire rubs against the
road forcing you to work against friction.  On the other hand, to turn
the wheel of a moving car you don't need to overcome friction. As long
as the tires don't slip no work is done against friction.  (It's 
possible to turn without slipping because tires are flexible.)
This seems to make sense. 

Al Tino