Re: Rolling, Static, and Kinetic Friction

["Richard L. Bowman" <rbowman@bridgewater.edu>]

On Fri, 12 Jul 1996, Richard L. Bowman made a mistake (not the first one
in my life!!):

> Now static friction will occur between the tires and the road (or incline)
> since the tire is instantaneously at rest with respect to the road
> surface.  That much is straightforward.  But what is the direction?  That
> is always a question that makes me stop and think a bit.  My best route to
> a solution is to figure out the direction that the tire would be rotating
> if it were "spinning out" or "peeling out" or whatever your students call
> it.  Static friction which is acting so as to inhibit this motion must be
> acting opposite to this direction.  
>
> So going downhill, kinetic friction is uphill and static friction is
> downhill.

My thinking was correct for a car undergoing a positive acceleration from
an internal engine but in the case of a car or ball simply rolling
downhill, I was wrong.  If we "oiled" the incline, the car's tires would
slide downhill; thus the friction which keeps them from doing this must be
uphill.

Thanks to all who caught my mistake in thinking!

Richard