Re: Rolling, Static, and Kinetic Friction

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

Now I will try a stab (hopefully without a mistake in this case) at
answering Lowell's additional questions about the taped/untaped tires.

I wrote: 
> > This situation is an excellent demonstration that in most (anything on
> > Teflon is a counter example) cases, the coefficient of static friction is
> > greater than the coefficient of kinetic friction for the same two surfaces
> > sliding with respect to each other.  The rotating wheel is experiencing
> > static friction at its interface with the incline while the taped wheel is
> > experiencing kinetic friction.

If we define "th" as the angle of the incline and "f" as the frictional
force and the positive direction downhill, then 

In all cases the force accelerating the car downhill is:  mg sin(th)

_Case 1:_  Rear Tires Taped; Front Tires Free-wheeling

The front tires experience a negative static friction (uphill) which is
greater than the negative kinetic friction experienced by the rear wheels
which are sliding.  Thus the rear wheels will have a greater positive
net force (downhill) and will make the car become unstable.  The small
amount of energy tied up in the rotating front wheels as compared to the
rear wheels actually makes the situation even worse.

_Case 2:_  Rear Tires Free-wheeling; Front Tires Taped

Now the rear tires have the lesser net force acting on them since they are
experiencing static friction which is greater than the kinetic friction
experienced by the front wheels.  Thus the rear wheels will behave nicely
being "dragged" downhill behind the front wheels.

Hope this does not make the whole thing more slippery!

Richard Bowman