Re: Rolling, Static, and Kinetic Friction

["Tom K. McCarthy" <mcca6300@spacelink.msfc.nasa.gov>]

> 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.

I think this pertains to the situation where the internal force of the 
engine is creating a force that the static friction is keeping in check, 
up to the point where it is greater than the static friction, then 
peeling begins.  With a Tonka truck, static friction provides the torque 
which turns the wheels, which in this case, whould require an up the ramp 
force.  In the case of a perfectly round wheel on a prefectly flat 
ground, with friction, friction will provide the torque in a conservative 
manner.

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TK McCarthy, PhD   Email:mcca6300@spacelink.msfc.nasa.gov