Re: Rolling, Static, and Kinetic Friction

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

On Fri, 12 Jul 1996, Lowell Herr wrote:

> Assume a toy car is rolling down an incline plane with all wheels rolling 
> freely (no wheels are sliding).  Should I be making a distinction between 
> rolling and static friction?  What is the direction of that friction?  Is 
> it down the incline in the direction the car is moving or opposite the 
> direction the car is moving?

"Rolling" friction will be present from friction in the bearings, or in
the case of your Tonka cars between the wheel and the axle or the axle and
the car depending upon which is rotating with respect to which, and to a
lesser degree from air friction.  So I like the term "kinetic" friction
better for both of these.

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.

> We had an interesting discussion about this problem today in our workshop 
> and I was interested in some new ideas.  The discussion arose out of a 
> demonstration where a toy Tonka car was allowed to move down an 
> incline, first with the rear wheels taped so they would not move.  
> Then the front wheels were taped and the rear wheels were allowed to 
> rotate.  When the rear wheels are taped, the car slides out of control 
> and falls off the incline but when the front wheels are taped to the 
> car, the toy car will ALWAYS slide straight down the incline.
> Your ideas are welcome.

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.

I like friction because it is present essentially everywhere in the
universe but our models for it can be readily seen even by the most
inexperienced student to be just that and not the absolute "truth" about
cases of the force of friction.

Sliding on out of here,
Richard
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Dr. Richard L. Bowman
Dept. of Physics                         e-mail:  rbowman@bridgewater.edu
Bridgewater College                                  phone:  540-828-5441
Bridgewater, VA  22812                                 FAX:  540-828-5479
  "http://www.bridgewater.edu/departments/physics/physics.html";
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