Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Rolling, Static, and Kinetic Friction

On Fri, 12 Jul 1996, Richard L. Bowman wrote:

"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.

For argument sake, let us assume the bearings are frictionless. I want
to concentrate on what is going on between the tires and the road
surface. In the case of the Tonka toy, the tires are very hard and do
not compress to any degree. The "road surface" was wood in our
demonstration.


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.

For both the front and rear set of wheels? Be more specific which
happens to which and further, state the case as to which set of wheels
are locked so they do not move.


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.

This has been my argument but was refuted in our discussion.

Lowell