Re: Rolling, Static, and Kinetic Friction

[palmer@sfu.ca (Leigh Palmer)]

> 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?

The frictional force which acts on a body in motion relative to another
body with which it is in contact does not always act in such a direction
as to oppose the motion. Rather than worrying about what to call these
forces one should ask "In which direction does THE force of friction act
ON THE CAR. The answer is always the same; the force of friction acts in
a direction tangential to the motion. If the car is decelerating on the
horizontal then the frictional force acts in the direction opposing the
motion; if the car is accelerating on the horizontal then the frictional
force acts in the direction of the motion.

Thus "frictional force" is just the name associated with the tangential
component of the contact force of interaction between the two bodies. If
the car rounds a turn on the horizontal the frictional force will be in
neither of the directions mentioned above. Should we introduce a term for
this "new" kind of frictional force? I don't think that will help the
student learn the physics, and helping the student is what teachers must
strive to do before all else. Anything "extra" which does not serve that
purpose is expendable. "Frictional force" should be all the names a
beginning student needs. Teaching the student that a car is, in some
sense, a particle will be enough to do.

I once found that my second year college class in mechanics had students
all of whom *knew* that F = ma, but most did not *believe* that F = ma in
practice! If you can get a high school student to believe F = ma you will
have done more than most have done in my experience, even after a first
year college mechanics course.

It is said: "A difference, to be a difference, must make a difference."
Since the dynamical effect is the same for rolling, static, and sliding
friction, why distinguish among them at all for the high school student?

Leigh