Re: Rolling, Static, and Kinetic Friction

[Lowell Herr <herr@catseq.catlin.edu>]

> 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.

The car was not on a horizontal surface.  The car was rolling down an 
incline.  Let's not complicate the original explanation.


> 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.

There are two different frictional forces involved in the original 
problem and that is why the car will go out of control in one situation 
and straighten out in the other situation.  To use a term "frictional 
force" for both front wheels and real wheels begs the question.

> 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

Why distinguish?  So the question can be answered.

Lowell Herr