Re: Rolling, Static, and Kinetic Friction

[Richard Langer <rlanger@slps.k12.mo.us>]

At 05:44 PM 7/13/96 -0700, you wrote:
> >        I said:
> > *************************************************************
> > I concede your point, but my plea was for dynamical effects. Static
> > friction can certainly change the kinetic energy of a car just as
> > surely as can sliding friction or rolling friction. That the latter
> > two dissipate energy* while the first changes it conservatively
> > is of secondary importance in an introductory course. Once the
> > students know what F = ma means, and believe it, Then one can go on
> > to more sophisticated topics.
> > **************************
>
> and Jack disagreed:
>
> >        I disagree.  Consider the following problem, which I always
> > include on friction quizzes and tests:
> >        A 75 Kg skier stands on horizontal, snowy surface.  Coef of
> > friction between skis and snow is .02.  The skiers friend pushes her
> > forward with a steady force of 1 N.  Calculate the acceleration of
> > the skier.
> >        About 80% of all students have the skier accelerating
> > backwards.
>
> Since nothing but his signature followed this, I will add my own
> interpretation. It is my concusion from the result Jack obtained
> that at least 80% of his students don't understand F = ma,
> something which, as I said before, does not surprise me, since
> few do. Yet we all know it is a very simple concept. We must ask
> ourselves "If it's so simple, why don't they learn it?" Perhaps
> the answer is that we are not helping them to do so.
>
> The fact that his students could not detect the absurdity of the
> result obtained by plug-and-chug means to me that those students
> believe that physics is an intellectual exercise which has no
> counterpart in the real world. Why does Jack not interpret it in
> the same way? Why does he continue to include this question on
> quizzes and tests when he expects that the students will not be
> able to understand it?
>
> Suppose you had told your students that the frictional force was
> equal in magnitude to the applied force of 1 N. Don't you think
> that many more would have got it right, and that they would have
> advanced their understanding a bit more than by missing it?
>
> Leigh

What error are the students making?  And what caused the error?

I used to teach that Static Friction = mu*Normal Force - if the kids believe
that, then the Frictional Force is about -1.5N in comparison with the Push
of 1N - sure enough, the calculation appears to indicate that skier will
accelerate backwards!  

I recently noticed that the sheet of equations for the AP Physics exam gives
the equation F<= (less than or equal to) mu*Normal Force.  That is what I
now teach, even though my text book uses an EQUAL sign. This means that the
-1.5 N is an upper limit on the force of Friction - and if the use of the
upper limit as the force leads to a nonsense answer, scale it down!

One of the extremely important things that I try to teach my students is
that they are intelligent folks who have been making observations on the
world around them for all their lives, and if their answer contradicts what
they KNOW to be right - their answer MAY be wrong.

THINK!!!
Richard M. Langer                  Gateway High School,
Physics Teacher                    5101 McRee Ave.
rlanger@dtd1.slps.k12.mo.us        St. Louis, MO 63110