Re: Rolling, Static, and Kinetic Friction

[palmer@sfu.ca (Leigh Palmer)]

>        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