Re: Rolling, Static, and Kinetic Friction

[palmer@sfu.ca (Leigh Palmer)]

>        I think that the result indicates that they do understand
> F=ma, what they don't understand, despite much preaching and
> exercise, is the physics of friction.  In particular they don't
> understand the physical implication of the statement that the
> frictional force from static friction is >= mu*f_normal, with
> emphasis on the ">".  The students who are conscientious and do
> their homework do, generally, get this one right.


It is not hard to get right, but getting it right is no indication
that a student understands F = ma. Remember, I get to teach all the
students who got it right, and it seems they know that F = ma, but
they don't believe it. You'd be surprised, I think, at how few
students believe deep down that physics relates to the real world.

>        Your statement about what I expect is a non-sequitur.
> I continue to give this one because when we get to the review,
> many of the students get the point.  One is supposed to think
> while doing calculations.

I'm glad you had a purpose in assigning a question they would
likely miss. I do that too, but my reason is different. I have to
assign grades, and questions of that sort are good discriminators
between those who thingk and those who don't. My students often
call such questions "trick questions". That is because they don't
know that the "trick" is thinking. I give the higher grades to
the students who demonstrate that they have learned to think.

>        Then, again, you say,
> *******************************************************
> 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?
> *****************************
>        Yes and no, respectively.  It is part of my teaching style
> to give problems that penalize the students who calculate without
> thinking.  Some students tell me later that they appreciate the
> challenge of such problems.

They are, of course, the ones who did not get fooled.

>        I hope that you are not suggesting that students who "get
> it right" because you gave them the answer have "advanced their
> understanding".  I have always learned much more from the times
> that I let myself get "suckered" than from the times that my
> routines "worked".  "Once bitten, twice shy" is, I believe, a
> valid pedagogical precept.  Unfortunately, however, we are no longer
> permitted to strike unresponsive students on the side of the head
> with a board, so we must resort to subtler means.

If you present the problem the way I stated it you will still get
absurd answers. Some students will get it right, but some will get
it wrong because they will neglect Nature and add one and one and
get two! My point here is that those students will have made an
error in their physics; a student who mistakenly plugs into
f = mu * N in your problem is not making a physics error, he's
making a formula error. He picked the wrong formula, and that is
the way he will look at it later, because above all else he wants
to get the answer you think is right, whether it makes sense or
not in the world.

It is unfortunate in a way that we who teach physics never had much
difficulty learning the concepts when we were in school. In many ways
I think that the best teacher of physics might be the contientious,
intellectually scrupulous English literature graduate who finds
himself somehow forced to teach (and therefore to learn) physics.

Leigh