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Re: Rolling, Static, and Kinetic Friction



Hi Leigh and all-
Leigh, you write:
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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.
****************************
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.

You add:
**************************************************************
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?
*************
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.

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.
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.
Regards,
Jack

"These several facts prove nothing, for one cannot deduce a principle from so
few examples, but they do at least indicate that the ability to learn to spell
correctly is a gift; that it is born in a person, and that it is a sign of
intellectual inferiority. By parity of reasoning, its absence is a sign of
great mental power."
Mark Twain, "Extract from Eve's Diary'.