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Re: Rolling, Static, and Kinetic Friction



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.

I don't understand your question. You ask me what error the students are
making when they use F = mu * N, and then immediately explain their error,
which, as I said, is that they will think to be that they employed the
wrong formula. In fact their error was ignoring Nature; clearly that answer
*is* (not MAY be) wrong. Nature follows physics follows Nature! We are
supposed to teach students how we describe the real world mathematically.
Any teaching which produces conspicuously unnatural results will be
counterproductive. Counterintuitive results, followed by demonstration that
they are indeed natural, do not fit in the category of counterproductive
activities; they are my favorites in fact.

Does your AP equation sheet qualify that formula (which is the
traditionally correct (TC) one) by noting that it is of a stature
substantially different from that of F = m a ? And why, may I ask, did
you *ever* teach that other formula? It is probably even more confusing
to students than the TC one. Of course I, too, am constrained somewhat
by my textbook, but I have never felt that extended to ignoring errors
in it (and there are many in the elementary texts I have used).

Leigh