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Re: When Physical Intuition Fails



Well the third article "When physical intuition fails" in the Nov. issue is
the one posing the problem. The question was posed in a quiz to the
students and was given in a 15 min oral interview to the faculty. No hints
were given. The article is very interesting and shows how seemingly simple
problems can even stump the experts. Of course the author is trying to make
the case that explicitly teaching problem solving is important. I doubt
that it will convince the skeptics. I was already convinced. The article
may be justly called a stealth PER article.

Incidentally the UMass Amherst PER group had a similar problem that stumped
many experts and students alike. It was a car rolling down a slight incline
vs a car rolling into a hole and then to the same final location. It was
surprising how many were surprised to see that the longer path yielded a
shorter time. They showed the result with movies and simulations.

We will see what the reaction is. Maybe everyone who can solve it has
already seen the problem before?

Regards,
John


John Clement wrote:

The recent issue of AJP posed a problem which NO physics
professors (20) or
students (67) could solve. I wonder if anyone on this list can solve it?
Please no peeking at the answer key. I have already peeked so I am
disqualified.

Ignore the retarding effect of air resistance. A rigid wheel is spinning
with an angular speed of W0 about a frictionless axis. The
wheel drops on a
horizontal floor, slips for some time, and then rolls without slipping.
After the wheel starts rolling without slipping, the center of the mass
speed is Vf. How does Vf depend on the kinetic coefficient of
friction mu
between the floor and the wheel?

I've used a somewhat more difficult variant of this question
alternatively as an in-class example or on exams for many years.
Mine posits simultaneously giving a marble some forward motion and
back-spin by squeezing it out from under a thumb on a horizontal
surface. I typically ask for a general formula for the final
velocity in terms of the radius of the marble and the initial
translational and angular velocities.

Certainly most students have difficulty with the problem even though
I give an important hint (nay, THE important hint) when I put it on
an exam. But I thought it was pretty well known to most physics
faculty.

--
John Mallinckrodt mailto:ajm@csupomona.edu
Cal Poly Pomona http://www.csupomona.edu/~ajm

This posting is the position of the writer, not that of SUNY-BSC,
NAU or the AAPT.


This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.