Re: When Physical Intuition Fails

[John Mallinckrodt <ajm@CSUPOMONA.EDU>]

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.