Re: Hewitt blew it again (? ? ? )

[Jack Uretsky <jlu@HEP.ANL.GOV>]

See below

Adam was by constitution and proclivity a scientist; I was the same, and
we loved to call ourselves by that great name...Our first memorable
scientific discovery was the law that water and like fluids run downhill,
not up.
                          Mark Twain, <Extract from Eve's Autobiography>

On Thu, 6 Jan 2000, Chuck Britton wrote:

> At 11:42 AM -0800 1/6/00, Leigh Palmer wrote:
>
>         Something VERY strange is going on down here in the former colonies.
>
>         We just used a 2.8 meter long chain and saw and heard a SIGNIFICANT
>         time interval before the plastic ball arrived at the floor.
>
>         (isn't THIS result in keeping with the AJP video analysis?????)
>
> I repeat:
>
>
> JUST BECAUSE HEWITT SAYS IT, *DOESN'T* GUARANTEE THAT IT IS WRONG!!!!!!!
>
>
> > Thanks to Oren Quist I'm in the loop again. Oren faxed me the latest
> > "Figuring Physics" by the author of "Conceptual Physics", Paul
> > Hewitt. It's wrong, of course, or I wouldn't be cackling here.
> >
> > Since everyone (except me) has seen this before I won't set up the
> > problem. Hewitt's analysis (if one can call handwaving without
> > physical reasoning "analysis") is certainly not transparent to me,
> > and since the result is incorrect for an ideal model, I can only
> > assume that the error lies in that analysis. As Oren noted the
> > situation is not analogous to a falling pole. The end of a falling
> > pole is *rigidly* attached to the rest of the pole. An ideal chain
> > will not support compression and is also devoid of tension when
> > freely falling. The only part of the chain which is not freely
> > falling in this case is the standing part, and it can only exert
> > forces *upward* on pieces of the chain which are making the
> > transition between falling and stopping. The gravitational forces
> > thus can be the only forces acting on the block and on each of the
> > freely falling links. As Galileo knew, the resulting acceleration of
> > each of the freely falling bodies is the same, namely g downward.
_______________________________________________
        I just found my January issue.
        Ignoring the part of the chain that "is making the transition"
is like asking "other than that, Mrs. Lincoln, how was the show?"
That's the source of the whipping action that Hewitt talks about.
        I haven't sat down to do the analysis -  it involves a falling
catenary - but I can see where there is a resemblance to the falling
stick.
        Incidentally, the falling stick analysis in the same issue seems
to miss the point, since the author gives the equation for theta-dot-dot.
One obtains directly for the acceleration of the falling end:
        a=(3/2)g cos(theta).  When cos^2(theta) exceeds 2/3, the vertical
acceleration of the falling end exceeds g.
        It is not necessary to discuss solutions of the differential
equation.
                        Regards,
                                Jack