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Re: Hewitt blew it again (? ? ? )



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.

Because I believe that Nature should be allowed an opinion I did the
necessary experiment this morning. The two blocks arrive at the
bottom together. I used a zero mass block on the chain in order to
observe the largest effect possible, and of course there is no effect.
One can add a block of arbitrary mass to the end of the chain and
observe that superposition works perfectly: the added block and the
free end of the chain both accelerate at g whether the chain is
linked to the block or not.

I and many others on this list, including Oren, can easily recognize
nonsense when we are alerted to look for it. Unfortunately not
everyone can do so. The Physics Teacher, a publication to which I
subscribe and which I read when it finally arrives here in the Great
White North, is often taken as authoritative when, in fact, it
frequently publishes erroneous physics. I understand the problem. It
would take a full time editor to clean it up, and the editor could
use some help from colleagues as well, because none of us knows all
of physics.

Leigh

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Chuck Britton Education is what is left when
britton@academic.ncssm.edu you have forgotten everything
North Carolina School of Science & Math you learned in school.
(919) 286-3366 x224 Albert Einstein, 1936