Re: When Physical Intuition Fails

[John Mallinckrodt <ajm@CSUPOMONA.EDU>]

Brian Whatcott wrote:

> I find it easy to visualize a spur wheel spinning at 1 rpm suddenly engaging
> a fixed rack with no friction, in a fully elastic way.
> Is this a limitation of my imagination, or of yours, Bob?  :-)

I think probably yours.  I found Bob's analogy to be highly
compelling and wished that I had found the same words in my previous
post.

I think you may be confusing two different meanings of the word
"elastic."  The sense of the word that is relevant to the current
discussion is "nondissipative."  I suspect you may be using the word
in a mechanical engineering or materials science context, one in
which it means "nonpermanently deforming."  That sense of the word is
not, however, particularly relevant to this discussion.

> When I visualize a mass suddenly hung onto a steel strip,
> I see a fully elastic extension of the strip's length. Do you?

I don't, but I can see how you might.

If the extension were fully elastic (in the sense that means
nondissipative), the mass would oscillate forever between its lowest
point and the original point of sudden attachment.  It seems to me
that the dissipation that prevents that outcome is immediate and
obvious.

On the other hand, as long as the mass is not too large, when it is
removed the strip may well return to its original length free of
permanent deformation.  One inclined more toward engineering analyses
might even say that the deformation was "elastic."

--
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.