Re: Simple explanations. Was: what are the labs for?

["Rick Tarara" <rbtarara@sprynet.com>]

Let me respond with my very first thoughts on this.  WHO CARES?  OK, such
phenomenon are interesting to many of us as puzzles in and of themselves,
but certainly there are far more 'meaningful' (at least to students) things
to worry about than a falling spring which is being stretched only by it's
own weight (once released).  Perhaps, if the answer really demonstrates some
important aspect of forces that is applicable in other situations, then the
effort is worthwhile.  However, I am skeptical, and worry that if we are
presenting this kind of thing to all but our most dedicated Physics-major
students, then we are likely to elicit (not to our faces) the response
above.

Of course I don't really know Donald's intentions toward this
question--maybe just for his own edification or to challenge the rest of us
on this list.

Rick

-----Original Message-----
From: Donald E. Simanek <dsimanek@eagle.lhup.edu>

> Here's one which is bugging me today. Maybe some of you can enlighten me.
> It deals with a very simple and easy-to observe phenomena.
>
> Stand on a lecture desk, holding one end of a Slinky (TM) spring. Wait
> till the spring quits bouncing. Predict what will happen when you release
> the spring. In particular, what will be the motion of the lower end.
>
> a) The lower end rises as the upper end falls, the center of mass
> falling, and when the spring fully closes the whole thing falls with
> acceleration g.
>
> b) The lower end remains at the same level, while the upper end falls.
> When the spring fully closes it falls with acceleration g.
>
> c) All parts of the spring fall, the spring closing as it falls. The upper
> end falls faster than the lower end until the spring closes.
>
> Now for the analysis. Why does the spring behave this way and not some
> other way? Simple, yet correct, analysis, please. Would the same result be
> seen with two balls on the end of a spring made of stretched rubber bands?
> Would the rubber band demo behave the same if the two balls were of
> different mass? What if just one ball is at the lower end of a string of
> rubber bands? In this case the result is quite different than it was for
> the Slinky. Even with the slinky, does the spring constant matter? Would
> we get the same result with a limp spring as with a stiff one? Must the
> particular spring constant, length, and mass of the slinky be exactly
> right for the observed result?