Simple explanations. Was: what are the labs for?

["Donald E. Simanek" <dsimanek@eagle.lhup.edu>]

I'm only responding to a couple of small things in Greg's
thought-provoking post, so I changed the header line. 

On Fri, 6 Feb 1998, kifer/belk wrote:

> As Feynman recognized when he was unable to develop a
> freshman level lecture explaining QED - if you can not explain an idea
> in simple terms then you really don't understand the underlying idea. 

I've come to suspect that fairly good and simple explanations can be found
for most phenomena of everyday physics--if you are willing to do the long
and hard work required to find them. Usually, it's easier to do the math.
That's why it's best to master math before undertaking physics.

Unfortunately, I haven't had the time in my 30 plus years of teaching to
find all of these simple explanations, and I'm not impressed with most
which appear in textbooks.

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? 

And, if you think you've got this one licked, try an explanation of this:
A heavy rubber band is stretched horizontally between two nails on a
board. A cm or so beyond nail A a light object is placed. Stretch the
other end (B) of the band back, let it go. It it stopped by the nail B, of
course, but does the end of the band at the other nail (A):

a) Remain in contact with the nail until the band stops.

b) Move forward as the band contracts, knocking over the light object.

c) Move forward, knocking over the object, till the rear end contacts its
nail (B), then the band contracts.

Again, its not enough to pick the right answer, but to have a full
explanation of *why* it's right and the others are wrong. And the answer
should allow quantitative predictions of *how much* the ends of the spring
(or band) move, and precisely when, under given conditions of the nature
of the spring or band. 

You can find the descriptions of these, with a picture of the rubber band
demo, in the document of physics demonstrations in the physics section of
my web page (address below). But I've suppressed my poor explanations for
a while, till you folks supply me with better ones.

	-- Donald

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Dr. Donald E. Simanek                            Office: 717-893-2079
Professor of Physics                                FAX: 717-893-2048
Lock Haven University, Lock Haven, PA. 17745
dsimanek@eagle.lhup.edu                 http://www.lhup.edu/~dsimanek
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