Re: Inertia DEMO

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

On Fri, 28 Nov 1997, Derek Chirnside wrote:

> Cut some small paper loops.  (Say 14 ml wide, 50ml radius)  Hang 
> a ruler from two of them (from the finders of two students waering 
> safety glasses.  Destry the ruler with another ruler sword style 
> (with a sudden loud Ye ha!!!!!!!)  The ruler will break leaving the 
> loops intact.  (Repeat s-l-o-w-l-y and break the loops)

Here are some others of this same character.

Have a glass or beaker of water sitting very near one end of a long strip
of paper (cash register paper is good). Yank the paper out quickly. The
glass doesn't topple. (Poor man's version of the tablecloth trick.) Of
course the bottom of the glass should be dry. Pull the paper slowly, the
glass slides along with it. Pull it moderately fast and the glass topples.
All three results need to be discussed.

Same idea with a lighted candle on the paper.

A stack of poker ships on the paper.

A smooth-edged coin (nickel) on the paper, in a plane perpendicular to the
length of the paper. Now do it with the coin in a plane parallel to the
length of the paper. Takes some practice.

A tall and narrow stick, smooth bottom. A stick which will topple at the
slightest touch. Cheat on these last two--use smooth surface paper, like
Christmas wrapping paper. 

A trick for making the tricks work. Hold the paper out tautly from the
edge of the table, and do a karate chop downward on the paper. Why does
this help? Some use a wet finger for the chop. 

Suspend a long stick (hardware store yardstick) by light strings from two
supports. Break it by a swift blow to the center of the stick, without
breaking the strings. Try suspending the light strings from your ears
(test it using your fingers first). I am not responsible for any Vincent
van Gogh outcomes. To make this meaningful, determine the force required
to break the strings by static loading. Determine the force to break the
stick by static loading. Compare. Draw some conclusions. Try heavier
sticks. This can be done to break a broomstick, but requires practice (and
a lot of broken broomsticks) to ensure that the blow is swift enough and
done with a rigid instrument. Of course you will mention the Karate
connection: the demo of breaking boards with the edge of the hand, the
boards being stacked with thin wooden strips under the ends. Why don't you
ever see it done without the wooden strips? Why don't you see it done with
3/4 inch plywood? (Better also mention cutting the boards so that the
grain doesn't run end-to end, but side-to-side. And baking the wood
thoroughly dry in a slow oven in advance of the demo. Ah, details.)

I submit that with measurements included as indicated, this is a
respectable and worthwhile laboratory exercise.

And what useful physics do we learn about inertia from any of these? 
Nothing. Do these demos answer these questions: "Why don't the strings
break?" "Why doesn't the candle, or beaker, or chip stack topple?" To give
the answer "Because of inertia" is no explanation at all. These demos cry
out for more detailed analysis, and that analysis and the resulting
explanations will involve forces, momentum and the impulses due to forces. 

All of these raise an important question. "Why does the speed of the pull
matter?" How often do students raise this question? How often do
demonstrators mention it--and do the things to reveal what's going on? THAT
is an important feature of these demos, and it has nothing to do with
inertia. If you call this an inertia demo, then the speed of the paper
yank is a complicating factor, distracting attention from the inertia
question. The speed of the yank is demonstrating some good and important
physics, and to ignore that is to throw away a valuable part of this demo. 

	-- Donald

......................................................................
Dr. Donald E. Simanek                            Office: 717-893-2079
Prof. of Physics                    Internet: dsimanek@eagle.lhup.edu
Lock Haven University, Lock Haven, PA. 17745          CIS: 73147,2166 
Home page: http://www.lhup.edu/~dsimanek            FAX: 717-893-2047
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