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Re: Inertia demos.





On Sat, 29 Nov 1997, Tom McCarthy wrote:

I believe they all have some educational value, but this, as Donald
points out depends on the investment of answering a number of critical
questions. I brought up this topic because I started second guessing
the worth of these fun demos after hearing someone say they aren't
demonstrating what they were intending to. In the desire of fostering a
better understanding, I would be willing to drop certain tricks of the
trade and look for others to replace them. But, I need to be convinced
they are not serving their purpose. Like the Lenz's Law demo, I have so
much fun with this one that I'd hate to give it up if it is really much
more complicated than I am making it out to be.

There's an old saw in physics: "Every experiment demonstrates something.
If it doesn't demonstrate what you intended, it demonstrates something
else." My quibble with this demo was the claim that it is effective in
giving students a better understanding of *inertia*. Actually, once you
refocus its purpose more broadly, and take the time to do the tablecloth
pull with slow, fast and jerk, then analyze the different outcomes, it
becomes an excellent demonstration of the properties of impulse Ft, and is
therefore still worth doing. Indeed, the analysis is effective and
valuable even if the word "inertia" is never uttered.

And I admit, sheepishly, that I hadn't realized *this* value of the demo
until you raised the question, Tom. I had dismissed it as mere
showmanship, and thought the real explanation might be too messy to
discuss. Then when you made us think about it I realized what a neat demo
if impulse it was, and there are darn few good demos of impulse which
separate the effects of F and t in Ft.

But unless these matters are brought up and discussed with the class, I
think it *is* mere showmanship.

-- 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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