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Re: Inertia and Lenz's Law





On Thu, 27 Nov 1997, Tom McCarthy wrote:

I was at a talk where a physicist stated that pulling a table cloth out
from beneath a set table was not truly a demonstration of the property of
inertia. What is your sense of this?

This discussion should be interesting.

First, we ought to ask, "What is *the property of inertia*?" If we wish to
demonstrate something, we ought to first clearly define what it is.

The measure of inertia is mass. If we are going to demonstrate anything
about interia, we must show the dependence of something on mass. The
tablecloth demo doesn't conclusively do this. Objects of different mass on
the tablecloth may respond differently, but this usually isn't mentioned,
noticed by the audience, or measured. So, I have to agree, it tells us
little about inertia.

So what *does* this demonstrate? It seems to me that any demonstration
worth doing should have an explanation, in physical terms, at a level
understandable to the student. To do a demo without a proper explanation
afterward (perhaps as a result of student discussion) is a waste of class
time. We waste a lot of class time in order to entertain students. This is
why I refuse to have anything to do with demo "shows" for students in
which no detailed explanations and discussion are included. They are
hardly more than "stage magic" shows. Such extravaganza shows for physics
teachers are ok, for one presumes the teachers know the explanations
without need for discussion. Or do they?

I was tempted to say it demonstrates Newton's first law: "A body at
rest..." But that would only be strictly true if the friction of the cloth
were zero. It isn't.

Now, if properly done, one should always first do the experiment of
pulling the cloth slowly, showing that the objects on the cloth move with
it, and do not slide. (How many times have I seen this demo performed with
that important first step omitted!) Then jerk the cloth, and they stay
put (more or less). Aha! the rapidity of the impulse has something to do
with it! Starting friction vs. sliding friction? No, the difference
between the two coefficients isn't *that* great. This demo's explanation
is not so simple. Could this be a demo of a property of the "jerk"? :-)

Also, in my opinion (not shared by many), we should do demos in which the
better students have a good chance of figuring out the outcome and the
explanation (using physics) even before the demo is performed even if they
have never seen it before. In this case, about a third of college students
will have seen it before, but probably won't be able to give a real
explanation, other than to parrot the meaningless slogan "It's because of
inertia."

And the better demos should be clean and uncomplicated by confusing
factors. In this case the coefficient of friction and the law of friction
come up. The center of mass height and base dimensions of the objects on
the cloth will influence their probability of falling over.

How often we do demos without using them as a vehicle for critical
thought. Someone should write a book on the use *and misuse* of
demonstrations.

Oh, yes. An explanation. The force due to friction on an object on the
cloth is dependent on the object's mass, and the coefficient of friction,
but not appreciably on the velocity of the cloth. Thus the *impulse* (Ft)
given to the object by the cloth is dependent on the time the cloth is in
contact. Slide it out quickly and the impulse is small, causing little
change in momentum of the object on it. So it demonstrates (sort of) that
sliding friction isn't very dependent on velocity.

-- 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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