Re: Inertia and Lenz's Law

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

On Thu, 27 Nov 1997, Tom McCarthy wrote:

>     If anyone has a great inertia demo they wish to share, I would like
> to hear about it.  I have one that astounds students.  I get flat poker
> chips, and stack them in alternating colors (using school colors, of
> course).  With a steel rule that has a thickness that is less than a
> chip, I rapidly wave it back and forth, each time knocking out the
> bottom chip.  It sorts them and astonishes the students.  But, like the
> tablecloth, is this not really a demo of the Property of Inertia? 

It can be, if presented in that light. I turn to one of the books in my
"inspirational" physics library, _Demonstrations in Physics_ by Julius
Sumner Miller (Ure Smith, Sydney, 1969). In this book the demonstrations
are described, with pictures, and several questions are posed about each.
No answers are given. Here's the questions he asks about this one.

1. If a ruler or table-knife blade strikes the lower-most checker smartly,
horizontally, that checker may be dislodged without the stack toppling
over. Why is this?

2. If this operation is continued is the success likely to be greater or
less? Explain this.

3. What determines the _least_ velocity the ruler may have to drive a
checker out?

4. The same may be accomplished with a string held in both hands and
pulled smartly against the bottom checker. Examine the tension which
arises in the string.

Now *this* is the way to make a demonstration meaningful--to put some
physical thinking into it. 

Question 2 gets at the inertia of the stack above the one being knocked
out.

Question 3 gets at the fact that the impulse depends on the time it takes
to slide the bottom chip out, and that depends on the velocity of the
ruler. The faster ruler slides it out quicker, and the impulse Ft is
smaller, since F depends only on the coefficient of friction and the
weight of the stack. The minimum impulse required to topple the stack
depends upon the mass of the stack, i.e., its inertia. Now we get to
rethink our answer to Question 2. 

As an afterthought to the original question, I wondered, "What *is* the
law of inertia?" I frankly didn't know. None of the textbooks I use
mention it by that name. I had to go to Hewitt's book to even find it
mentioned.  It is, he says, Newton's first law. This made me think of our
old bugaboo, objective tests. If I took an exam in which someone asked me
about the law of inertia, I'd miss that question. Does that mean that I'm
ignorant of that part of physics and couldn't do a problem in inertia? I
think not.  Was that a good question? I think not. If someone asked "What
is the measure of inertia?" I'd have answered "Mass." And if someone had
asked "What laws allow us to measure inertia (mass)?" I'd have to answer
"Newton's *second* law." Newton's first *and* second laws are required to
understand inertia, and any demonstration of inertia must necessarily
involve the second law. Come to think of it, any demo which involves *two*
bodies must necessarily require Newton's third law as well. 

To speak of the "law of inertia" as synonymous with Newton's *first* law
is misleading and useless, so I'm not surprised that Hewitt does just that
(Thinking Physics, p. 16.). If someone wants a demo of *that*., that's
simple.

Put a heavy object on the table. Point to it and say. "There's no net
force on it. You will notice that it's not moving. Q.E.D."

Of course you are then obligated to demonstrate that there's no net force
on it, and someone may point out that it, and everything in the room is
moving.

	-- 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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