Re: [Phys-L] inertia and the tablecloth demo

[John Denker <jsd@av8n.com>]

On 08/19/2016 03:42 PM, Anthony Lapinski wrote:
> A book rests on a
> table. If the book's weight (true weight, mg) is the action force, then the
> reaction force is the ___ on the ____ acting _____.

That's a fill-in-the-blanks question.
As written, it's ill-posed.

On 08/19/2016 07:12 PM, Anthony Lapinski wrote:

>  It's a MC question with four choices.

Well, that's a different question.  Much depends on what
the four choices are.  Maybe they narrow the scope to the
point where what remains is well-posed ... or maybe they
don't.  Details matter.

> weight is Earth pulling down on the book

That seems like lawyer games to me.

When you weigh something, do you measure the weight?
Most people would say yes.  Nitpickers might say no,
but I always try to discourage that kind of nitpicking.
If you say yes, then weight is the force on the scale,
to a good approximation.

Maybe in the teacher's mind the emphasis was on weight
as in the gravitational interaction, or maybe it was on
weight as in weighing.  The student cannot figure this
out without reading the teacher's mind.

So I say this is an ESP exam, not a physics exam (unless
the as-yet-undisclosed MC answers are constructed to
sidestep one of the interpretations).

------------

More importantly:  I try to design questions so that the
question and the answer are interesting and important.
I just don't see the importance here.  If the book/table
force is numerically equal to the book/earth force, why
make a fuss about the distinction???  In the real world,
the usual goal would be to calculate one of those forces.
The natural real-world question would ask about either
the book/table force or the book/earth force, naming the
desired force unambiguously ... not calling it "the
reaction force".

I still say that a student could understand everything
that is important and interesting about this physical
situation and still get the question wrong.

I might add that operationally, you would have a hard
time measuring the gravitational force on the earth in
the field of the book.  What are you going to do, measure
the acceleration of the earth?  This is another reason why
a student who focuses on what's important will gravitate
toward the book/table interaction.

Suggestion:  When designing or vetting questions, ask
 -- which forces do we really care about, and
 -- which forces are really measurable, operationally?

-------

At this point the teacher may say, "I needed to have a
question about action and reaction, and this is the best
I can do."

I sympathize with that, up to a point.  It does not
reflect a weakness of the teacher, but rather a weakness
of the action/reaction formalism.

Seriously, there are lots of real-world situations where
/nobody/ can keep track of the action/reaction pairs.
Fluid dynamics is an example that springs to mind.  The
conventional and sensible approach is to formulate it
in terms of momentum conservation and momentum transfer,
not in terms of action/reaction.  Once just for fun I
tried to rederive the equations using action/reaction,
and it turned into a horror show.

--------

We tell ourselves that the basic laws are important
because they always apply.  Well, yes and no.  Just
because a certain formulation of a certain law can be
applied doesn't mean it should be applied.  There may
be other formulations and/or other laws that are more
useful.