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

[John Denker <jsd@av8n.com>]

On 08/19/2016 08:46 PM, Anthony Lapinski wrote:
> I first teach about true weight (mg -- downward) and apparent weight (scale
> reading, normal force -- upward). 

>  I'm sure others teach weight differently as this discussion has
> come up before on this listserv.

I for one would have a hard time with the idea of an "upward" direction
assigned to the apparent weight, or to any other kind of weight.

Where I come from, the weight vector /defines/ what we mean by "down".

> The MC answers are constructed so there
> is no ambiguity (earth on book, book on earth, table on book, book on
> table).

That looks ambiguous to me.

I predict that students who do well on other parts of the test but
get this question wrong choose "table on book" significantly more
than 33% of the time.

Rationale:  weight = book on table, counterforce = table on book.

Another rationale:  The table-on-book force is /equal/ to the
book-on-earth force.  Mathematically they are the same vector,
since they have the same magnitude and same direction.  To consider
one of them right and the other wrong is literally a distinction
without a difference.

============

I see this sort of thing all the time.  Usually it is intentional:
I assign a question that is known to have eight different correct
answers.

Occasionally it is unintentional:  Suppose I really want and expect
a particular answer and a particular method of solution, to make an
important point or exercise an important technique.  If some student
finds an unexpected solution, I cheerfully mark it correct.  (I reserve
the right to ask followup questions so that the important points get
covered eventually.)

What I /expect/ has no bearing on the correctness of the student's
work.  It's not an ESP exam.