[Phys-L] thrust > drag

[John Denker <jsd@AV8N.COM>]

On 07/01/05 16:04, Chuck Britton wrote:
> [0] For an airplane cruising at constant velocity - which force is greater -
> a)      the drag produced by the air
> b)      the thrust produced the jet engine.
> c)      neither is greater - they are equal.
>
> This is Newton's number ONE law. (which some pedagogs dismiss as a
> trivial special case of the Second Law.)

Hmmmmm....

The question does not look to me like a statement
of Newton's first law ... nor even a corollary.

Let's see what students would have to do, in order
to connect question [0] with Newton's laws.  For
starters, they would have to answer several background
questions:
   1) What's the definition of drag?
   2) What's the definition of thrust?
   3) What's the definition of cruise?
   4) How is the thrust line oriented in typical aircraft?
   5) How does the atmosphere affect flight?

Pray tell, how is the student supposed to know these
things?  I'd be willing to wager that most of the
teachers who assign question [0] don't know the
right answers to questions (1) through (5).

By the way, under ordinary conditions, on average,
the correct answer to question [0] is (a).  (Maybe
thrust is not much greater than drag, but the question
didn't ask about "much greater".)

If you like answer (c), you'll have to re-word the
question.

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

Questions like this give "story problems" a bad name.
That's a shame, because story problems are super-
important.  Real life feeds us a steady diet of story
problems.

In any case, if/when you assign story problems, make
sure the students have -- or can obtain -- enough
domain knowledge to be able to attack the problem.

Story problems on homework, or during in-class
discussion, are not so much of a problem, because
students can hunt up additional domain knowledge if
needed.  In contrast, story problems on closed-book
multiple-choice tests are very hard to design, and
very hard to grade.  If somebody answers (a) to
question [0], how do you interpret this:
  -- non-understanding of Newton's laws?
  -- too little domain knowledge?
  -- too much domain knowledge?

Also:  If you're asking questions about the real world,
don't ask narrow, brittle questions.  Suppose thrust
is 0.1% greater than drag .... Do you put on your
"practical purposes" hat and call them equal, or do
you put on your "strict constructionist" hat and call
them unequal?  I recommend neither of the above;  I
recommend asking a more robust question, so that the
responses don't require fussy interpretation.

As a related thought:  Don't confuse story problems
with laws of nature.  The laws of physics in particular
are often very precise and/or idealized.  The real
world is much messier.
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