Re: [Phys-L] The Make-Believe World of Real World Physics

["Craft, Peter" <PETER.CRAFT@det.nsw.edu.au>]

In the spirit of JD's suggestion "More generally, we
should look very hard at any and all questions that consistently
stump the students.  I hope other folks will post some more stumpers."

The following question has been discussed by the science teachers at our school, arguing about whether it is a good test question or not.  I am very interested in other's input. As i cannot put graphics in the post it will require a very little imagination.

At that top of the page is a graph showing the output on a CRO for a constantly rotating coil in a particular generator. The question specifically states the emf is being measured. The graph is a uniform sine curve with 4 or 5 complete cycles shown. Beneath the graph is a statement saying that it is impossible to state with certainty that the output is being produced by an AC generator, that it may be a DC generator causing the output on the CRO. They are then asked to justify this statement.

I am putting this up for comment, as my staff disagree about the validity of the question.

Regards

Peter Craft
Head Teacher Science
Corowa High School

Phone 02 6033 1889
Fax 02 6033 1889
________________________________________
From: Phys-l [phys-l-bounces@phys-l.org] On Behalf Of John Denker [jsd@av8n.com]
Sent: Tuesday, 30 July 2013 9:13 AM
To: Phys-L@phys-l.org
Subject: Re: [Phys-L] The Make-Believe World of Real World Physics

You might be a redneck physicist if you always keep
a loaded trebuchet in the back of your truck.



On 07/29/2013 07:29 AM, William Maddox wrote:

> If you do not like the
> questions on Lapinski's list perhaps you could provide some questions
> you have asked on an introductory physics test.

In some sense I do like that list of questions.
They are very thought-provoking.  They are exactly the sort of
thing we should be discussing in this forum.  More generally, we
should look very hard at any and all questions that consistently
stump the students.  I hope other folks will post some more stumpers.

IMHO at the end of the course, physics should be intuitive, so we
really need to understand why students find some questions hard
and/or counterintuitive.

Sometimes questions are hard for a good reason ... but sometimes
they are hard for silly reasons, such as ambiguous terminology,
in which case we should rephrase the questions.  I have already
suggested ways of rewriting some of them.  Continuing down that
road:

On 07/29/2013 12:52 PM, Chuck Britton wrote:
> Folks on the street indeed are likely to think of acceleration =
> speeding up and deceleration = slowing down. therefore acc at top =
> zero.

Yes!  That's true and important.

My point is that the FotS (Folks on the Street) are not wrong.
 1) Velocity is v;  the rate-of-change of v is called acceleration.
 2) Speed is |v|, the rate-of-change of |v| is called acceleration.
  This definition is different, but not wrong.  The concept of
  scalar acceleration / deceleration is perfectly reasonable and
  widely used.

The fact that we have one word with two different reasonable
meanings is not the students' fault!

On 07/29/2013 02:19 PM, Anthony Lapinski wrote:
> a student in a physics class should answer it differently
> (correctly). Acceleration = rate of change of velocity = gravity =
> constant (in free fall).

Let's try looking at it from a slightly different angle.

On what basis "should" the student give answer (1) as opposed to (2)?

 a) There's always the old "because I said so" rule.  Students will
  find this arbitrary and non-intuitive.

 b) I find it very hard to defend the claim that the definition (2)
  is wrong.  Different yes; wrong no.  People -- including physicists --
  use the word this way every day.

 c) AFAICT the best answer is that the laws of physics are *usually*
  more conveniently expressed in terms of v and (d/dt)v than in terms
  of |v| and (d/dt)|v|.  This strikes me as an important, non-arbitrary
  physics concept.

  My point is, the original question did not focus on this point.
  The important conceptual point was hiding behind an unimportant
  terminological snafu.

  What's worse, the original question asked about one-dimensional
  motion, where the concepts of speed and velocity are very nearly
  equally convenient, so in some sense this is the worst possible
  scenario for fishing for concept (c).

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

General suggestion:  If you want to get a better score, play a different
game.  Ask questions that probe the concepts in a way that doesn't get
tangled up in the terminology ... and especially avoids terminology
that is known to be ambiguous.

Specific suggestion:  If you are fishing for the conceptual answer (c),
ask a question that focuses much more tightly on this concept.  As an
extreme example:
  Question:  The laws of physics are usually more conveniently formulated
  in terms of
   1) the position x, the velocity v, and the rate-of-change of velocity.
   2) the position x, the speed |v|, and the rate-of-change of speed.

In general I cringe at multiple-guess questions, and in absolute terms
this is surely a bad question, but in relative terms it strikes me as
better than the original.  In particular, if somebody gets the wrong
answer to the original question, I don't know how to interpret the
answer ... whereas with this question, if somebody it wrong, at least
we know where we stand.


Similar suggestion:  If on the other hand you are fishing for the concept
that the acceleration of gravity, in the usual terrestrial reference frame,
is substantially constant over typical laboratory length scales, ask a
question that more directly probes for this concept.

For example:  An object of mass m is thrown vertically upward.  At times
a and b, it is on the way up.  At time c, it is turning around.  At times
d and e, it is on the way down.  The usual spherical-cow assumptions apply:
terrestrial laboratory reference frame, laboratory length-scales, negligible
friction, negligible buoyancy, et cetera.

                 c
                 |
                 |
                b|
                 |d
                 |
                a|
                 |e
                 |
                 |

Describe the force of gravity (direction and magnitude) at each of these
times (a,b,c,d, and e).

It seems to me, if a student gets the wrong answer to this one, we know
it is a conceptual problem, not a superficial terminological problem.

If this question seems too easy, there are lots of ways of making it
harder ... but I do /not/ recommend making it harder by hiding it
behind terminology that is known be both artificial and ambiguous.
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