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

[John Denker <jsd@av8n.com>]

In the context of:

[1]   Michael McCloskey
        "Intuitive Physics"
        http://homepage.psy.utexas.edu/homepage/faculty/Markman/PSY394/McCloskey_IntuitivePhysics.pdf

[2] Eric Mazur
        The Make-Believe World of Real-World Physics
        http://mazur.harvard.edu/sentFiles/MazurTalk_1396.pdf
        Millikan prize lecture, sharply critical of reference [1].

[3]  Stephanie Chasteen
        A blog entry praising reference [2].


On 07/09/2013 12:07 PM, David Craig wrote:

> A recent post highlighted this blog entry on a presentation by
> Mazur:

  (reference [3])

>  I invite you all to consider the example from the presentation the
> blog highlights under the heading "Physics is Different".    I have
> some fairly strong opinions concerning the supposed "lessons" of this
> example and ones like it, but before I inject my views, I'd be
> interested in some discussion by the list.

I have a preference for primary sources.  I will mainly discuss what
is actually said in references [1] and [2] ... rather than what other
folks have said about it.

David Craig calls our attention to the diagram wherein a walking or
running person drops a ball.  The figure originated in reference [1].
The discussion on pages 41-80 of reference [2] discusses this figure,
citing it as an example of wrong physics and bad pedagogy.

=======

Before going on, let me foreshadow several points I would like to
make:

  1) Check the work.
  2) Check your work.  This is the cornerstone of critical thinking.
  3) Also check the other guy's work.
  4) Also check for errors in the errata.  If guy "B" says that guy
   "A" is wrong, it may be that they are both wrong.  Life is not a
   multiple-guess test.  Manichean multiple-guess thinking rots the
   brain.
  5) Being right about the broad principles does not give somebody
   a license to be wrong about the details ... or vice versa.
  6) Most people are really bad at handling ill-posed problems,
   especially in a classroom setting.  Even people who could handle
   such problems in a real-world setting go haywire in the classroom.
  7) Check the work already.

Ironically, both reference [1] and reference [2] are in the same
boat: Each is right as to the broad principles, but wrong as to the
details.

In particular, Mazur is right that there is a treeeeemendous amount
of nonsense in typical physics classes and physics textbooks.  I'm
talking about
 a) some stuff that is just plain wrong,
 b) some stuff that is 100 years out of date,
 c) some stuff that is ludicrously impractical, for no good reason,
 d) some discussions and diagrams that are confused and confusing,
 *) et cetera.

The part of reference [1] that Mazur chooses as his flagship example
is wrong; however, it is not wrong in the way he says it is.

If you pay attention to the details, you discover that reference [1]
describes the person in the diagram as walking, not running.  This
means that reference [1] is flagrantly wrong about the physics.
There is no way that a dropped ball lands two paces farther down the
road (unless there is a howling tailwind or other wildly abnormal
circumstances).

However, Mazur's version of the quiz-question is different.  He uses
the same figure, but changes the description.  He describes the person
as running (rather than walking).

Reference [3] attributes to Mazur the assertion that
  "The runner would have to be running at /25 miles per hour/"
in order to obtain result (C) as shown in the diagram.  Emphasis in
the original [3].  Perhaps this is a misquotation, but I doubt it,
because the key point is that this is an impossible speed.  Alas, when
I do the calculation, I get a speed closer to half that ... which is
by no means impossible.

  Details: In particular, let's say the ball is dropped from a height
  h.  It appears that h is on the order of 1 meter, but my conclusions
  are only weakly sensitive to the exact value of h.  The ball travels
  forward by an amount 2.66 h, as you can verify by measuring the
  diagram.  A fast runner could easily travel 2.66 meters in the time
  it takes something to fall 1 meter.  The problem with Mazur's
  version of the quiz-question is that we don't know how fast the
  runner is going.  So the question is seriously underspecified.
  Also, if we have a fast runner and a low-density ball, air
  resistance becomes a nontrivial issue.

  Reference [3] attributes to Mazur the assertion that
    "The runner would have to be running at /25 miles per hour/"
  in order to obtain result (C) as shown in the diagram.  Perhaps this
  is a misquotation, but I doubt it, because the key point is that
  this is an impossible speed.  Alas, when I do the calculation, I get
  a speed closer to half that ... which is by no means impossible.

  At the next level of nit-picky detail, one could argue that it would
  be hard to run fast using the gait shown in the diagram, because the
  stride is too short.  This would be hard ... but again not impossible.

  ==> Assuming the intent is to illustrate Galileo's principle of
   relativity (or -- equivalently -- the first law of motion), you
   could make this quiz question very much better by specifying that
   the runner is going very fast.
  ==> Then you could improve the diagram by showing the runner with a
   much longer stride.  This would be super-easy to do.
  ==> You could also specify a dense, heavy ball.

Because the question is so underspecified, it becomes a telepathy
exam.  That is to say, the only way to figure out the intent of the
question is to read the mind of the person who cobbled it up.  Mazur
claims that "B is closest" to the right answer.  Alas, this is true
under some assumptions but not others.

This explains why laypersons prefer one answer while physicists prefer
another: They make different assumptions about the state of mind of
the person asking the question.  I see no clear evidence that they
make different assumptions about the laws of physics ... although
there could be some of that also.

Mazur is however right about the larger point: Examples like this make
physics seem weird and disconnected from the real world.  Even so,
this does not give him a license to get the details wrong.  He should
criticize the original version of the question for what is *actually*
wrong with it.

Both versions of the question could be fixed so as to correctly
illustrate the principles of physics as intended.  However, that is
not the whole story.

As I see it, the inescapable issue here is that situations where
something is passively dropped from a moving platform are not
particularly common.

  --> Dropping bombs from an airplane is a real-world example
   in this category, but most people have very little experience
   with this.

  --> The quintessential case concerns motion within a uniformly
   moving vessel or vehicle.  This has been central to physics since
   Day One of modern science (1638).

   However, this case is so easily analyzed in the comoving frame that
   laypersons rarely think about how the situation looks in the lab frame.

To summarize:
 *) The original question in reference [1] is definitely wrong physics.
 *) It would be interesting to conduct a survey using the original
  ("walking") version, to see if physicists can get the right answer.
 *) The criticism found in reference [2] is off-target and incorrect
  as to details.
 *) The modified ("running") question as found in reference [2] is so
  underspecified that we cannot be sure whether or not the physics is
  wrong.  In any case, it is a telepathy exam, and the analysis in
  reference [2] is very incomplete.

Mazur could have made his point about wrong physics using other
examples, of which there are many:
 -- billiards problems written by people who obviously have
  never seen (let alone played) a game of billiards
 -- sig-figs rules vehemently emphasized by people who obviously
  don't know the first thing about numerical methods
 -- airplane problems written by people who obviously don't
  know the first thing about airplanes
 -- thermodynamics problems written by people who obviously
  don't know the first thing about entropy
 -- bogus electrical laws routinely "proved" by people who obviously
  have no clue about the principles involved
 -- et cetera.

There are thousands of these things running around.  Just now I
flipped open Knight to a random page, in this case page 162, and
the first thing that caught my eye was a velocity-versus-time
profile that requires infinite acceleration.  Gack.

In addition, there are things that are not strictly wrong, but
are just 100 years out of date.  These might as well be wrong,
because they have to be unlearned before the modern, useful ideas
can be learned.

On top of all that is a problem that is slightly less acute but more
pervasive, namely the category of problems that aren't wrong physics,
but are nevertheless ridiculously hokey.  The runner dropping a ball,
even if you get the physics right, is hokey, and therefore bad
pedagogy.

There are thousands of these things running around, too. For example,
on page 163 of Knight there is a pulley arrangement that is just
ridiculous.  It pretends to be a real-world rich-context example, but
it's three times as complicated and incomparably less useful than the
arrangement that is used for practical applications.  I have some
sympathy when slight errors are introduced in the name of pedagogical
simplification, but that's not what's going on here.

And then there is the archetypal example: Monkey-shooting.  Yeah,
things could happen that way, but no sane person would allow them to
happen that way.

Physics principles actually are important ... so why not illustrate
them with /important/ examples rather than ludicrously contrived,
impractical, and irrelevant examples?