Re: [Phys-L] kinematics objectives

[John Denker <jsd@av8n.com>]

On 05/09/2013 08:41 AM, Philip Keller wrote:
> Maybe I should have said "build intuition" rather than "target
> misconception".  

Good.

> Because in many cases, my students come in with no
> sense of the subject at all.  They have no misconceptions about
> momentum, energy, charge, electric fields, vectors...for all of those
> things, they are a clear slate, 

That's about 99% right.

As Joe B. pointed out, there is never a /completely/ clean slate.
Indeed, you wouldn't want one if you could get one.  The trick is 
to find "something" good on the slate -- however vague and inchoate --
something good that you can reinforce and then build upon.

> But they most certainly do believe that objects prefer to be at rest.

The following is a somewhat eccentric opinion, but let me mention it
anyway:  IMHO it's not even a misconception to say that objects in
motion tend to come to rest;  it is at worst an ambiguity.  Consider
the contrast:
  a) If you are a flagellate bacterium, then you live in a world
   with a verrrry low Reynolds number.  Friction is dominant, and
   inertia is an utterly negligible correction.
  b) If you are an aircraft, your Reynolds number is much higher.
   Inertia is dominant, and friction is a relatively minor
   correction term.

In the introductory physics class, we choose to start from the
low-friction case.  Students' intuition about the high-friction 
case is not wrong;  it's just incompatible with our chosen starting
point.

========

I take a similarly tolerant attitude toward acceleration.
  A) there is a vector velocity
  B) there is also a scalar speed
  A') there is a vector acceleration, i.e. a change in velocity
  B') There is also a scalar acceleration, i.e. a change in speed

Textbooks like to pretend that B' is crazy wrong, but it's not.
It's just ambiguous.  I hear professional physicists use the terms 
acceleration and deceleration routinely, to refer to changes in
/speed/.

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

There is a pedagogical / psychological dimension to this.  There is
a mountain of evidence suggesting that established ideas are virtually 
never truly unlearned, not on any pedagogically relevant timescale 
anyway.  Instead the best you can hope for is to hide the bad ideas
behind a wall of better ideas, so that in any given context the right
idea is more likely to be recalled, instead of wrong (or merely 
inapplicable!) ideas.

A direct attack on the idea that objects in motion tend to come to
rest will never be successful, because the idea has too much supporting
evidence.  The best you can hope for is to place /limits/ on the
validity of the idea, to restrict it to tiny objects moving slowly
through a sticky medium.

By telling students their ideas are not crazy wrong, just /restricted/,
they are less likely to get defensive.  It gives them a framework
that accounts for *all* the data.  This upholds one of the core 
principles of critical reasoning:  account for *all* the data.