Re: acceleration

["John S. Denker" <jsd@AV8N.COM>]

On 11/26/2003 11:20 PM, Steve Clark wrote:
> > In the worst cases they reverse sign conventions at the top!
>
> I don't [teach that]  but they do it anyway.

Right.

> I don't know why.

Allow me to state the obvious hypothesis/conjecture:

Some students are naturally attracted to analyzing
things in the frame comoving with the object.  The
typical 15 year old student has 15 years experience
of riding in chars and 10 years of experience riding
bicycles.  They also know that astronauts are
weightless in the frame comoving with the space
station.

If you ride a bike to the north end of the block and
back, you don't face north and go forwards on the
outbound trip and face north and go backwards on
the return trip.  You turn around so as to always
face the direction you are going.

This is not just a matter of habit, there are good
reasons why the bicycle (and the rider == observer)
face the direction they are going.  Also note that
the rider is not particularly interested in the
forces as perceived in the lab frame ... the rider
has good reasons for knowing and wanting to know
the forces in the comoving frame.

I am still not repeat !!not!! suggesting that we
spend time teaching intro-level students how to
analyze things in the frame of the moving object.
I am merely observing that there are good reasons
why the students would naturally want to attempt
it.  Everybody on this list knows that for most
questions it is easier to get accurate answers in
the lab frame ... but the students don't know that
until *after* they've been taught.

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

A tangentially-related remark:  When teaching students
to use the lab frame, please don't overdo it.
 -- Bad:  "There is no such thing as an accelerated
reference frame"
 ++ Better:  "Accelerated reference frames are
beyond the scope of this course."
 -- Bad:  "There is no such thing as a centrifugal
field."
 ++ Better: "There is no centrifugal field in a
nonrotaing frame of reference."

I have been in the following situation:
 --- The HS football coach uses the term "centrifugal
force."
 --- The bright young student says "Mr. Wilberforce
the physics teacher told us 100 times, there is no
such thing as centrifugal force."

I hate being in the situation of telling the student
that the football coach is right and the physics
teacher is wrong.

This situation is easily preventable.  "There is
no centrifugal field in a nonrotating frame."
...                  ^^^^^^^^^^^^^^^^^^^^^^