Re: inertia demo

[David Bowman <dbowman@TIGER.GEORGETOWNCOLLEGE.EDU>]

Regarding John D.'s comment:

> ...
> A 10kg block of dry ice has noticeable inertia and (if
> you do it right) has impressively low friction.  But cars
> have pretty low friction, too, and you can borrow a
> megagram of car more easily than you can lay hands on
> large pieces of dry ice.

I suspect that the desirable goal of a low friction/resistance
condition for the car could be significantly enhanced if the car had
recently been driven so as to minimize the problem of cold tires
having a rigid flat spot on the bottom which would serve to impede
the initial motion.  Also significantly overinflating the tires could
help serve to reduce the rolling resistance.  Having a flat paved
gravel-free surface is also desirable.

Having the students push on a railroad car on a flat track in a rail
yard would be nice if such a thing could be arranged.  But this is
very doubtful.  It's hard to imagine the liability minded lawyers for
a railroad (or a school district) giving a pass to such a field
trip scheme.  But the small contact deformation and low rolling
resistance of steel wheels on steel rails is hard to beat (short of
going maglev).  For a railroad car with decently unworn round wheels
I would expect that the largest source of friction/resistance may
even come from shearing the stiff axle grease in the wheel bearings
(unless the rails have some unfortunately mismatched joints nearby or
if the wind is blowing too strongly, or if the track is curved and
the inner lip of the wheels is rubbing against the inside of the
outer rail).

Overall I suspect a big chunk of dry ice may work better than the car
in terms of being a hands-on example of big frictionless inertia, but
as John says, the car is probably easier to come by.

> *) Terminology: Note that I said "slow down" rather than
> "decelerate".  I'm becoming more and more convinced that
> "deceleration" is a dirty word.

Why is this word any dirtier than the phrase 'slowing down'?

> Any (any!) change in velocity is an acceleration.  An acceleration
> in the direction opposite to the velocity is still an acceleration;
> it causes a decrease in speed.

I realize that John has not advocated banning the word 'decelerate'
from physics classes; he just claimed it was a "dirty word".  But I
want to take the opportunity to spout off, anyway, on the practice
of banning certain words from the physics classroom.  I'm sort of
dubious of attempts to forbid the use of certain vernacular terms
(heat, decelerate, centrifugal force, etc.)  My attitude is that it
is best to defuse the potential of vernacular words for harboring
or abetting misconceptions by confronting head-on any possible
confusion that may be contained in the word or caused by a mismatch
between a word's vernacular meaning and its technical physics
meaning.  I prefer to render the potentially offending word
(misconceptionally) impotent by *carefully defining* its meaning
with sufficient emphasis.  In the case of the word 'decelerate' I
would prefer to *define* it as meaning exactly slowing down or
decreasing the speed.  The opportunity can then be used to explain
how such a situation happens when the acceleration vector and the
velocity vector have an obtuse angle between them (i.e. a negative
dot product).  Similarly, I would explain how speeding up occurs
when these vectors have an acute angle/positive dot product.  I
would certainly *emphasize* that the vernacular use of the word
'acceleration' and the physics use of the same word are *different*.
The vernacular meaning of speeding up *must* be distinguished (in the
students' minds) from the physics technical meaning of the rate of
change of the velocity.  I would prefer to expend instructional
energy on emphasizing the definitional nuances that lead to confusion
(if they go unnoted) to expending it on enforcing a ban on vocabulary
terms that have been placed on some index of forbidden words.
Besides, once the students leave the protective confines of their
physics classes, they will co back to the outside world where the
banned words and vernacular meanings are still used all the time
anyway.

> This posting is the position of the writer, not that of Moe, Curly, or
> Shemp.

I was hoping this example would have come up soon.  But what about
Larry?

David Bowman

This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.