Re: acceleration

[Rick Tarara <rtarara@SAINTMARYS.EDU>]

While it is clear that there are many approaches and many starting points
and many philosophies being expressed here, discussions of teaching about
motion almost always touch on the ball at the top of it's trajectory as a
major stumbling block.  While I personally can't see any advantage to
describing the motion with momentum versus acceleration, with scalar
acceleration versus vector acceleration, I do see this particular motion as
a very important example and assessment tool for student understanding.

The first semester of my gen-ed course is sub-titled 'A study of motion' and
the primary motion IS a ball thrown vertically upwards.  (I always come to
class armed with a bowling ball--primarily used to wake up the class by
dropping it--and a 6 cm super-ball.)  I throw the small ball up and catch it
hundreds of times during the semester.  In my curriculum we start with
trying to describe the motion--speed first, changing speed next, direction
and velocity, then change in velocity--acceleration.  I develop the idea of
instantaneous velocity through a thought experiment to measure the speed of
a car as it passes mile-post 65 on the near-by toll road.  Then this idea is
brought to analyzing the motion of the ball around the peak.  We plot
position versus time, the velocity versus time.  We work in lab on motion
graphs and the meaning of slopes.  We start right up front with the vector
nature of acceleration--things speed up when the acceleration is in the
direction of motion, things slow down when the acceleration is opposite the
direction of motion (and reserve changes in direction for later).  Then we
introduce forces and go back to the ball.  Now we look at the force acting
on the ball throughout the holding, throwing, rise, fall, and catch.  We now
have a rationale for WHY the ball is accelerating downwards throughout its
flight.  Then comes work and energy.  We look at the work being done on the
ball--in detail.  We look at the work to kinetic to potential to kinetic to
work to thermal transfers.  We can also look at the whole motion from the
point of view of momentum armed with the knowledge that this is an alternate
way (in fact Sir Isaac's way) to discuss Newton's laws of motion.

For two-dimensional motion, we do experiments to try and see that the
vertical and horizontal motions can be separated and then for trajectory
motion we go right back to the motion of the ball thrown up--now
superimposed on a constant horizontal motion.

So, for a very large part of a standard curriculum study of motion, the
vertically thrown ball can be used over and over as a prime example.  The
final exam will see one multi-part question that deals with all of these
topics (speed, velocity, acceleration, force, work, energy, momentum) all
applied to this motion they have seen over and over.  I won't pretend that
100% get it--even after 15 weeks of this--but MOST do.

[Dropping the bowling ball with the super-ball balanced in one of the finger
holes provides an exciting demonstration of collision phenomena.  ;-) ]

Rick

*********************************************************
Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, Indiana
rtarara@saintmarys.edu
********************************************************
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********************************************************

----- Original Message -----
Subject: Re: acceleration


> At 04:52 PM 11/19/2003,  Matt Jusinski, you wrote:
>
> [J Denker]
> > > In my book, there is such a thing as scalar
> > > acceleration
> > > which means an increase in speed (|v|).
>
> > If the scalar acceleration is zero locally, and the
> > speed is locally zero, Why does a projectile fall back
> > down?
> >
> > It seems to me that scalar acceleration is not much
> > help in explaining most common phenomenon.
> >
> >
> > Matt Jusinski  Morris Hills Regional District
>
>
> Yes, Matt is using a short-hand, but still, a nit-pick.
> To abstract the thought:
> "If a [math construct] is zero, why does a physical
> effect happen?"
> ...Because, the models of schoolmasters do not control
> reality. One can hope they illuminate aspects of reality,
>   is all - and accept their sphere of influence - never
>   universal.