[Phys-L] Re: Energy is primary and fundamental? (was RE: First Day Activities or Demos)

[rlamont at POSTOFFICE.PROVIDENCE.EDU (rlamont)]

> So far all you've done is to criticized the Modeling approach
to handling
> the concept of energy.  How about offering something concrete
as a
> superior
> alternative, and I'll see if I can pick some holes in it?

As you can probably tell, I don't start my course by addressing
energy. But I do get to it in due time.

My opening exercise is to have the students whack at a bowling
ball with a soft foam waffle ball bat. One student strikes the
ball while calling out a steady rhythm of 1 and 2 and 3 and 4,
etc., striking the ball when each number is spoken. The other
students line the corridor. As the ball rolls by them, the
student nearest the ball when a 5, 10, 15, etc., is called out
places a coin on the floor where the ball was at that time. We
then record the number of tiles the ball has moved versus the
number of hits or "time". When the students can do this
reasonably well, we repeat the experiment, but this time the ball
is batted when both the numbers and the words "and" are spoken,
doubling the rate at which the batting force is applied.

The student now spend the rest of the class (and part of the next
one) analyzing the motion. I have the students plot the speed of
the ball (in tiles/hit). I give them no definitions of how to
calculate or plot the speed. This usually results in a lively
discussion amongst the work groups as to whether they should plot
the whole distance traveled versus the total number of hits, or
only the distance traveled in one 5 hit period versus a 5 hit
time interval. They invariably conclude that the 5 hit approach
makes more sense to them. I stay out of that argument.

I then have them come up with their own definition of
acceleration and then have them plot it versus time. They usually
find that the acceleration is fairly constant - implying that a
constant force produces a constant acceleration. They also
analyze the case where the ball was batted twice as often. Very
nicely, they find that the acceleration doubles - hence Newton's
2nd Law of motion.

Later on in the course, we revisit the data. They find that the
value of v squared is twice as large at the end of the corridor
than it was half way down the corridor. They also find that the
value of v squared is twice as large for the case when the ball
was batted twice as often. This leads to the quantity v squared
being related to the product of force and distance. This
introduces kinetic energy. The usual free fall experiments then
naturally bring in the relation between falling distance and v
squared - and the whole potential, kinetic, and total energy
conservation thing.

It's all physical and "hands-on". Energy becomes a simple
bookkeeping system that easily allows making predictions about
the dynamics of a system without fussing with the details of
vectors, constraint forces, time variation of forces, etc.

Bob at PC