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[Phys-L] Re: Energy is primary and fundamental?



Whoa! Ten yard penalty on that one. I doubt that an arbitrary
assignment of a constant g to something as fundamental as
"energy" is going to be satisfying to a student first
encountering all this. That's kind of an "It works because I say
so" approach.

Bob at PC

-----Original Message-----
From: Forum for Physics Educators [mailto:PHYS-
L@list1.ucc.nau.edu] On Behalf Of Ludwik Kowalski
Sent: Wednesday, August 17, 2005 4:06 PM
To: PHYS-L@LISTS.NAU.EDU
Subject: Re: Energy is primary and fundamental?

On Wednesday, Aug 17, 2005, at 14:50 America/New_York, rlamont
wrote:

What is your approach to the following simple problem? A ball
is
dropped from rest and falls for 5 seconds. Without using
acceleration, just energy, calculate its speed at the end of
the
5 seconds?

To answer this question one must know a relation between the
time if
fall t and the initial elevation, y. Suppose that after
performing many
experiments (in a vacuum tube) one discovers that y is
proportional to
t^2, and that the coefficient of proportionality is 4.9 m/s^2.
I am not
calling the coefficient of proportionality "acceleration." The
rest is
trivial. t=5 s leads y=122.5 m. Then, from m*g*h=m*(v^2)/2 one
gets
v=49 m/s^2. What prevents me from saying that g is constant
chosen
to
define potential energy? We simply like it to be be two times
larger
than the empirical 4.9 constant of proportionality. And the
fact that
the unit of g is the same as the unit of our constant is a pure
coincidence. No, I am not suggesting that physics should be
reduced
to
that kind of nonsense. What is wrong with teaching kinematics
first and
with defining energy in terms of work later, as in most
textbooks?

Ludwik Kowalski
Let the perfect not be the enemy of the good.