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

[kowalskil at MAIL.MONTCLAIR.EDU (Ludwik Kowalski)]

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.