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Re: Work/Energy theorem ?



Dario wrote:
A few days ago, JGreen asked * is the work/energy absolute ?

I was no able to answer since I have no idea what * absolute *
means. However I share with you a question I invented and use
in home-works. It may be related to Green's question. But even
if it does not, it is fun.
-----------------------------------------------------------
THE QUESTION:

Charles in riding a train and watching the outside panorama.
He notices a specially beautiful tree and been a student of
physics, thinks: "that tree has kinetic energy".

After a while, the train stops and looking the same tree again,
Charles thinks: "Now the tree is at rest ! Since there are no
changes in the potential energy of the tree, where dit it's
kinetic energy go?"
-------------------------------------------------------------

Dario Moreno

Ancora Bravo!
Another good example of the pseudo problems one will struggle to
solve if one endows every mathematical entity (energy, force . . . )
with a substantive existence so that it becomes endowed with properties
such as position and velocity. Eg.: the velocity v in P=F.v is a
property of the object experiencing F; it is not a property of F (which
is not a substantive entity).

....and let me add my compliments! A farting fly can achieve the power*
of a railroad locomotive when seen to emit while moving sufficiently
rapidly! I have to teach my students that energy is insubstantial; that
there is no such thing as "pure energy". I wish they had not picked up
that particular concept. Caloric does not belong in a modern curriculum.

If you want to see energy done well, see Feynman Volume 1 on the
conservation of energy. He has a parable (Dennis and his blocks) which,
as usual, is conceptually right on the money. I always read this
section verbatim to my introductory students, knowing full well they
will not look it up if I recommend it**. The math should not scare
even those delicate sensibilities Hewitt tries to protect. Try it;
you'll like it! Please compare it to Hewitt and let us know what you
think.

Leigh

*A rocket ship is "powered" by an engine which exerts a thrust (reaction
force) F. If the rocket is moving with speed v it gains kinetic energy
at a rate P = F.v

**But a physics teacher who hasn't read it surely will look it up.