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Re: "Dissipation" (was Re: momentum conservation(2))



John,
I was delighted with your original question and your defense of
the use of the word "dissipation". I join others in your camp.

At 4:25 PM -0800 3/14/00, John Mallinckrodt wrote:
I want to second Jahn Barrer's eloquent defense of the word "dissipation"

Arons in "Introductory Physics Teaching" makes statements like "energy
dissipated as frictional interactions..." (p. 105)

In *Physics Today* I just browsed an article title "Experiments Reveal How
Heat is Mixed..in Ocean" with statements like "the depth integrated rate
of kinetic energy dissipation" (March p.19) is a different stretch of
the same idea [that there is energy not specifically accounted for
but tracked as missing]

I thought of your concern with this "dissipated energy" in your described
experiment. [I envision it as two PASCO carts, one initially stationary,
that experience an inelastic collision]. There is a clearly measured
conservation of momentum (I typically see results with less than 6%
discrepency). The energy conservation is not so clear. A large sum
of the KE is gone and you label it as "dissipated"

The PASCO carts, I think, have a significant amount of what you call
"dissipated" energy in the form of potential energy that "locks" the
two carts together. The "velcro" strips between the carts do this.
To test my idea you could take the locked carts and carefully (with
one stationary) pull one away from the other with a thread. You will
find that once the cart "tears free" it is launched with a small
velocity. Measuring the velocity of this "launched cart" may reveal
a major portion of your energy labeled as "dissipated". Just an idea.
Hopefully I'm not muddling this too badly. (The pull is like an activation
energy in Chemistry).

In any case the inelastic collision works to change the position of matter
(and in this case the velcro barbs are bent etc.) Cars wrap about each
other in real inelastic collisions (lots of position changes there!)
Finally you can look at collisions in terms of energy and momentum. All
interactions conserve both momentum and energy (of course) but the momentum
is always easily measured and is more "apparently" conserved in all
types of collisions. The more work done changing positions (like
velcro barbs etc.) the more inelastic the collision is and....the less
KE there is.

I hope I haven't caused too many of my students to go down the wrong
path and hurt their futures too badly. When attending the recent
wedding of an ex-student (taught him in '91) who is now working on
his dissertation (Ph.D.) at Georgia Tech he turned to his new wife and said
"You know all the hours I have spent in the lab the last two years?
Well you can blame him. He started it my sophomore year at Murphy High."
The professors probably spent a lot of time undoing all my damage!

John....keep hammering away. I hope you don't get to waves!

Tim Burgess
UMS-Wright Preparatory School
Mobile, AL 36606