Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-l] "inelastic"



Not to keep opening the same can of worms - but instead of the mystical
"storage" of KE as PE, why not look at the forces. If the collision is truly
elastic then the compressional forces act to reduce the KE. In a
conservative system, after maximum compression all parts of the system
retrace exactly the same paths they took during the compression, but in
reverse. So exactly the same sequence of forces unfold and the colliding
objects end up with exactly the same KE as before.

In an inelastic collision, the microscopic pieces take on randomized thermal
motions and the original paths of the pieces cannot be retraced exactly
during the expansion phase (hence nonconservative)- resulting in the final
KE being less than the original.

Appealing to PE with introductory students is suspiciously like avoiding the
mechanics of the interaction.

Bob at PC

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of Carl Mungan
Sent: Friday, October 13, 2006 3:56 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] "inelastic"

One other detail: Am I the only one bothered by expressions such as
"KE is conserved" in elastic processes? To me, that sends students
the wrong message. I'd rather say "mechanical energy is conserved
throughout the process, KE being stored partially or entirely as PE
during the collision and is then converted back to KE after the
particles separate, *because* the interaction forces are
conservative, namely they're elastic, gravitational, or
electrostatic". Otherwise students have a hard time with some of the
usual textbook problems such as maximum compression of a spring
bumper during collision of two air carts, gravitational slingshot of
a satellite around a planet (what is the "magic" that happens while
the satellite is near the planet?), interactions of point charges
(they'd have to be infinitely far away before you can say E_mech is
only KE), etc.
--
Carl E Mungan, Assoc Prof of Physics 410-293-6680 (O) -3729 (F)
Naval Academy Stop 9c, 572C Holloway Rd, Annapolis MD 21402-5002
mailto:mungan@usna.edu http://usna.edu/Users/physics/mungan/
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l