[Phys-l] conservation versus constancy

[John Denker <jsd@av8n.com>]

In the "inelastic" thread, on 10/13/2006 03:55 PM, Carl Mungan wrote:

> One other detail: Am I the only one bothered by expressions such as 
> "KE is conserved" in elastic processes? 

Certainly not the only one!

I would like to pick on the "KE is conserved" statement for reasons only
tangentially related to the other thread.

Among other problems, the quoted statement abuses the notion of
conservation.
-- One might argue that in context this is just a terminological nit-pick,
since an astute reader can figure out that the statement probably meant
"KE is constant" rather than "KE is conserved".
-- OTOH, for pedagogical reasons it is important to keep straight the
distinction between conservation and constancy ... and to leap at
almost any opportunity to reinforce the conceptual distinction.

Local conservation of energy says that the energy in a region is constant
/except/ insofar as energy flows across the boundary.

You can have quantities that are non-constant yet conserved. You can
have quantities that are constant yet non-conserved. For details, including
diagrams, see
http://www.av8n.com/physics/conservative-flow.htm#sec-conservation+-constancy