Re: [Phys-L] let's define energy

[John Denker <jsd@av8n.com>]

On 09/29/2015 01:09 PM, Marty Weiss wrote:
> I meant "lost" in the sense that it is no longer useful to do
> something.   Why does the car stop?  The energy is "lost" to heat,
> etc.  Students can understand conservation but they need something
> concrete to be able to say why the car stopped.  Thus I use "lost" in
> quotes to indicate the energy is not useful.

Maybe I'm being overly sensitive, but still I suggest
that "lost" is not the optimal description.  Usually
"transferred" is better.

It pays to be somewhat sensitive, because in another
part of this thread, we have Vokos and Scherr explicitly
talking about "energy degradation and usefulness".  Ick.
  http://spu.edu/depts/physics/documents/Daane.PhysRevSTPER.10.pdf

In more constructive terms:  I would prefer to say 
that the energy was tossed overboard, i.e. transferred
outward across the boundary of the system ... *IF* that
was what actually happened.
 -- If the car went up a hill and coasted to a stop, 
  the energy is not "lost".  The energy is still there,
  in the car or rather in the earth/car system.
 -- If it's an electric car and it stopped via regenerative
  braking, the energy is not "lost";  it's right there in
  the battery.
 -- In a game of bumper cars, if the car stopped because
  it transferred all of its energy to another car via an
  elastic collision, the energy is not "lost".  It was
  just transferred outward across the boundary.
 -- If the car stopped by applying the brakes, and the
  brakes have not yet cooled off, the energy is still
  there and has not even been transferred across the
  boundary.  Physics won't even tell you want fraction of
  the energy is not "useful".  Somebody could hook up the
  brake pads to a tiny heat engine and use it to do some
  useful work.  Insofar as the car is not set up to do
  that, it's an engineering choice, not a law of nature.

Also, as David Bowman and others have pointed out, as
soon as you start talking about "useful" energy you get
stuck with unanswerable questions about useful to whom
for what purpose.

Not to mention the fact that I think that questions about
"why" are almost always unphysical.  It would be better
to ask, /How/ do we account for the energy and momentum as 
the car comes to a stop?  /How/ do we reconcile that with
everything else we know?
  http://www.av8n.com/physics/causation.htm#sec-know-happen
  http://www.av8n.com/physics/causation.htm#sec-1638

My suggestion:  As for the physics energy, talk about 
energy inside the boundary and energy crossing the 
boundary.  This is what the conservation law cares about.

Also keep track of momentum and other relevant quantities.
If you care about irreversibility, keep track of the entropy.