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: ENERGY WITH Q



----- Original Message -----
From: "Jim Green" <JMGreen@SISNA.COM>
To: <PHYS-L@lists.nau.edu>
Sent: Tuesday, November 27, 2001 12:08 PM
Subject: Re: ENERGY WITH Q


Robert Cohen wonders as follows:

Is energy conserved (i.e., energy that does not include "pretend"
energies
like PE)?

Integrate N#2 --> Int F*ds = Int ma ds

We _define_ Int F*ds as work and Int ma ds = 0.5mv^2 as KE == energy --
by
definition.

Some of this work we call PE and we lump the PE and the KE together --
This
is not always helpful in an intro class.

Now N#3 says that the F on the system in question = -F on the applicator

Thus the work done on the system = - the work done on the applicator.

And it then follows that energy increase of the system = the decrease of
the energy of the applicator. Total KE is conserved -- always -- even
for a
complicated interaction with multiple particles.
***************************************************
Not so! even if F1=-F2, there are many cases where ds1 is not equal to
ds2...
*********************************************************

But KE + PE is _not_ always conserved.

Now some on this list will want to use "enhanced" language but this is
quite adequate for an intro class -- and beyond.

But if you try to say that energy flows, you do a great disservice to
your
students.

Surely we all know all of the above; we just need to be reminded from
time
to time how simple this is.

Jim Green
mailto:JMGreen@sisna.com
http://users.sisna.com/jmgreen

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor