Re: ENERGY WITH Q

[Jim Green <JMGreen@SISNA.COM>]

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.

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