Re: CONSERVATION OF ENERGY

[LUDWIK KOWALSKI <kowalskil@alpha.montclair.edu>]

On Thu, 24 Jul 97 15:12:59 Al Bachman <BACHMAN@rfmh.org> wrote:

>  The ambiguity here seems to be the TWO ways of calculating the energy
>  transferred as (a) the loss of KE of the object, and (b) the work done
>  by friction.

I assume that this was a comment on the conversation between two teachers
which triggered this thread. Here how the beginning of the dialog (between
A and B) was described in a message posted yesterday: 

> > ......................................................................
> > A: Suppose that a puck, pushed along a horizontal floor with the 
> >    initial kinetic energy K comes to rest. It is stopped by a constant 
> >    frictional force, R. The sliding distance, x, should be
> >
> >                              x = K / R
> >
> > B: This can not be exactly correct because part of the initial kinetic 
> >    energy is converted into thermal energy (temperature goes up). The 
> >    value of x should be less than K/R. 
> > .......................................................................

If [force*distance] and [c*(m+M)*dT] the TWO ways of calculating the 
energy converted from K to dU then two experssions must be identical 
"by definition". No experiment is necessary to verify that the two 
quantities are numerically equal. The prediction made by B can not 
possibly be correct. I would very much like to see what other people 
have to say about this. Wouldn't many of us think, initially, that the 
argument presented by B is valid?

Two ways of referring to the same thing? How can this be justified?
Recall that the ideal student also predicted that [ ] and [ ] must be
equal but his argument was based on the nonsensical model according
to which work and heat are forms of energy. His way of thinking was
based on the analogy with what happens to a rock falling from a known
elevation. The initial potential energy and the final kinetic energy
(of the rock) are equal, the total mechanical energy is conserved.
**********************************************************************

>  The puck ... moving with KE across a rough horizontal floor loses 
>  energy to internal forms ... If the frictional force is known, then 
>  the energy so transferred is calculated as the work done by that force.

I assume that "energy transfer" was a slip of the tongue and that "energy 
transformation" was intended (from internal-macroscopic to internal-thermal). 
Nothing is transferred through the boundary of the m+M system. Sorry for
being picky; Jim Green reminded us that terminology must be as correct as 
possible.
                                Ludwik Kowalski