Re: work done against what?

["Eugene P. Mosca" <mosca@nadn.navy.mil>]

On Wed, 11 Sep 1996 jmclean@chem.ucsd.edu wrote:

> Eugene P. Mosca says:
> > As an object undergoes an upward displacement the gravitation force on it
> > does negative work, but the work done by the reaction force, which acts on
> > the planet, does no work.  (I am assuming we are in a reference frame in
> > which the planet is stationary.)  In this case the work done by the
> > matching force is zero..  
> What if you consider the opposing force (rather than the reaction force),
> i.e., the force which the lifting person applies.  Doesn't that force do
> positive work?

The work done by the force applied by the person is not equal to the
negative of the work done by the gravitational force, except when the
change in the barbell's kinetic energy is zero.

gene

> >  The total work total work
> > done by an action-reaction pair is non zero only when there is a change
> > in the configuration of the system, and the negative of this total work
> > represents the corresponding change in the system's potential energy. 
> I think that perhaps this is how to define potential energy?  Once that's
> done, to consider both the potential energy and the work done by the
> action-reaction pair seperately is to double count them;  they are the same
> thing by a different name.... I think.  I get confused about the details of
> this stuff.
>
> -- 
> --James McLean
> jmclean@chem.ucsd.edu
> post doc
> UCSD

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