Re: What Flows? & work done against what?

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

Hi Gordon, you said!  

> If mechanical work is defined as equal to a force times the applied
> distance, and since forces are supposed to always occur in pairs, work
> might always be considered as being done by opposing the matching
> force that's against it. 

I am not sure what you mean when you say "work might always be considered
as being done by opposing the matching force that's against it."  However,
consider the following.  

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..  

>        As for applying that to an ambiguous case, consider:
>   a) a person lifts a weight in a stationary elevator 5 ft.
> b) the elevator rises 5 ft and the person holds the weight against their
>  waist. The person applied force against the weight while it rose. 
>           (They applied a force over a distance, right?)
>   c) the person "lifts" the weight 5ft while the elevator actually goes
>      down 5ft. The weight's actual altitude thusly did not change. A
>  camcorder fixed in the elevator would show the same view in a) or c).
>
> > In which case(s), a,b, & c, was work actually done and by whom or
> > what? Have you ever considered this situation or something equivalent?

I have considered such situations.  To me, to understand the results it
seems helpful to realize that while the work done by a force is frame
dependent, the total work done by an action-reaction force pair is frame
independent (and so is a galilean invariant).  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. 

Gene


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Eugene (Gene) P. Mosca           Phone 410-293-6659 (Fax 3729)
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