Re: What Flows? & work done against what?

[SHEPHERDG@gtcc.cc.nc.us]

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> I am going to change the subject a bit, from terminology about heat to
> terminology about work.  The work done by a force on a particle is the
> dot product of the force and the displacement of the object.  (Obviously
> a non-calculus definition.)  Occasionally one hears about the" work
> done against" a force.  For example, "A man lifts a known weight
> through a known change in height.  How much work does he do
> against gravity?"  What does this mean?  I have never seen a good
> definition of "work done against."  From the contect I presume that the
> work done against gravity is merely nothing more than the negative of
> the work done by the gravitational force on the weight.  Is that correct? 
> Wouldn't it be best to avoid using the phrase "work done against?" 

> ----------------------------------------------------------------
> Eugene (Gene) P. Mosca           Phone 410-293-6659 (Fax 3729)
> Physics Department                     410-267-0144  Home
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> Annapolis MD 21402               email: mosca@nadn.navy.mil
>
> ----------------------------------------------------------------

Hi Gene!
       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.
        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?
Gordon Shepherd
Guilford Tech. Comm. Coll
Jamestown, NC 27282