Re: positive and negative work

[Rick Tarara <rtarara@SAINTMARYS.EDU>]

----- Original Message -----
From: "Michael Edmiston" <edmiston@BLUFFTON.EDU>


> But I also explain to my students there is a big sign problem when we try
to
> use the "work-energy theorem" because the work-energy theorem itself does
> not specify whether the agent doing the work done and the thing changing
> energy are the same.  So, when I write the work-energy theorem I simply
> write W = +/- delta-E and tell the students they have to figure out which
> sign is appropriate on a case-by-case basis.
>
> Conclusion, I believe there is a firm sign convention for what I consider
> the formal definition of work (integral force dot ds).  But there is no
> formal convention for the sign used in the work-energy theorem because the
> theorem does not specify whether both sides of the equation are referring
to
> the same body/agent or not.

It seems to me that at least in most introductory applications the
convention is:

The Net Work done _on an object_ = the change in Kinetic Energy _of the
object_.

The Net Work is positive if the KE increases, negative if it decreases.

Since I never teach beyond this level  I don't immediately recognize what
the other choices might be.

Rick

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Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, IN 46556
rtarara@saintmarys.edu

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