Re: positive and negative work

[Rick Tarara <rtarara@SAINTMARYS.EDU>]

If we look at the HS level (pretty much the same as our 'liberal arts'
level), I'm not sure that I see that keeping with a simple (but standard)
sign convention complicates things or is counter-productive.  We've just
finished this unit.  The formulation was easy:

For NET-Work ON an object---positive if the KE increased, negative if it
decreased.

For WORK done BY an agent--positive if force and displacement are in the
same direction, negative if opposite directions.

That's it!  Keep the examples simple--as with most of the semester, a ball
thrown straight up and later caught at the original height provides ample
physics with which to work--and this should be unambiguous.

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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----- Original Message -----
From: "Scott Goelzer" <sgoelzer@EARTHLINK.NET>


> on 11/9/01 9:39 AM, Michael Edmiston at edmiston@BLUFFTON.EDU wrote:
>
> snip
> > I would answer this by saying it's not important, don't bother to teach
> > students any arbitrary conventions.  The only reason we would need to
> > memorize arbitrary conventions would be if we ever intended to
communicate
> > clearly with other people.  If we are only concerned with understanding
> > things ourselves and we never intend to discuss our understanding with
other
> > people, then each of us can adopt whatever conventions work for us.  All
> > that each of us has to do is be internally consistent, and we each can
> > figure out how the world works, right?
> > Don't limit your decision to quit teaching arbitrary conventions to just
+/-
> > signs on work and energy.  Don't memorize any arbitrary conventions.
For
> > example, don't memorize colors.  Heck, green is just an arbitrary name
we
> > give to the color that is traditionally used for signaling cars to
proceed
> > at a traffic light.  If you personally want to call that red, go right
> > ahead.  But don't talk to my daughter (who is just learning to drive)
> > because she is using a different convention for colors, and if you tell
her
> > she should proceed through the intersection when she sees the light is
red,
> > you're going to jeopardize my daughter's life (and others as well).  So
go
> > have whatever color conventions you want, just don't talk to my
daughter.
> Unfair comparison to the question I was addressing.
> I need to know standard colors to be granted the privilege of driving with
> everyone else who drives. I need to know the standard sign conventions to
be
> granted an engineering license or physics degree so that I can communicate
> and design. Does a HS student need to know the sign convention for work to
> understand energy conservation? Naaa. Gets in the way.
>
> > All language conventions are arbitrary.  Who needs them?  Only those
people
> > who want to communicate with each other.
> I developed this problem in college when majoring in chemistry and
physics.
> Many of sign conventions required for communication were NOT standardized
> (still have to think for 2 minutes when sorting my anodes from my
cathodes)
> My primary goal for HS students is conceptual understanding. Putting a
> question up such as "Is the box doing neg. or pos. work" seems counter
> productive.