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Re: positive and negative work



Exactly. Its the vector nature of displacement and force that
complicates most of the standard "text" problems, not whether energy is
increasing or decreasing when work is done on the system.

-----Original Message-----
From: Rick Tarara [mailto:rtarara@SAINTMARYS.EDU]
Sent: Friday, November 09, 2001 12:57 PM
To: PHYS-L@lists.nau.edu
Subject: Re: positive and negative work


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.