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Re: [Phys-L] defining energy

I teach 11th graders. For the energy topic, I begin with W = Fd, then
follow with P = W/t. I mention about how work relates to energy.

Next day I derive PE = mgh (amt of work needed to lift an object, etc...),
then KE = 0.5mv2 (amt of work needed to get object to speed v). Then, we
derive the general work-energy theorem. This works fine for my students.

Later, we do conservation of energy.

Phys-L@Phys-L.org writes:
But if I define work as "the means by which energy is transferred", then
together my definitions are circular! I am not looking to overwhelm my
11th graders with formality, but I would like the definitions to be
logically sound.




On Tue, Oct 29, 2013 at 1:49 PM, Richard Tarara
<rtarara@saintmarys.edu>wrote:

If you just 'define' work as "the means by which energy is transferred
from one object to another and/or from one form to another" it is a
little
cleaner. Defining energy is still a problem--the 'ability to do work'
definition is pretty good for my world/national energy course, but
ultimately it boils down to a bookkeeping system for keeping track of
properties of nature that are conserved but moved and transformed by
work.
The Feynman 'Dennis the Menace' story is useful at this level (and
beyond),
and appears in Volume//one of the lectures but also in the Kirkpatrick
and
Francis textbook. I'm sure someone will provide an incomprehensible (to
11th graders) more formal definition! ;-)

rwt

On 10/29/2013 11:48 AM, Philip Keller wrote:

Hello,

I am going to be teaching this topic to my 11th graders soon. I have a
question about the definition of energy. I know that the "ability to
do
work" definition runs into trouble when you consider heat, engines and
2nd
law issues. But what if I turn the definition around. Instead of
saying
"energy is the ability to do work", I want to say:

Work is defined to be the product of force and displacement (in the
same
direction). Then, in different contexts, you can show that work =
delta
(some quantity). Any such quantity is referred to as [blank] energy.
Fill
in the blank with an adjective that fits the context.

So "energy" is not the ability to do work, but energies are the
quantities
that are changed by work.

Does this definition pass muster?
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--
Richard Tarara
Professor of Physics
Saint Mary's College

free Physics educational software at

www.saintmarys.edu/~rtarara/**software.html<http://www.saintmarys.edu/~rtarara/software.html>

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