Re: [Phys-L] defining energy

[Paul Lulai <plulai@stanthony.k12.mn.us>]

For the first general shot at it, i go with...
If you want to speed something up, slow it down, bend it, break it, change its temperature, make it glow, change its mass, change its direction, it is going to cost you.  What it costs, is energy.

Then how you pay for it in energy has just a few options.

-----Original Message-----
From: Phys-l [mailto:phys-l-bounces@phys-l.org] On Behalf Of Philip Keller
Sent: Tuesday, October 29, 2013 1:13 PM
To: Phys-L@phys-l.org
Subject: Re: [Phys-L] defining energy

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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