Re: [Phys-l] Work and Energy: which first?

["Scott Hill" <shill@wso.williams.edu>]

On 6/12/06, Jeffrey Schnick <JSchnick@anselm.edu> wrote:
> John Decker wrote:
>
> > Here's how I define it:
> >  http://www.av8n.com/physics/thermo-laws.htm#sec-energy
>
> Thanks for sharing your clear definition of energy.  Your recursive
> definition (the statements that a few different kinds of energy are
> energy and that anything that can be converted into any of these or into
> that which can be converted into any of these is energy) makes a lot of
> sense but it does raise a question: For the systems you used to argue
> that the capacity to do work definition is wrong (2 hot potatoes and a
> heat engine vs. a hot potato, a cold potato, and a heat engine); how
> would you convert the energy into one of the forms you listed
> (translational kinetic energy of an object of mass m, gravitational
> potential energy, spring energy, capacitive energy, and inductive
> energy)?

I'd also like to thank everyone for their useful responses.  My
interpretation of the recursive approach is that anything which either
*creates* or *destroys* any of the known types of energy is itself a
source or sink of energy.  Thus, for example, kinetic energy can be
converted to thermal energy via friction, which can warm the potatoes.
It isn't necessary that the thermal energy be convertible back to
kinetic energy.  In a way, the conservation of energy is a defining
characteristic of energy; without the conservation law, you don't have
a unified concept of energy.

As this is a recursive definition, one must start with a base case, so
I'm planning to start with kinetic energy and build up a list of other
types of energy from there.  You can also get at the expression for
work this way, using the kinematic relation v^2=v0^2+2a dx.

We'll see if it works; I'll be introducing it to my class either
tomorrow or Wednesday.

/
:@-) Scott Hill
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