Re: [Phys-l] teaching energy

[John Denker <jsd@av8n.com>]

On 10/02/2006 11:04 AM, Bob LaMontagne wrote:

> > A) In the classroom:
> >  Book on high shelf = high potential energy.
> >  Book on lower shelf = lower potential energy.
>
>
> How does height become magically transformed to motion? 

Calling it magic doesn't make it magic.

> Height and motion
> are entirely different entities. It's actually the gravitational force
> producing the change in KE.

We have here the makings of a holy war between the big-endians
and the little-endians.

We are all quite aware that the motion *can* be analyzed in terms
of force and work.  If you *choose* to do that, fine, it's your
choice.

However, please keep in mind that others may choose differently.
Please keep in mind that the motion *can* be analyzed in terms of
energy exchange.

>  PE only provides a convenient calculational tool
> that allows the student to easily compute the change in KE when the
> elevation of the object changes and gravity is the only force involved. 

No!  That has moved from being narrow (but tolerable) opinion to
outright error.  PE does not "only" provide that.  In particular,
energy exchange provides a very convenient tool for analyzing what
happens in cases where gravity is *not* the only force involved.

For example, when I am mountain-biking with little kids, at the
top of a hill I tell them they have lots of potential energy, and
that they will have lots of kinetic energy when they get to the
bottom, and if they play their cards right they can use most of
that to get them back up the hill on the far side, converting KE
back to PE.  I don't mention forces.  They don't know about forces,
and indeed I haven't thought much about the forces involved.  The
force depends on the detailed shape of the hill, but the energy
analysis does not.  The energy exchange is indeed a "convenient
calculational tool" but it is more than that.  It is one of the
most fundamental principles of physics.  It is tremendously useful
in a broad range of applications.  Additional examples are available
on request.

> To
> me, that is the essence of energy - a scalar calculational tool that acts as
> a very convenient surrogate for Newton's 2nd Law. I find it ill conceived to
> encourage a student to somehow turn potential energy into some kind of a
> substance that can (with the proper incantations) magically appear as KE.

It is not magic and it is not a surrogate for anything.

You can open your eggs on your preferred end.  That's your choice.

I open my eggs on the small end *or* the large end, depending on
circumstances.