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Re: [Phys-L] defining energy



On 10/29/2013 11:25 AM, Paul Lulai wrote:
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.


That's not the route I would have taken.

For starters: As previously mentioned, there is already a
terrible problem because of conflicting definitions of
energy:
-- The physics energy, versus
-- the "DoE" energy aka "available" energy or "useful"
energy ... which is an important concept, but emphatically
not the same as the physics energy.

As a related point: The "cost" of energy is *not* the same
as the amount of energy. Physics deals with the amount of
energy. Questions about the price / cost / value of energy
belong to microeconomics, which is very much uglier than physics.

Therefore, it seems to me that introducing energy in terms of
the "cost" of this-or-that is doing the devil's work for him,
reinforcing some pernicious misconceptions.

=======

At a more nit-picky level, several of the given examples are
not even reliably true. For a coil spring, bending it does
not require energy; rather, straightening out the bends
requires the input of energy.

Perhaps more importantly, it's not at all clear that this
"costs" energy in any technical sense, because you can
get back virtually all of the energy you put into the
spring. In microeconomic terms, lending money is not the
same as spending money, not even close. Ditto for energy.