Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-L] let's define energy

/Physics/ energy is just mass. The usual objection to that is that the kinetic energy of a particle is not its mass. To answer that you have to address the question, "the mass of what?" The kinetic energy "of a particle" in a particular reference frame is one contribution to the mass of any system that includes the particle and, whose center of mass is at rest in the reference frame in question. So, for instance, if you have a .511 MeV photon as viewed in some reference frame, what is generally referred to as the kinetic energy of the photon is one contribution to the mass of the system consisting of that photon and another .511 MeV photon moving in the exact opposite direction. This implies that what we call the kinetic energy of the photon is really one contribution to the kinetic energy of the SYSTEM. In interactions, what matters is the kinetic energy of the system in the reference frame in which the center of mass is at rest. If two cars collide, one of which is going 55 mph and the other 60 mph relative to the road, the amount of damage doesn't depend on the kinetic energy in the reference frame in which the road is at rest but rather the kinetic energy in the two-car center of mass frame. So if the car going 60 mph rear-ends the other one while they are both moving in the same direction there is less damage done than if they collide head on. In a two-particle collision, the sum of the rest masses of the products is limited by the center of mass energy of the two-particles. If a person standing at rest on the earth throws a ball straight up and one wants to know what the maximum separation between the ball and the earth is going to be, the relevant reference frame is the ball plus earth center of mass frame which to a fantastic approximation is the earth frame. As the ball goes up, the kinetic energy of the earth plus ball system is converted into gravitational potential energy of the earth plus system. In both cases, what is relevant is the energy of the system in the rest frame of the system. For accounting purposes it is convenient to assign both energies to the ball and call the gravitational potential energy of the system the gravitational potential energy of the ball and call the kinetic energy of the system the kinetic energy of the ball. But both are energies of the system and contribute to the mass of the system.

-----Original Message-----
From: Phys-l [mailto:phys-l-bounces@www.phys-l.org] On Behalf Of John
Denker
Sent: Friday, September 25, 2015 1:34 PM
To: Phys-L@Phys-L.org
Subject: [Phys-L] let's define energy

On 09/25/2015 07:04 AM, Paul Nord wrote:

I heard some good debate on the idea of making an "Energy Concepts
Inventory" something like the FCI (Force Concepts Inventory). The
conclusion seemed to be that the sciences use the term energy in so
many different ways that science needs to sort this out before we can
expect students to clearly articulate these concepts.

1) Is this debate documented anywhere? In particular, is there something
that interested parties could read and then cite?

2) How many different definitions of /physics/ energy are we talking about?
Is there more than one serious contender?

I've been wrong about this sort of thing before, but I would hope the
community could come to a consensus on how to define the /physics/
energy.

This stands in contrast to things like:
-- "adiabatic", where there are two long-established
meanings, neither of which is particularly better or
worse than the other.
-- "heat", where there are at least four long-established
and widely-used meanings, each of which has some merit
but also some serious problems. (Not to mention various
vernacular and/or metaphorical usages.)
-- etc. etc. etc.

3) Within "science" broadly, I know of two or three definitions of «energy»
... only one of which is the /physics/ energy.

a) The /physics/ energy, as I understand it, is unique, well
defined, and well behaved. Here's how I explain it:
https://www.av8n.com/physics/thermo/energy.html
or equivalently
http://www.av8n.com/physics/thermo/energy.html

b) Meanwhile, there is also the Department of Energy «energy».
This involves some notion of "available" or "useful" energy.
This is important, but it's not the /physics/ energy.
Definitely not. When the DoE says "please «conserve» «energy»"
they are not using the physics notion of energy *or* the physics
notion of conservation.

c) In dictionaries and in third-grade science books you often
see energy defined as "the ability to do work". This is a
rough approximation to the DoE «energy». It is absolutely
not the physics energy. For details on this, see
https://www.av8n.com/physics/thermo/energy.html#sec-workability

Note that the DoE «energy» is important. I'm not saying it's wrong; I'm just
saying it's not the /physics/ energy.
Context matters. In physics context you have to use the physics definition.

==========================

One of the things I don't like about the FCI is that it plays word games. It sets
up situations where it would make sense to apply the vernacular definition of
force, and then penalizes students for doing so. I can imagine a cockamamie
Energy Concept Inventory that pulled the same sort of trick: setting up
situations where it would make sense to apply the DoE definition of
«energy» and then penalizing students for doing so. Please please please
let's not go there.

==========================

I leave it as a question:

1) Does anybody have a problem with the definition and
explanation of energy as I have presented it?
http://www.av8n.com/physics/thermo/energy.html

2) Does anybody know of any other viable, useful, or even
plausible ways of defining the /physics/ energy?

3) Does anybody know of any other vernacular notions
that commonly interfere with students' understanding
of physics energy? (I don't want a long list of
misconceptions, just the truly /important/ ones. Keep
in mind the Anna Karenina principle: Each student who
is confused is confused in his own way.)

_______________________________________________
Forum for Physics Educators
Phys-l@www.phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l