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Re: [Phys-L] Conservation of Energy vs Constant Energy



Isn't it just a definition? So tell them that the definition is that the
energy final = energy initial + sum of the energy inputs - sum of energy
outputs

If they object tell them that you will expect them to use this definition
only. After all various authors are always using their own definitions.
The definition of weight is a perfect example where there are multiple
definitions in various textbooks, and some standards agencies say
weight=mass.

The ILDs have an interesting demo where they introduce friction and look at
E_g and E_k for a cart on a ramp. Each graph alone is not constant but the
sum of the two graphs is constant so mechanical energy = E_g + E_k is
conserved. Then they introduce friction and mechanical energy is no longer
conserved, but if you include E_I (internal) then you can say that the all 3
together are conserved. This is perhaps a good way to introduce the
definition of conservation. In either case bar charts can be helpful in
firming up the definition you wish them to understand. The cart with
friction is also an interesting case because you can have friction such that
E_k remains constand, but E_g is decreasing, so is E_k really conserved?
You are continually transferring energy from E_g to E_k and from E_k to E_i.

Actually the term is not as important as the idea that the energy always
goes somewhere and is neither created nor destroyed. So in an open system
you have to account for the energy inputs and outputs, while in a closed
system you just have to account for how energy is moved from one place to
another.

John M. clement
Houston, TX


Thanks John.
I eventually stopped debating. I think it will come up again.
I was shocked that energy conservation =/= energy is constant
was a one vs the room debate. It seemed like some were slowly
starting to wonder.
Baby steps i guess.
Have a good one, and thanks for the help.
Paul.


.:. Sent from a touchscreen .:.
Paul Lulai



-------- Original message --------
From: John Denker
Date:05/08/2015 6:53 AM (GMT-06:00)
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] Conservation of Energy vs Constant Energy

On 05/04/2015 07:51 AM, Paul Lulai wrote:

I am in an awkward spot. I am trying to convince a group of hs
physics teachers that energy is always conserved.

Just now I upgraded my screed on this topic.
https://www.av8n.com/physics/conservation-continuity.htm
or equivalently
http://www.av8n.com/physics/conservation-continuity.htm

In particular, I added a new subsection and a couple of new
diagrams to explain why /locality/ is important, and why
locality requires us to think in terms of conservation, not
just constancy.

https://www.av8n.com/physics/conservation-continuity.htm#sec-locality

I also made it somewhat less disorganized and fixed a few
typos. The reasoning can be summarized as follows:

* To be useful, the law must be local. Furthermore,
special relativity says that a non-local law cannot
possibly be correct.
* Locality requires us to consider small systems.
* Small systems are generally not closed.
* Therefore a law that applies only to closed systems
is nowhere near strong enough to be a fundamental
law of physics.
* The fact is, energy is /always/ conserved ... even
in small, open regions where the energy is not constant.


--------------------------------
As a separate matter:

Do folks have authoritative resources I can point to?

Rather than asking for an authoritative reference, they ought
to be asking for convincing evidence.

I know they're not scientists, and you're not going to make
them scientists overnight ... but you can at least remind
them that appeal to authority is risky and unscientific. It
is very, very weak evidence.
https://www.av8n.com/physics/authority.htm
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