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] The "why" questions



On 11/29/2010 08:27 AM, Philip Keller wrote:
This issue continues to puzzle me. I just taught a lesson on the
work-energy theorem.

Net work = delta K.

delta K = Net work.

But did one CAUSE the other? Or are they only associated with each
other?

I don't know if it helps, but there are actually four possible
versions of this:

a: W does not cause ΔKE b: W causes ΔKE
nor vice versa and not vice versa

c: ΔKE causes W d: bidirectional causation:
and not vice vera W causes ΔKE
*and* vice versa

Option (d) uses an expanded, weaker, less-strict notion of cause
and effect. I'm not sure I recommend it, but if you decided to
take that route I would not object.

Almost nobody is happy with option (d). Most people want there
to be a strong, strict, unidirectional arrow of causation flowing
from cause to effect.

I think this may be a question about language and grammar, more than
about physics. Our language works this way: subjects of sentences
perform actions indicated by verbs. The engine did work on the
rocket. The kicker kicked the ball. It FEELS right to say that the
work caused the energy change. "How do I know" that the energy
changed is not the same question.

Well, sometimes it is just language, but sometimes it is far
deeper than that. Language reflects the way people think ...
and language often /shapes/ the way people think.
-- If I thought everybody understood it, I wouldn't care at
all what language they used to talk about it.
-- Alas, if somebody who understands the issue uses sloppy
language, it can cause everybody else to get the wrong idea.
-- In this case, it is far worse than that, because some
people have profoundly wrong ideas about the physics, and
are communicating their wrong ideas with perfect clarity.

"How do I know that the rocket's energy changed?" ---> Because I
observe that v-final > v-initial.

"Why did the rocket's energy change?" ---> Because the engine did
work.

I agree that those are two different expressions.
*) SOMETIMES they are meant to be taken literally, such that one
is physics and the other is metaphysics.
*) SOMETIMES one is physics and the other is just a sloppy way of
expressing the same physics.

Constructive suggestion: When students ask a "why" question, push
them to rephrase it as a "how do we know" question. Usually they
are perfectly happy with this. It is the question they meant to
ask all along.

In contrast, if there is some wiseacre who really insists on
asking the metaphysical questions, quote Galileo and Newton to
him. Modern science began when people /stopped/ fussing over
the metaphysics and started concentrating on things that actually
matter.

Causation is a real thing. It is a serious topic of the utmost
importance. (Remember the example: Mayo Clinic versus Love Canal.)
Causation is not, however, related to F=ma. Claiming that F causes
ma (and not vice versa) demonstrates a profound misunderstanding of
what causation is.

That the equations are symmetric seems undebatable. And yet...???

Well, the operation of the rocket engine is highly irreversible.
Entropy and all that.

Causation (in the strict sense) is asymmetric and irreversible.

It is a reasonable question to ask whether our understanding of
causation can be traced back to the second law of thermodynamics.

It is 100% unreasonable to connect causation to the first law of
thermodynamics (i.e. conservation of energy) or to similarly
reversible ideas such as F=ma.