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Re: Words and Concepts Physics( was causation)



On 11/16/2003 09:40 AM, SSHS KPHOX wrote:
> I, for one, find the idea of explaining centrifuges as some sort of
> field fascinating, but would hesitate to use that model with high
> school students. It is an interesting thought, however, and could be
> a unifying theme for the entire course. I would need to understand
> why that field is there though. What causes it ? ;-)

We have three cases to consider:

1) What causes the acceleration field near a planet?

2) What causes the acceleration field in the cabin of
a rocketship in interplanetary space, if the engine
is producing straight-line acceleration?

3) What causes the acceleration field in the cabin of
a centrifuge?

Of these, the first is far and away the most mysterious,
but we manage to deal with it in elementary courses.
At some point you have to say "hypotheses non fingo",
it is what it is, and postpone deeper investigation
to a later course.

The other two are incomparably less mysterious. Just
write down the laws of motion in the non-accelerated
frame and then perform the change of coordinates.

Item (2) is trivial.

Item (3) is trivial for objects not moving relative
to the rotating frame. For moving objects, there is
also a Coriolis effect. Deriving the latter requires
either simple calculus or some intricate, careful
hand-waving. There are two separate contributions,
and your average garden-variety hand-waver is likely
to overlook one or the other, producing a result that
is off by a factor of two.

http://www.av8n.com/how/htm/motion.html#sec-coriolis-effect