Re: Hot air rising and automobile thermometers

[Robert A Cohen <bbq@ESU.EDU>]

On Tue, 27 Jul 1999, Ludwik Kowalski wrote:

> While on the subject let me ask a hypothetical question.
> Suppose you have access to a machine shop and other kind
> of technical support. And that money is not a problem. You
> want to buid an AAPT apparatus competition item which
> clearly demonstrates that due to Coriolis forces wind
> velocities can be parallel, not perpendicular to the "isobaric
> lines". You start with the large rotating disk idea. Then what?

I'll take a crack at this...

How about spinning a parabolic surface about its center?  At a particular
rotation rate, a ball placed on its surface and at rest relative to the
surface will stay stationary relative to the surface.  At this speed, the
normal force (acting centripetally) will balance the centrifugal force
(or, in other words, the normal force will be providing the centripetal
force necessary for the ball to follow the circular path prescribed by the
moving surface...whew!).  Thus, any motion relative to the surface will
appear to be influenced by the coriolis force only (not the centrifugal
force), as is the case with the geostrophic winds.

If we drill a hole in the surface and attach the ball (that is residing on
the surface) to an object that is free to hang down through the hole, the
hanging bob will provide a force on the ball that will attract it to the
hole, much like a low pressure center does on the atmosphere.  From the
point of view of a camera mounted to rotate with the surface, the ball
will appear to spiral around the hole, rather than go directly into it.
This should be sufficient to demonstrate what you are looking for.

By the way, the direction of the spiral will be the same direction
(clockwise or counter-clockwise) as the spin of the surface.  From the
"rest" frame, the stationary ball is spinning around the hole and so as it
approaches the hole it spins faster, conserving angular momentum.

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| Robert Cohen               Department of Physics       |
|                            East Stroudsburg University |
| bbq@esu.edu                East Stroudsburg, PA  18301 |
| http://www.esu.edu/~bbq/   (570) 422-3428              |
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