Re: A bawl in a rotating dish (was Hot air rising ...)

[Leigh Palmer <palmer@SFU.CA>]

Dear Robert,

I won't go into detail, but I think your first question (and
consequently your answer) are ill conceived. You are raising
the specter of the mysterious "centripetal force" (I know,
you said "component") again. If you don't think this confuses
students and high school teachers then you haven't been
teaching very long.

Clearly the winds feel the Centrifugal force in the same sense
that they feel the Coriolis force. *Both* forces are balanced
by dissipative forces if the wind is steady. You imply to the
naive reader that the evidence that the Coriolis force is felt
is that the winds *move* in response to it. You are being an
Aristotelean mechanic here. Forces produce *accelerations*.

Leigh

At 03:44 -0700 9/12/99, Robert A Cohen wrote:
> I was cleaning up my mail account and noticed a post I made a couple of
> months ago in which I raised some questions but never bothered to answer
> them.  For completeness, in case (or in the hope that) someone happens to
> chance upon the questions (in the archive?) and wants to know the answers,
> here they are.
>
> First the questions...
>
> > P.S. A good physics question is to ask why the winds feel only a coriolis
> > force and not a centrifugal force.  Another good one is to ask how one
> > might roll a ball down a plane and have it *not* experience a coriolis
> > force (granted, the coriolis force would be small in any case, but there
> > is a way to get it to go to zero).
>
> 1. The winds do not "feel" the centrifugal force because we the normal
> force (of the surface) has a slight centripetal component that cancels out
> the centrifugal force.  In other words, we define a horizontal surface as
> that surface for which there is no horizontal forces on an object at rest.
> That is, a horizontal surface is *not* the surface of a sphere.  If it
> was, everyone would be leaning slightly poleward (to counteract the
> centrifugal force) just as someone leans toward the center of the circle
> when walking in a circle.