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

On Wed, 28 Jul 1999, Ludwik Kowalski wrote:

Clever. But I do not see why should the surface be parabolic?

Robert A Cohen wrote:

How about spinning a parabolic surface about its center?

I'm not sure if I understand the problem, but here is my thinking. If you
take a dish of liquid and rotate it then for "solid body" rotation (i.e.,
the fluid is rotating as a solid body rather than sloshing around) the
surface will be parabolic. A cork can be placed on the surface (as
it rotates) and, if the cork is moving with the fluid, it will remain
stationary relative to the fluid. Thus, from the cork's point of view,
there are no horizontal (relative to the fluid surface) forces on it, no
matter where it is placed on the fluid (close to axis, far from axis,
etc.).

If we replace the fluid with a solid surface and the replace the cork with
a ball, the ball will remain stationary (relative to the surface) no
matter where the ball is placed as long as the ball is initially
stationary (relative to the surface). From the ball's point of view,
there are no "apparent" horizontal forces acting on it as long as it
doesn't move (relative to the surface). As soon as it starts moving,
there will be a Coriolis force.

I haven't done the math - I'm just guessing that the surface exerts a
normal force that has a component directed equal but opposite to the
centrifugal force, no matter where the ball is placed. I'm guessing that
this is what you want, since you want to observe the effect of the
coriolis force. Centrifugal forces will complicate things for you.

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).

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