That stuff about water going down drains opposite ways in the
hemispheres is a myth. In practice, tiny irregularities in real drains
or little accidental swirling imparted to the water play a much bigger
role, evcen when you're far from the equator. The Coriolis forces,
which do twist the water different ways north and south of the equator,
become important on a very large scale, in driving weather and
persistent currents.
The basic idea behind the effect is this. Angular momentum is
conserved. We all have some angular momentum due to the Earth's spin.
If you were to keep spinning around with the Earth, your angular
momentum would decrease as you approached the center of the Earth, just
like the angular momentum of a skater would go down as she pulled her
arms in- if her spinning rate didn't change. The angular moment that
would be lost as something fell partly goes into a spinning motion of
the thing falling.
That's the effect- but like I said it's small for smallish objects.
Mike W.
(published on 10/22/2007)
