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Question
Are coriolis forces strong enough to cause an open basin of water to create whirlpools moving in opposite directions when the basin is transported 20 feet either side of the equator? I saw this demonstrated
in Kenya (for money), but I wonder if there wasn't a trick.
Asked by: Eric Seder
Answer
Your suspicions about being tricked are well founded. The Coriolis effect depends on a
change in the distance from the Earth's polar axis as mass moves in a north/south
direction.
A basin only a few feet in size is too small to exhibit the effect, just a marble rolling
on a flat surface to the north or south does not curve. In fact, even a rifle bullet fired
north or south does not appear to curve.
It takes distances on a scale of miles for the Coriolis effect to be noticeable. Artillery
shells, for example, ARE influenced by the Earth's rotation and must be aimed with that
factor accounted for. A shell fired northward in the northern hemisphere begins its
flight with an eastward motion due to the Earth's rotation. As it travels north, it
maintains that eastward velocity while the Earth's eastward velocity declines (until it
reaches zero at the north pole). Conversely, a shell fired southward in the northern
hemisphere begins with a LOWER eastward velocity. As it travels south, the increasing
eastward velocity of the Earth again makes the shell curve to the right relative to the
Earth's surface.
The Coriolis effect explains why areas of high atmospheric pressure rotate clockwise, and
low pressure areas counterclockwise, in the northern hemisphere. While large air masses in
the atmosphere, and large water masses in the ocean, feel the same effects as an artillery
shell, water in a basin, just like a rolling marble, does not cover a large enough
north/south distance to be affected. Instead, small currents initially in the basin will
determine which way, if any, the emptying water circulates.
Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor
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