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


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