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Coriolis force is a type of fictitious force imparted on a body moving in a rotating direction. Also referred to as 'the Coriolis effect,' it is called a fictitious force because it does not, in fact, exert any real force on its own. Rather, it is a measurable output of a rotating object, a consequence of inertia, and is observed by measuring coordinates on a rotating coordinate system.
Coriolis force is one of three forces that emerge when Newton's laws of motion, which are based on the idea that an observer is at rest, are translated onto a rotating frame of reference. The two others, both fictitious forces as well, are centrifugal force and the Euler effect. Coriolis force is most obviously evident when it comes to observing moving objects on the surface of the Earth, which is the largest rotating frame of reference most people will likely ever encounter.
In the northern hemisphere of the planet, above the equator, objects moving on the surface can be observed veering to the right. In the southern hemisphere objects veer to the left. Meteorological events, such as hurricanes, typhoons, and jet streams, are good examples of phenomena influenced by Coriolis force.
Coriolis force owes a degree of notoriety, in popular culture, to the television series The Simpsons. It was used as a plot device in the episode "Bart vs. Australia," in which a debate between characters Bart and Lisa, over which way the water flows when a toilet is flushed, sparks a collect call to Australia. Lisa explains that in the northern hemisphere water always flows in a counter-clockwise fashion, due to the Coriolis effect. This prompts Bart to call the southern hemisphere to verify her claim, igniting a dispute with the Australian government.
Lisa's assertion, while not incorrect, is in fact imprecise. While it has been shown that Coriolis force can impart an effect on bodies of water as small as a sink or toilet, the direction that water drains in these cases has much more to do with the direction the water comes into the bowl. Only after several hours of standing can the Coriolis effect overcome other forces, such as the mild current imparted by the placement of faucets.
In limited cases, the Coriolis effect is observable completely independent of the Earth as the frame of reference. Flies and moths, for instance, leverage Coriolis force when flying, by using special antennas — known as halteres — as gyroscopes. As the insect changes direction in flight or rotates on its axis, a Coriolis force develops on the vibrating haltere, which the insect can sense and adjust to for stability.