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| {{For|the psychophysical perception effect|Coriolis effect (perception)}}
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| [[File:Corioliskraftanimation.gif|frame|right|In the inertial frame of reference (upper part of the picture), the black ball moves in a straight line. However, the observer (red dot) who is standing in the rotating/non-inertial frame of reference (lower part of the picture) sees the object as following a curved path due to the Coriolis and centrifugal forces present in this frame.]]
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| In [[physics]], the '''Coriolis effect''' is a deflection of moving objects when they are viewed in a [[rotating reference frame]]. In a reference frame with clockwise rotation, the deflection is to the left of the motion of the object; in one with counter-clockwise rotation, the deflection is to the right. Although recognized previously by others, the mathematical expression for the '''Coriolis force''' appeared in an 1835 paper by French scientist [[Gaspard-Gustave Coriolis]], in connection with the theory of [[water wheel]]s. Early in the 20th century, the term ''Coriolis force'' began to be used in connection with [[meteorology]].
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| [[Newton's laws of motion]] describe the motion of an object in a (non-accelerating) [[inertial frame of reference]]. When Newton's laws are transformed to a uniformly rotating frame of reference, the Coriolis and [[centrifugal force (rotating reference frame)|centrifugal]] forces appear. Both forces are proportional to the [[mass]] of the object. The Coriolis force is proportional to the rotation rate and the centrifugal force is proportional to its square. The Coriolis force acts in a direction perpendicular to the rotation axis and to the velocity of the body in the rotating frame and is proportional to the object's speed in the rotating frame. The centrifugal force acts outwards in the radial direction and is proportional to the distance of the body from the axis of the rotating frame. These additional forces are termed inertial forces, [[fictitious force]]s or ''pseudo forces''.<ref>{{Cite book|title = Classical Mechanics: With introduction to Nonlinear Oscillations and Chaos|last = Bhatia|first = V.B.|publisher = Narosa Publishing House|year = 1997|page = 201|isbn = 81-7319-105-0}}</ref> They allow the application of Newton's laws to a rotating system. They are correction factors that do not exist in a non-accelerating or inertial reference frame.
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| Of course, the most commonly encountered rotating reference frame is the Earth. The Coriolis effect is caused by the rotation of the [[Earth]] and the [[inertia]] of the mass experiencing the effect. Because the Earth completes only one rotation per day, the Coriolis force is quite small, and its effects generally become noticeable only for motions occurring over large distances and long periods of time, such as large-scale movement of air in the atmosphere or water in the ocean. Such motions are constrained by the surface of the earth, so only the horizontal component of the Coriolis force is generally important. This force causes moving objects on the surface of the Earth to be deflected in a clockwise sense (with respect to the direction of travel) in the [[Northern Hemisphere]] and in a counter-clockwise sense in the [[Southern Hemisphere]]. Rather than flowing directly from areas of high pressure to low pressure, as they would in a non-rotating system, winds and currents tend to flow to the right of this direction north of the [[equator]] and to the left of this direction south of it. This effect is responsible for the rotation of large [[Cyclone#Structure|cyclones]] <!--Don't add tornadoes here; the Coriolis effect is not directly responsible for tornadoes-->(see [[#Meteorology|Coriolis effects in meteorology]]).
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| ==History==
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| Italian scientists [[Giovanni Battista Riccioli]] and his assistant [[Francesco Maria Grimaldi]] described the effect in connection with artillery in the 1651 ''Almagestum Novum'', writing that rotation of the Earth should cause a cannonball fired to the north to deflect to the east.<ref>{{Cite journal|doi=10.1063/PT.3.1195|title=Coriolis effect, two centuries before Coriolis|year=2011|last1=Graney|first1=Christopher M.|journal=Physics Today|volume=64|page=8|bibcode= 2011PhT....64h...8G }}</ref> The effect was described in the [[Theory of tides|tidal equations]] of [[Pierre-Simon Laplace]] in 1778.
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| [[Gaspard-Gustave Coriolis]] published a paper in 1835 on the energy yield of machines with rotating parts, such as [[waterwheel]]s.<ref name=corps>{{Cite journal|author=G-G Coriolis|title=Sur les équations du mouvement relatif des systèmes de corps|journal=J. de l'Ecole royale polytechnique|volume=15|pages=144–154|year=1835}}</ref> That paper considered the supplementary forces that are detected in a rotating frame of reference. Coriolis divided these supplementary forces into two categories. The second category contained a force that arises from the [[cross product]] of the [[angular velocity]] of a [[coordinate system]] and the projection of a particle's [[velocity]] into a plane [[perpendicular]] to the system's [[axis of rotation]]. Coriolis referred to this force as the "compound centrifugal force" due to its analogies with the [[centrifugal force]] already considered in category one.<ref>Dugas, René and J. R. Maddox (1988). ''[http://books.google.com/books?id=vPT-JubW-7QC&pg=PA374 A History of Mechanics]''. Courier Dover Publications: p. 374. ISBN 0-486-65632-2</ref><ref>{{Cite book|title=A Treatise on Infinitesimal Calculus : Vol. IV. The dynamics of material systems|author=Bartholomew Price|publisher = Oxford : University Press|pages=418–420|year=1862|url=http://books.google.com/?id=qrMA0R_0TPEC&pg=PA420}}</ref> The effect was known in the early 20th century as the "[[acceleration]] of Coriolis",<ref>{{Cite book|title=The Dynamics of Particles and of Rigid, Elastic, and Fluid Bodies|author=Arthur Gordon Webster|publisher=B. G. Teubner|year=1912|page=320|url= http://books.google.com/?id=zXkRAAAAYAAJ&pg=PA320|isbn=1-113-14861-6}}</ref> and by 1920 as "Coriolis force".<ref>{{Cite journal|title=Space, Time, and Gravitation|journal=The Scientific Monthly|volume=10|author=Edwin b. Wilson|editor= James McKeen Cattell|publisher=American Association for the Advancement of Science|year=1920|page=226|url= http://books.google.com/?id=xYUZAAAAYAAJ&pg=PA226}}</ref>
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| In 1856, [[William Ferrel]] proposed the existence of a [[Ferrel cell|circulation cell]] in the mid-latitudes with air being deflected by the Coriolis force to create the [[westerlies|prevailing westerly winds]].<ref>{{Cite journal|author = William Ferrel|url = http://www.aos.princeton.edu/WWWPUBLIC/gkv/history/ferrel-nashville56.pdf|title = An Essay on the Winds and the Currents of the Ocean|journal = Nashville Journal of Medicine and Surgery|volume = xi|issue = 4|date = November 1856|pages = 7–19}} Retrieved on 1 January 2009.</ref>
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| Understanding the kinematics of how exactly the rotation of the Earth affects airflow was partial at first.<ref>{{Cite journal|author = Anders O. Persson|url = http://www.aos.princeton.edu/WWWPUBLIC/gkv/history/persson_on_coriolis05.pdf|title = The Coriolis Effect:Four centuries of conflict between common sense and mathematics, Part I: A history to 1885}}</ref> Late in the 19th century, the full extent of the large scale interaction of [[pressure gradient force]] and deflecting force that in the end causes air masses to move 'along' [[isobar (meteorology)|isobar]]s was understood.{{Citation needed|date=July 2009}}
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| ==Formula==
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| {{See also|Fictitious force}}
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| {{Unreferenced section|date=January 2012}}
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| In non-vector terms: at a given rate of rotation of the observer, the magnitude of the Coriolis acceleration of the object is proportional to the velocity of the object and also to the sine of the angle between the direction of movement of the object and the axis of rotation.
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| The vector formula for the magnitude and direction of the Coriolis acceleration <ref name=D.Hestenes>{{cite book|last=Hestenes|first=David|title=New Foundations for Classical Mechanics|year=1990|publisher=Kluwer Academic Publishers|location=The Netherlands|isbn=90-277-2526-8 <!--(pbk)-->|pages=312}}</ref> is
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| : <math>\boldsymbol{ a}_C = -2 \, \boldsymbol{ \Omega \times v}</math>
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| where (here and below) <math>\boldsymbol{ a}_C</math> is the acceleration of the particle in the rotating system, <math>\boldsymbol{v}\,</math> is the velocity of the particle in the rotating system, and '''Ω''' is the [[angular velocity]] vector which has magnitude equal to the rotation rate ω and is directed along the axis of rotation of the rotating reference frame, and the '''×''' symbol represents the [[cross product]] operator.
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| The equation may be multiplied by the mass of the relevant object to produce the Coriolis force:
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| : <math>\boldsymbol{ F}_C = -2 \, m \, \boldsymbol{\Omega \times v}</math>.
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| See ''[[fictitious force]]'' for a derivation.
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| The ''Coriolis effect'' is the behavior added by the ''Coriolis acceleration''. The formula implies that the Coriolis acceleration is perpendicular both to the direction of the velocity of the moving mass and to the frame's rotation axis. So in particular:
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| *if the velocity is parallel to the rotation axis, the Coriolis acceleration is zero.
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| *if the velocity is straight inward to the axis, the acceleration is in the direction of local rotation.
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| *if the velocity is straight outward from the axis, the acceleration is against the direction of local rotation.
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| *if the velocity is in the direction of local rotation, the acceleration is outward from the axis.
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| *if the velocity is against the direction of local rotation, the acceleration is inward to the axis.
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| The vector cross product can be evaluated as the [[Cross product#Matrix notation|determinant of a matrix]]:
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| :<math>\boldsymbol{\Omega \times v} = \begin{vmatrix} \boldsymbol{i}&\boldsymbol{j}&\boldsymbol{k} \\ \Omega_x & \Omega_y & \Omega_z \\ v_x & v_y & v_z \end{vmatrix}\ = \begin{pmatrix} \Omega_y v_z - \Omega_z v_y \\ \Omega_z v_x - \Omega_x v_z \\ \Omega_x v_y - \Omega_y v_x \end{pmatrix}\ ,</math>
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| where the vectors '''''i''''', '''''j''''', '''''k''''' are unit vectors in the ''x'', ''y'' and ''z'' directions.
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| ==Causes==
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| The Coriolis effect exists only when one uses a rotating reference frame. In the rotating frame it behaves exactly like a real force (that is to say, it causes acceleration and has real effects). However, Coriolis force is a consequence of [[inertia]], and is not attributable to an identifiable originating body, as is the case for electromagnetic or nuclear forces, for example. From an analytical viewpoint, to use [[Newton's second law]] in a rotating system, Coriolis force is mathematically necessary, but it disappears in a non-accelerating, inertial frame of reference. For example, consider two children on opposite sides of a spinning roundabout ([[carousel]]), who are throwing a ball to each other. From the children's point of view, this ball's path is curved sideways by the Coriolis effect. Suppose the roundabout spins counter-clockwise when viewed from above. From the thrower's perspective, the deflection is to the right.<ref name="CalvinandHobbs">{{cite book|title=Atmospheric Science: An Introductory Survey|author=John M. Wallace and Peter V. Hobbs|pages=368–371|year=1977|publisher=Academic Press, Inc.|ISBN=0-12-732950-1}}</ref> From the non-thrower's perspective, deflection is to left. ''For a mathematical formulation see [[Fictitious force#Mathematical derivation of fictitious forces|Mathematical derivation of fictitious forces]].''
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| An observer in a rotating frame, such as an astronaut in a rotating space station, very probably will find the interpretation of everyday life in terms of the Coriolis force accords more simply with intuition and experience than a cerebral reinterpretation of events from an inertial standpoint. For example, nausea due to an experienced push{{clarify|date=March 2012}} may be more instinctively explained by Coriolis force than by the law of inertia.<ref name=Ebenholtz>{{Cite book|title=Oculomotor Systems and Perception |author=Sheldon M. Ebenholtz |url=http://books.google.com/?id=1W7ePrvrRyYC&pg=PA151
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| |isbn=0-521-80459-0 |year=2001|publisher=Cambridge University Press}}</ref><ref name=Mather>{{Cite book|title=Foundations of perception |author=George Mather |url=http://books.google.com/?id=LYA9faq3lt4C&pg=PA73
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| |isbn=0-86377-835-6 |year=2006 |publisher=Taylor & Francis}}</ref> See also [[Coriolis effect (perception)]]. In meteorology, a rotating frame (the Earth) with its Coriolis force proves a more natural framework for explanation of air movements than a non-rotating, inertial frame without Coriolis forces.<ref name=Barry0>{{Cite book|title=Atmosphere, Weather and Climate |author=Roger Graham Barry, Richard J. Chorley |page= 113 |isbn=0-415-27171-1 |year=2003 |publisher =Routledge |url=http://books.google.com/?id=MUQOAAAAQAAJ&pg=PA115}}</ref> In long-range gunnery, sight corrections for the Earth's rotation are based upon Coriolis force.<ref name=Taylor0>The claim is made that in the Falklands in WW I, the British failed to correct their sights for the southern hemisphere, and so missed their targets. {{Cite book|title=A Mathematician's Miscellany |author=John Edensor Littlewood |page= 51 |url=http://www.archive.org/details/mathematiciansmi033496mbp |year=1953 |publisher=Methuen And Company Limited}} {{Cite book|title=Classical Mechanics |author=John Robert Taylor |page= 364; Problem 9.28 |url=http://books.google.com/?id=P1kCtNr-pJsC&pg=PA364 |isbn=1-891389-22-X |year=2005 |publisher=University Science Books}} For set up of the calculations, see{{Cite book|author=Donald E. Carlucci, Sidney S. Jacobson |title =Ballistics |page= 225 |url=http://books.google.com/?id=pX9Tzs7VuSoC&pg=PA225 |isbn=1-4200-6618-8 |year=2007 |publisher=CRC Press}}</ref> These examples are described in more detail below.
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| The acceleration entering the Coriolis force arises from two sources of change in velocity that result from rotation: the first is the change of the velocity of an object in time. The same velocity (in an inertial frame of reference where the normal laws of physics apply) will be seen as different velocities at different times in a rotating frame of reference. The apparent acceleration is proportional to the angular velocity of the reference frame (the rate at which the coordinate axes change direction), and to the component of velocity of the object in a plane perpendicular to the axis of rotation. This gives a term <math>-\boldsymbol\Omega\times\boldsymbol v</math>.
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| The minus sign arises from the traditional definition of the cross product ([[right hand rule]]), and from the sign convention for angular velocity vectors.
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| The second is the change of velocity in space. Different positions in a rotating frame of reference have different velocities (as seen from an inertial frame of reference). In order for an object to move in a straight line it must therefore be accelerated so that its velocity changes from point to point by the same amount as the velocities of the frame of reference. The effect is proportional to the angular velocity (which determines the relative speed of two different points in the rotating frame of reference), and to the component of the velocity of the object in a plane perpendicular to the axis of rotation (which determines how quickly it moves between those points). This also gives a term <math>-\boldsymbol\Omega\times\boldsymbol v</math>.
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| ==Length scales and the Rossby number==
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| {{Further|Rossby number}}
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| The time, space and velocity scales are important in determining the importance of the Coriolis effect. Whether rotation is important in a system can be determined by its [[Rossby number]], which is the ratio of the velocity, ''U'', of a system to the product of the [[Coriolis parameter]],<math>f = 2 \omega \sin \varphi \,</math>, and the length scale, ''L'', of the motion:
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| :<math>Ro = \frac{U}{fL}.</math>
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| The Rossby number is the ratio of inertial to Coriolis forces. A small Rossby number signifies a system which is strongly affected by Coriolis forces, and a large Rossby number signifies a system in which inertial forces dominate. For example, in tornadoes, the Rossby number is large, in low-pressure systems it is low and in oceanic systems it is around 1. As a result, in tornadoes the Coriolis force is negligible, and balance is between pressure and centrifugal forces. In low-pressure systems, centrifugal force is negligible and balance is between Coriolis and pressure forces. In the oceans all three forces are comparable.<ref name=Lakshmi>{{Cite book|title=Numerical Models of Oceans and Oceanic Processes |author=Lakshmi H. Kantha & Carol Anne Clayson |publisher=Academic Press |isbn=0-12-434068-7 |year=2000 |page=103 |url=http://books.google.com/?id=Gps9JXtd3owC&pg=PA103
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| }}</ref>
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| An atmospheric system moving at ''U'' = {{convert|10|m/s|mph|sp=us|abbr=on}} occupying a spatial distance of ''L'' = {{convert|1000|km|0|sp=us|abbr=on}}, has a Rossby number of approximately 0.1.
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| A baseball pitcher may throw the ball at U = {{convert|45|m/s|mph|sp=us|abbr=on}} for a distance of L = {{convert|18.3|m|ft|sp=us|abbr=on}}. The Rossby number in this case would be 32,000.
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| Needless to say, one does not worry about which hemisphere one is in when playing baseball.
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| However, an unguided missile obeys exactly the same physics as a baseball, but may travel far enough and be in the air long enough to notice the effect of Coriolis. Long-range shells in the Northern Hemisphere landed close to, but to the right of, where they were aimed until this was noted. (Those fired in the Southern Hemisphere landed to the left.) In fact, it was this effect that first got the attention of Coriolis himself.<ref name=Butz>{{Cite book|title=Science of Earth Systems |author=Stephen D. Butz |page=305 |url=http://books.google.com/?id=JB4ArbvXXDEC&pg=PA304
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| |isbn=0-7668-3391-7 |year=2002 |publisher=Thomson Delmar Learning}}</ref><ref name=Holton>{{Cite book|title=An Introduction to Dynamic Meteorology |page=18 |url=http://books.google.com/?id=fhW5oDv3EPsC&pg=PA18
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| |author=James R. Holton |isbn=0-12-354015-1 |year=2004 |publisher=Academic Press}}</ref><ref name=Jacobson>{{Cite book|title=Ballistics: Theory and Design of Guns and Ammunition |author=Donald E. Carlucci & Sidney S. Jacobson |pages=224–226 |url=http://books.google.com/?id=pX9Tzs7VuSoC&pg=PA224
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| |isbn=1-4200-6618-8 |publisher=CRC Press |year=2007}}</ref>
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| ==Applied to Earth==
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| An important case where the Coriolis force is observed is the rotating Earth. Unless otherwise stated, directions of forces and motion apply to the Northern Hemisphere.
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| ===Intuitive explanation===
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| As the Earth turns around its axis, everything attached to it turns with it (imperceptibly to our senses). An object that is moving without being dragged along with this rotation travels in a straight motion over the turning Earth. From our rotating perspective on the planet, its direction of motion changes as it moves, bending in the opposite direction to our actual motion. When viewed from a stationary point in space above, any land feature in the Northern Hemisphere turns counter-clockwise, and, fixing our gaze on that location, any other location in that hemisphere will rotate around it the same way. The traced ground-path of a freely moving body traveling from one point to another will therefore bend the opposite way, clockwise, which is conventionally labeled as "right," where it will be if the direction of motion is considered "ahead" and "down" is defined naturally.
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| ===Rotating sphere===
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| [[File:Earth coordinates.PNG|thumb|Coordinate system at latitude φ with ''x''-axis east, ''y''-axis north and ''z''-axis upward (that is, radially outward from center of sphere).]]
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| Consider a location with latitude ''φ'' on a sphere that is rotating around the north-south axis.<ref name=Menke>{{Cite book|title=Geophysical Theory |author=William Menke & Dallas Abbott |pages=124–126 |url=http://books.google.com/?id=XP3R_pVnOoEC&pg=PA120
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| |isbn=0-231-06792-5 |year=1990 |publisher=Columbia University Press}}</ref> A local coordinate system is set up with the ''x'' axis horizontally due east, the ''y'' axis horizontally due north and the ''z'' axis vertically upwards. The rotation vector, velocity of movement and Coriolis acceleration expressed in this local coordinate system (listing components in the order east (''e''), north (''n'') and upward (''u'')) are:
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| :<math>\boldsymbol{ \Omega} = \omega \begin{pmatrix} 0 \\ \cos \varphi \\ \sin \varphi \end{pmatrix}\ ,</math> <math>\boldsymbol{ v} = \begin{pmatrix} v_e \\ v_n \\ v_u \end{pmatrix}\ ,</math>
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| :<math>\boldsymbol{ a}_C =-2\boldsymbol{\Omega \times v}= 2\,\omega\, \begin{pmatrix} v_n \sin \varphi-v_u \cos \varphi \\ -v_e \sin \varphi \\ v_e \cos\varphi\end{pmatrix}\ .</math>
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| When considering atmospheric or oceanic dynamics, the vertical velocity is small, and the vertical component of the Coriolis acceleration is small compared to gravity. For such cases, only the horizontal (east and north) components matter. The restriction of the above to the horizontal plane is (setting ''v<sub>u</sub>'' = 0):
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| :<math> \boldsymbol{ v} = \begin{pmatrix} v_e \\ v_n\end{pmatrix}\ ,</math> <math>\boldsymbol{ a}_c = \begin{pmatrix} v_n \\ -v_e\end{pmatrix}\ f\ , </math>
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| where <math>f = 2 \omega \sin \varphi \,</math> is called the Coriolis parameter.
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| By setting ''v<sub>n</sub>'' = 0, it can be seen immediately that (for positive φ and ω) a movement due east results in an acceleration due south. Similarly, setting ''v<sub>e</sub>'' = 0, it is seen that a movement due north results in an acceleration due east. In general, observed horizontally, looking along the direction of the movement causing the acceleration, the acceleration always is turned 90° to the right and of the same size regardless of the horizontal orientation.
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| As a different case, consider equatorial motion setting φ = 0°. In this case, '''Ω''' is parallel to the north or ''n''-axis, and:
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| :<math>\boldsymbol{ \Omega} = \omega \begin{pmatrix} 0 \\ 1 \\ 0 \end{pmatrix}\ ,</math> <math>\boldsymbol{ v} = \begin{pmatrix} v_e \\ v_n \\ v_u \end{pmatrix}\ ,</math> <math>\boldsymbol{ a}_C =-2\boldsymbol{\Omega \times v}= 2\,\omega\, \begin{pmatrix}-v_u \\0 \\ v_e \end{pmatrix}\ .</math>
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| Accordingly, an eastward motion (that is, in the same direction as the rotation of the sphere) provides an upward acceleration known as the [[Eötvös effect]], and an upward motion produces an acceleration due west.
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| {{Selfref|For additional examples in this article see [[#Cannon on turntable|cannon on turntable]] and [[#Tossed ball on a rotating carousel|tossed ball]]. In other articles, see [[rotating spheres]], [[Centrifugal force (rotating reference frame)#Apparent motion of stationary objects|apparent motion of stationary objects]], and [[Fictitious force#Crossing a carousel|carousel]].}}
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| ===Distant stars===
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| The apparent motion of a distant star as seen from Earth is dominated by the Coriolis and centrifugal forces. Consider such a star (with mass m) located at position '''r''', with [[declination]] δ, so '''Ω · r''' = |'''r'''| Ω sin(δ), where '''Ω''' is the Earth's rotation vector. The star is observed to rotate about the Earth's axis with a period of one [[sidereal day]] in the opposite direction to that of the Earth's rotation, making its velocity '''v = –Ω × r'''. The fictitious force, consisting of Coriolis and centrifugal forces, is:
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| : <math>
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| \begin{align}
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| \boldsymbol {F_f} & = -2 \, m \, \boldsymbol{\Omega \times v} - m \, \boldsymbol{\Omega \times { (\Omega \times r)}} \\[8pt]
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| & = +2 \, m \, \boldsymbol{\Omega \times (\Omega \times r)} - m \, \boldsymbol{\Omega \times {(\Omega \times r)}} \\[8pt]
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| & = m \, \boldsymbol{\Omega \times (\Omega \times r)} \\[8pt]
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| & = m \, \boldsymbol{(\Omega (\Omega \cdot r)} - \boldsymbol{r (\Omega \cdot \Omega))} \\[8pt]
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| & = - m \, \Omega^2 \, \boldsymbol{ ( r} - \mid\boldsymbol{r}\mid \sin(\delta)\boldsymbol{u_\Omega)},
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| \end{align}
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| </math>
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| where '''u<sub>Ω</sub>''' = Ω<sup>−1</sup>'''Ω''' is a unit vector in the direction of '''Ω'''. The fictitious force '''F<sub>f</sub>''' is thus a vector of magnitude m Ω<sup>2</sup>|'''r'''| cos(δ), perpendicular to '''Ω''', and directed towards the center of the star's rotation on the Earth's axis, and therefore recognizable as the centripetal force that will keep the star in a circular movement around that axis.
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| {{clear}}
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| ===Meteorology===
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| [[File:Low pressure system over Iceland.jpg|thumb|This [[Low pressure area|low pressure system]] over [[Iceland]] spins counter-clockwise due to balance between the Coriolis force and the pressure gradient force.]]
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| [[File:Coriolis effect10.svg|thumbnail|Schematic representation of flow around a '''low'''-pressure area in the Northern Hemisphere. The Rossby number is low, so the centrifugal force is virtually negligible. The pressure-gradient force is represented by blue arrows, the Coriolis acceleration (always perpendicular to the velocity) by red arrows]]
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| [[File:Coriolis effect14.png|thumb|Schematic representation of inertial circles of air masses in the absence of other forces, calculated for a wind speed of approximately {{convert|50|to|70|m/s|mph|sp=us|abbr=on}}.]]
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| Perhaps the most important impact of the Coriolis effect is in the large-scale dynamics of the oceans and the atmosphere. In meteorology and [[oceanography]], it is convenient to postulate a rotating frame of reference wherein the Earth is stationary. In accommodation of that provisional postulation, the centrifugal and Coriolis forces are introduced. Their relative importance is determined by the applicable [[Coriolis effect#Length scales and the Rossby number|Rossby number]]s. [[Tornado]]es have high Rossby numbers, so, while tornado-associated centrifugal forces are quite substantial, Coriolis forces associated with tornados are for practical purposes negligible.<ref name=Holton2>{{Cite book|title=An Introduction to Dynamic Meteorology |year=2004 |author=James R. Holton |url=http://books.google.com/?id=fhW5oDv3EPsC&pg=PA64
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| |page= 64 |isbn=0-12-354015-1 |publisher=Elsevier Academic Press |location=Burlington, MA}}</ref>
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| High pressure systems rotate in a direction such that the Coriolis force will be directed radially inwards, and nearly balanced by the outwardly radial pressure gradient. This direction is clockwise in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere. Low pressure systems rotate in the opposite direction, so that the Coriolis force is directed radially outward and nearly balances an inwardly radial pressure gradient. In each case a slight imbalance between the Coriolis force and the pressure gradient accounts for the radially inward acceleration of the system's [[circular motion]].
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| ====Flow around a low-pressure area====
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| {{Main|Low-pressure area}}
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| If a low-pressure area forms in the atmosphere, air will tend to flow in towards it, but will be deflected perpendicular to its velocity by the Coriolis force. A system of equilibrium can then establish itself creating circular movement, or a cyclonic flow. Because the Rossby number is low, the force balance is largely between the [[pressure gradient force]] acting towards the low-pressure area and the Coriolis force acting away from the center of the low pressure.
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| Instead of flowing down the gradient, large scale motions in the atmosphere and ocean tend to occur perpendicular to the pressure gradient. This is known as [[geostrophic flow]].<ref name=Barry>{{Cite book|title=Atmosphere, Weather and Climate |author=Roger Graham Barry & Richard J. Chorley |url=http://books.google.com/?id=MUQOAAAAQAAJ&pg=PA115
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| |page=115 |isbn=0-415-27171-1 |year=2003 |publisher=Routledge}}</ref> On a non-rotating planet, fluid would flow along the straightest possible line, quickly eliminating pressure gradients. Note that the geostrophic balance is thus very different from the case of "inertial motions" (see below) which explains why mid-latitude cyclones are larger by an order of magnitude than inertial circle flow would be.
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| This pattern of deflection, and the direction of movement, is called [[Buys-Ballot's law]]. In the atmosphere, the pattern of flow is called a [[cyclone]]. In the Northern Hemisphere the direction of movement around a low-pressure area is counter-clockwise. In the Southern Hemisphere, the direction of movement is clockwise because the rotational dynamics is a mirror image there. At high altitudes, outward-spreading air rotates in the opposite direction.<ref>[http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17026 Cloud Spirals and Outflow in Tropical Storm Katrina] from [[Earth Observatory]] ([[NASA]])</ref> Cyclones rarely form along the equator due to the weak Coriolis effect present in this region.
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| ====Inertial circles====
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| An air or water mass moving with speed <math>v\,</math> subject only to the Coriolis force travels in a circular trajectory called an 'inertial circle'. Since the force is directed at right angles to the motion of the particle, it will move with a constant speed around a circle whose radius <math>R</math> is given by:
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| : <math>R= \frac {v}{f}\,</math>
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| where <math>f</math> is the Coriolis parameter <math>2 \Omega \sin \varphi</math>, introduced above (where <math>\varphi</math> is the latitude). The time taken for the mass to complete a full circle is therefore <math>2\pi/f</math>. The Coriolis parameter typically has a mid-latitude value of about 10<sup>−4</sup> s<sup>−1</sup>; hence for a typical atmospheric speed of {{convert|10|m/s|mph|sp=us|abbr=on}} the radius is {{convert|100|km|0|sp=us|abbr=on}}, with a period of about 17 hours. For an ocean current with a typical speed of {{convert|10|cm/s|mph|sp=us|abbr=on}}, the radius of an inertial circle is {{convert|1|km|1|sp=us|abbr=on}}. These inertial circles are clockwise in the Northern Hemisphere (where trajectories are bent to the right) and counter-clockwise in the Southern Hemisphere.
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| If the rotating system is a parabolic turntable, then <math>f</math> is constant and the trajectories are exact circles. On a rotating planet, <math>f</math> varies with latitude and the paths of particles do not form exact circles. Since the parameter <math>f</math> varies as the sine of the latitude, the radius of the oscillations associated with a given speed are smallest at the poles (latitude = ±90°), and increase toward the equator.<ref name=Marshall2>{{Cite book|title=p. 98 |author=John Marshall & R. Alan Plumb |url=http://books.google.com/?id=aTGYbmVaA_gC&pg=PA98
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| |isbn=0-12-558691-4 |year=2007 |publisher=Elsevier Academic Press |location=Amsterdam}}</ref>
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| ====Other terrestrial effects====
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| The Coriolis effect strongly affects the large-scale oceanic and [[atmospheric circulation]], leading to the formation of robust features like [[jet stream]]s and [[western boundary current]]s. Such features are in [[geostrophic]] balance, meaning that the Coriolis and ''pressure gradient'' forces balance each other. Coriolis acceleration is also responsible for the propagation of many types of waves in the ocean and atmosphere, including [[Rossby wave]]s and [[Kelvin wave]]s. It is also instrumental in the so-called [[Ekman layer|Ekman]] dynamics in the ocean, and in the establishment of the large-scale ocean flow pattern called the [[Sverdrup balance]].
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| {{clear}}
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| ===Eötvös effect===
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| {{See also|Eötvös effect}}
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| The practical impact of the "Coriolis effect" is mostly caused by the horizontal acceleration component produced by horizontal motion.
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| There are other components of the Coriolis effect. Eastward-traveling objects will be deflected upwards (feel lighter), while westward-traveling objects will be deflected downwards (feel heavier). This is known as the [[Eötvös effect]]. This aspect of the Coriolis effect is greatest near the equator. The force produced by this effect is similar to the horizontal component, but the much larger vertical forces due to gravity and pressure mean that it is generally unimportant dynamically.
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| In addition, objects traveling upwards or downwards will be deflected to the west or east respectively. This effect is also the greatest near the equator. Since vertical movement is usually of limited extent and duration, the size of the effect is smaller and requires precise instruments to detect.
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| ===Draining in bathtubs and toilets===
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| Contrary to popular misconception, water rotation in home bathrooms under normal circumstances is not related to the Coriolis effect or to the rotation of the earth, and no consistent difference in rotation direction between toilets in the Northern and Southern Hemispheres can be observed. The formation of a vortex over the plug hole may be explained by the conservation of [[angular momentum]]: The radius of rotation decreases as water approaches the plug hole, so the rate of rotation increases, for the same reason that an ice skater's rate of spin increases as they pull their arms in. Any rotation around the plug hole that is initially present accelerates as water moves inward.
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| Only if the water is so still that the effective rotation rate of the earth is faster than that of the water relative to its container, and if externally applied torques (such as might be caused by flow over an uneven bottom surface) are small enough, the Coriolis effect may determine the direction of the vortex. Without such careful preparation, the Coriolis effect may be much smaller than various other influences on drain direction<ref>{{cite book|url=http://books.google.com/books?id=8hOLs-bmiYYC&pg=PA168&f=false|pages=168–9|title=Physics: A World View|author=Larry D. Kirkpatrick and Gregory E. Francis|year=2006|publisher=Cengage Learning|ISBN=978-0-495-01088-3|accessdate=1 April 2011}}</ref> such as any residual rotation of the water<ref>{{cite journal | journal = Journal of Fluid Mechanics | title = Stationary bathtub vortices and a critical regime of liquid discharge | author = Y. A. Stepanyants and G. H. Yeoh | volume = 604
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| | issue = 1| pages = 77–98| year = 2008 | url = http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=1878300 |bibcode = 2008JFM...604...77S |doi = 10.1017/S0022112008001080 }}</ref> and the geometry of the container.<ref>{{cite book
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| |title = A Student's Guide to Earth Science: Words and terms | author = Creative Media Applications
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| |publisher = Greenwood Publishing Group| year = 2004 | isbn = 978-0-313-32902-9 | page = 22 | url = http://books.google.com/books?id=fF0TTZVQuZoC&pg=PA22}}</ref> Despite this, the idea that toilets and bathtubs drain differently in the Northern and Southern Hemispheres has been popularized by several television programs and films, including "[[Escape Plan (film)|Escape Plan]]", ''[[Wedding Crashers]]'', ''[[The Simpsons]]'' episode "[[Bart vs. Australia]]", and ''[[The X-Files]]'' episode "[[Die Hand Die Verletzt]]".<ref>Emery, C. Eugene, Jr. (May 1, 1995). [http://web.archive.org/web/20080105055918/http://www.encyclopedia.com/doc/1G1-16836639.html "''X-Files'' coriolis error leaves viewers wondering"]. ''[[Skeptical Inquirer]]''</ref> Several science broadcasts and publications, including at least one college-level physics textbook, have also stated this.<ref>{{cite web|last=Fraser|first=Alistair|title=Bad Coriolis|url=http://www.ems.psu.edu/~fraser/Bad/BadCoriolis.html|work=Bad Meteorology|publisher=Pennsylvania State College of Earth and Mineral Science|accessdate=17 January 2011}}</ref><ref>{{cite book|last=Tipler|first=Paul|title=Physics for Engineers and Scientists|year=1998|publisher=W.H.Freeman, Worth Publishers|isbn=978-1-57259-616-0|edition=4th|page=128|quote=...on a smaller scale, the coriolis effect causes water draining out a bathtub to rotate counter clockwise in the northern hemisphere...}}</ref>
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| In 1908, the Austrian physicist [[Ottokar Tumlirz]] described careful and effective experiments which demonstrated the effect of the rotation of the Earth on the outflow of water through a central aperture.<ref>{{Cite journal|title=Ein neuer physikalischer Beweis für die Achsendrehung der Erde|year=1908|last=Tumlirz|first=Ottokar|journal=Sitzungsberichte der math.-nat. Klasse der kaiserlichen Akademie der Wissenschaften IIa|volume=117|pages=819–841|authorlink=Ottokar Tumlirz}}</ref> The subject was later popularized in a famous article in the journal ''[[Nature (journal)|Nature]]'', which described an experiment in which all other forces to the system were removed by filling a {{convert|6|ft|m|sp=us|abbr=on}} tank with {{convert|300|USgal|L|sp=us|abbr=on}} of water and allowing it to settle for 24 hours (to allow any movement due to filling the tank to die away), in a room where the temperature had stabilized. The drain plug was then very slowly removed, and tiny pieces of floating wood were used to observe rotation. During the first 12 to 15 minutes, no rotation was observed. Then, a vortex appeared and consistently began to rotate in a counter-clockwise direction (the experiment was performed in [[Boston, Massachusetts]], in the Northern Hemisphere). This was repeated and the results averaged to make sure the effect was real. The report noted that the vortex rotated, "about 30,000 times faster than the effective rotation of the earth in 42° North (the experiment's location)". This shows that the small initial rotation due to the earth is amplified by gravitational draining and conservation of angular momentum to become a rapid vortex and may be observed under carefully controlled [[laboratory]] conditions.<ref>{{Cite journal|doi=10.1038/1961080b0|title=Bath-Tub Vortex|year=1962|last1=Shapiro|first1=Ascher H.|journal=Nature|volume=196|page=1080|bibcode = 1962Natur.196.1080S|issue=4859}}</ref><ref>[http://web.mit.edu/fluids/www/Shapiro/ncfmf.html (Vorticity, Part 1)]. Web.mit.edu. Retrieved on 8 November 2011.</ref>
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| ===Ballistic missiles and satellites===
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| Ballistic missiles and satellites appear to follow curved paths when plotted on common world maps mainly because the Earth is spherical and the shortest distance between two points on the Earth's surface (called a [[great circle]]) is usually not a straight line on those maps. Every two-dimensional (flat) map necessarily distorts the Earth's curved (three-dimensional) surface. Typically (as in the commonly used [[Mercator projection]], for example), this distortion increases with proximity to the poles. In the Northern Hemisphere for example, a ballistic missile fired toward a distant target using the shortest possible route (a great circle) will appear on such maps to follow a path north of the straight line from target to destination, and then curve back toward the equator. This occurs because the latitudes, which are projected as straight horizontal lines on most world maps, are in fact circles on the surface of a sphere, which get smaller as they get closer to the pole. Being simply a consequence of the sphericity of the Earth, this would be true even if the Earth didn't rotate. The Coriolis effect is of course also present, but its effect on the plotted path is much smaller.
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| The Coriolis effects became important in [[External ballistics#Coriolis drift|external ballistics]] for calculating the trajectories of very long-range [[artillery]] shells. The most famous historical example was the [[Paris gun]], used by the Germans during [[World War I]] to bombard [[Paris]] from a range of about {{convert|120|km|sp=us|abbr=on}}.
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| ==Special cases==
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| ===Cannon on turntable===
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| {{Technical|section|date=June 2010}}
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| {{See also|Fictitious force#Crossing a carousel}}
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| [[File:Target on turntable.PNG|thumb|300px|Cannon at the center of a rotating turntable. To hit the target located at position 1 on the perimeter at time ''t'' = 0s, the cannon must be aimed ahead of the target at angle θ. That way, by the time the cannonball reaches position 3 on the periphery, the target also will be at that position. In an inertial frame of reference, the cannonball travels a straight radial path to the target (curve ''y''<sub>A</sub>). However, in the frame of the turntable, the path is arched (curve ''y''<sub>B</sub>), as also shown in the figure.]]
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| [[File:Trajectory for three angles of launch.PNG|thumb|300px|Successful trajectory of cannonball as seen from the turntable for three angles of launch θ. Plotted points are for the same equally spaced times steps on each curve. Cannonball speed ''v'' is held constant and angular rate of rotation ω is varied to achieve a successful "hit" for selected θ. For example, for a radius of 1 m and a cannonball speed of 1 m/s, the time of flight ''t''<sub>''f''</sub> = 1 s, and ''ωt''<sub>''f''</sub> = ''θ'' → ''ω'' and ''θ'' have the same numerical value if ''θ'' is expressed in radians. The wider spacing of the plotted points as the target is approached show the speed of the cannonball is accelerating as seen on the turntable, due to fictitious Coriolis and centrifugal forces.]]
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| [[File:Vector relationships.PNG|thumb|300px|Acceleration components at an earlier time (top) and at arrival time at the target (bottom)]]
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| [[File:Cannon force components.PNG|thumb|300px|Coriolis acceleration, centrifugal acceleration and net acceleration vectors at three selected points on the trajectory as seen on the turntable.]]
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| The animation at the top of this article is a classic illustration of Coriolis force. Another visualization of the Coriolis and centrifugal forces is [http://www.youtube.com/watch?v=49JwbrXcPjc this animation clip].
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| Given the radius ''R'' of the turntable in that animation, the rate of angular rotation ω, and the speed of the cannonball (assumed constant) ''v'', the correct angle θ to aim so as to hit the target at the edge of the turntable can be calculated.
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| The inertial frame of reference provides one way to handle the question: calculate the time to interception, which is ''t<sub>f</sub>'' = ''R'' / ''v'' . Then, the turntable revolves an angle ω ''t<sub>f</sub>'' in this time. If the cannon is pointed an angle θ = ω ''t<sub>f</sub>'' = ω ''R'' / ''v'', then the cannonball arrives at the periphery at position number 3 at the same time as the target.
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| No discussion of Coriolis force can arrive at this solution as simply, so the reason to treat this problem is to demonstrate Coriolis formalism in an easily visualized situation.
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| The trajectory in the inertial frame (denoted ''A'') is a straight line radial path at angle θ. The position of the cannonball in (''x'', ''y'') coordinates at time ''t'' is:
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| :<math>\mathbf{r}_A (t) = vt\ \left( \cos (\theta ), \ \sin (\theta )\right) \ . </math>
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| In the turntable frame (denoted ''B''), the ''x''- ''y'' axes rotate at angular rate ω, so the trajectory becomes:
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| :<math>\mathbf{r}_B (t) = vt\ \left( \cos ( \theta - \omega t), \ \sin ( \theta - \omega t)\right) \ , </math>
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| and three examples of this result are plotted in the figure.
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| To determine the components of acceleration, a general expression is used from the article [[fictitious force]]:
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| :<math>
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| \mathbf{a}_{B} =
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| \mathbf{a}_A </math> <math>{}- 2 \boldsymbol\Omega \times \mathbf{v}_{B} </math> <math>{}- \boldsymbol\Omega \times (\boldsymbol\Omega \times \mathbf{r}_B ) </math> <math>{}- \frac{d \boldsymbol\Omega}{dt} \times \mathbf{r}_B \ ,
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| </math>
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| in which the term in '''Ω × v<sub>B</sub>''' is the Coriolis acceleration and the term in '''Ω × ( Ω × r<sub>B</sub>)''' is the centrifugal acceleration. The results are (let α = θ − ω''t''):
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| :<math> \boldsymbol{\Omega} \mathbf{\times r_B} </math> <math>= \begin{vmatrix} \boldsymbol{i}&\boldsymbol{j}&\boldsymbol{k} \\ 0 & 0 & \omega \\ v t \cos \alpha & vt \sin \alpha & 0 \end{vmatrix}\ </math> <math>= \omega t v \left(-\sin\alpha, \cos\alpha\right )\ ,</math>
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| :<math>\boldsymbol{\Omega \ \times} \left( \boldsymbol{\Omega} \mathbf{\times r_B}\right)</math> <math> = \begin{vmatrix} \boldsymbol{i}&\boldsymbol{j}&\boldsymbol{k} \\ 0 & 0 & \omega \\ -\omega t v \sin\alpha & \omega t v \cos\alpha & 0 \end{vmatrix}\ \ , </math>
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| producing a centrifugal acceleration:
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| :<math> \mathbf{a_{\mathrm{Cfgl}}}</math> <math>= \omega^2 v t \left(\cos\alpha, \sin\alpha\right )=\omega^2 \mathbf{r_B}(t) \ .</math>
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| Also:
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| :<math>\mathbf{v_B} = \frac{d\mathbf{r_B}(t)}{dt}=(v \cos \alpha + \omega t \ v \sin \alpha,</math> <math>\ v \sin \alpha -\omega t \ v \cos \alpha ,\ 0)\ \ ,</math>
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| :<math> \boldsymbol{\Omega} \mathbf{\times v_B} </math>  <math>= \begin{vmatrix}\! \boldsymbol{i}& \! \boldsymbol{j}& \! \boldsymbol{k} \\ 0 & 0 & \omega \\v \cos \alpha\quad &v \sin \alpha\quad &\quad \\ \; + \omega t \ v \sin \alpha & \; -\omega t \ v \cos \alpha & 0
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| \end{vmatrix}\ \ , </math>
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| producing a Coriolis acceleration:
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| :<math>\mathbf{a_{\mathrm{Cor}}} </math> <math>= -2\left[ -\omega v \left( \sin\alpha - \omega t \cos\alpha\right),\right.</math><math>\left. {\color{white}...}\ \omega v \left(\cos\alpha + \omega t \sin \alpha \right) \right]\ </math>
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| ::<math>=2\omega v \left(\sin\alpha,\ - \cos\alpha \right) </math> <math>-2\omega^2 \mathbf{r_B}(t) \ . </math>
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| These accelerations are shown in the diagrams for a particular example.
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| It is seen that the Coriolis acceleration not only cancels the centrifugal acceleration, but together they provide a net "centripetal", radially inward component of acceleration (that is, directed toward the center of rotation):<ref name=caution>Here the description "radially inward" means "toward the axis of rotation". That direction is ''not'' toward the [[osculating circle|center of curvature]] of the path, however, which is the direction of the true centripetal force. Hence, the quotation marks on "centripetal".</ref>
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| :<math>\mathbf{a_{\mathrm{Cptl}}} = -\omega^2 \mathbf{r_B}(t) \ , </math>
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| and an additional component of acceleration perpendicular to '''r<sub>B</sub>''' ''(t)'':
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| :<math>\mathbf{a_{C\perp}} </math> <math>= 2\omega v \left(\sin\alpha,\ -\cos\alpha \right) \ .</math>
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| The "centripetal" component of acceleration resembles that for [[circular motion]] at radius ''r''<sub>B</sub>, while the perpendicular component is velocity dependent, increasing with the radial velocity ''v'' and directed to the right of the velocity. The situation could be described as a circular motion combined with an "apparent Coriolis acceleration" of 2ω''v''. However, this is a rough labelling: a careful designation of the true centripetal force refers to a [[Centripetal force#Local coordinates|local reference frame]] that employs the directions normal and tangential to the path, not coordinates referred to the axis of rotation.
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| These results also can be obtained directly by two time differentiations of '''r<sub>B</sub>''' ''(t)''. Agreement of the two approaches demonstrates that one could start from the general expression for fictitious acceleration above and derive the trajectories shown here. However, working from the acceleration to the trajectory is more complicated than the reverse procedure used here, which, of course, is made possible in this example by knowing the answer in advance.
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| As a result of this analysis an important point appears: ''all'' the fictitious accelerations must be included to obtain the correct trajectory. In particular, besides the Coriolis acceleration, the [[Centrifugal force (rotating reference frame)|centrifugal force]] plays an essential role. It is easy to get the impression from verbal discussions of the cannonball problem, which are focussed on displaying the Coriolis effect particularly, that the Coriolis force is the only factor that must be considered;<ref name=Owen>{{Cite book|title=Fundamentals of Scientific Mathematics |author=George E. Owen |page=23 |url=http://books.google.com/?id=9dRxGCktg7QC&pg=PA22
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| |isbn=0-486-42808-7 |edition=original edition published by Harper & Row, New York, 1964 |year=2003 |publisher=Courier Dover Publications}}</ref> emphatically, that is not so.<ref name=Tavel>{{Cite book|title=Contemporary Physics and the Limits of Knowledge|author=Morton Tavel |page=88 |url=http://books.google.com/?id=SELS0HbIhjYC&pg=PA88
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| |isbn=0-8135-3077-6 |publisher=Rutgers University Press |year=2002}}</ref> A turntable for which the Coriolis force ''is'' the only factor is the [[#parabolic turntable|parabolic turntable]]. A somewhat more complex situation is the idealized example of flight routes over long distances, where the centrifugal force of the path and [[Lift (force)|aeronautical lift]] are countered by [[Orbit|gravitational attraction]].<ref name=Ogden>{{Cite book|author=James R Ogden & M Fogiel |title=High School Earth Science Tutor |page=167 |url=http://books.google.com/?id=fFmqhNXixLUC&pg=PA167
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| |isbn=0-87891-975-9 |publisher=Research & Education Assoc.|year=1995}}</ref><ref name= McCully2>{{Cite book|author=James Greig McCully |title=Beyond the moon: A Conversational, Common Sense Guide to Understanding the Tides |pages=74–76 |url=http://books.google.com/?id=RijQELAGnEIC&pg=PA76|isbn=981-256-643-0|year=2006|publisher=World Scientific}}</ref>
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| {{clear}}
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| ===Tossed ball on a rotating carousel===
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| [[File:Coriolis construction.JPG|thumb|300px|A carousel is rotating counter-clockwise. ''Left panel'': a ball is tossed by a thrower at 12:00 o'clock and travels in a straight line to the center of the carousel. While it travels, the thrower circles in a counter-clockwise direction. ''Right panel'': The ball's motion as seen by the thrower, who now remains at 12:00 o'clock, because there is no rotation from their viewpoint.]]
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| The figure illustrates a ball tossed from 12:00 o'clock toward the center of a counter-clockwise rotating carousel. On the left, the ball is seen by a stationary observer above the carousel, and the ball travels in a straight line to the center, while the ball-thrower rotates counter-clockwise with the carousel. On the right the ball is seen by an observer rotating with the carousel, so the ball-thrower appears to stay at 12:00 o'clock. The figure shows how the trajectory of the ball as seen by the rotating observer can be constructed.
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| On the left, two arrows locate the ball relative to the ball-thrower. One of these arrows is from the thrower to the center of the carousel (providing the ball-thrower's line of sight), and the other points from the center of the carousel to the ball.(This arrow gets shorter as the ball approaches the center.) A shifted version of the two arrows is shown dotted.
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| On the right is shown this same dotted pair of arrows, but now the pair are rigidly rotated so the arrow corresponding to the line of sight of the ball-thrower toward the center of the carousel is aligned with 12:00 o'clock. The other arrow of the pair locates the ball relative to the center of the carousel, providing the position of the ball as seen by the rotating observer. By following this procedure for several positions, the trajectory in the rotating frame of reference is established as shown by the curved path in the right-hand panel.
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| The ball travels in the air, and there is no net force upon it. To the stationary observer the ball follows a straight-line path, so there is no problem squaring this trajectory with zero net force. However, the rotating observer sees a ''curved'' path. Kinematics insists that a force (pushing to the ''right'' of the instantaneous direction of travel for a ''counter-clockwise'' rotation) must be present to cause this curvature, so the rotating observer is forced to invoke a combination of centrifugal and Coriolis forces to provide the net force required to cause the curved trajectory.
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| {{clear}}
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| ===Bounced ball===
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| [[File:Points of view.PNG|thumb|300px|Bird's-eye view of carousel. The carousel rotates clockwise. Two viewpoints are illustrated: that of the camera at the center of rotation rotating with the carousel (left panel) and that of the inertial (stationary) observer (right panel). Both observers agree at any given time just how far the ball is from the center of the carousel, but not on its orientation. Time intervals are 1/10 of time from launch to bounce.]]
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| The figure describes a more complex situation where the tossed ball on a turntable bounces off the edge of the carousel and then returns to the tosser, who catches the ball. The effect of Coriolis force on its trajectory is shown again as seen by two observers: an observer (referred to as the "camera") that rotates with the carousel, and an inertial observer. The figure shows a bird's-eye view based upon the same ball speed on forward and return paths. Within each circle, plotted dots show the same time points. In the left panel, from the camera's viewpoint at the center of rotation, the tosser (smiley face) and the rail both are at fixed locations, and the ball makes a very considerable arc on its travel toward the rail, and takes a more direct route on the way back. From the ball tosser's viewpoint, the ball seems to return more quickly than it went (because the tosser is rotating toward the ball on the return flight).
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| On the carousel, instead of tossing the ball straight at a rail to bounce back, the tosser must throw the ball toward the right of the target and the ball then seems to the camera to bear continuously to the left of its direction of travel to hit the rail (''left'' because the carousel is turning ''clockwise''). The ball appears to bear to the left from direction of travel on both inward and return trajectories. The curved path demands this observer to recognize a leftward net force on the ball. (This force is "fictitious" because it disappears for a stationary observer, as is discussed shortly.) For some angles of launch, a path has portions where the trajectory is approximately radial, and Coriolis force is primarily responsible for the apparent deflection of the ball (centrifugal force is radial from the center of rotation, and causes little deflection on these segments). When a path curves away from radial, however, centrifugal force contributes significantly to deflection.
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| The ball's path through the air is straight when viewed by observers standing on the ground (right panel). In the right panel (stationary observer), the ball tosser (smiley face) is at 12 o'clock and the rail the ball bounces from is at position one (1). From the inertial viewer's standpoint, positions one (1), two (2), three (3) are occupied in sequence. At position 2 the ball strikes the rail, and at position 3 the ball returns to the tosser. Straight-line paths are followed because the ball is in free flight, so this observer requires that no net force is applied.
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| ===Bullets at high velocity through the atmosphere===
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| Because of the rotation of the earth, a bullet does not fly straight from the barrel to a target. The Coriolis effect minutely changes the trajectory of a bullet, curving the path of the projectile. This effect only affects accuracy at extremely long distances and is therefore accounted for by accurate long-distance shooters, such as snipers or contestants in long-distance shooting competitions.<ref name=Taylor0 />
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| ==Visualization of the Coriolis effect==
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| [[File:Coriolis effect11.jpg|thumb|right|A fluid assuming a parabolic shape as it is rotating]]
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| [[File:Forces in a Parabolic Dish.svg|thumb|right|The forces at play in the case of a curved surface.<br>''Red'': gravity<br>''Green'': the [[normal force]]<br>''Blue'': the resultant [[centripetal force]].]]
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| [[File:Parabolic dish ellipse oscill.gif|frame|right|Object moving frictionlessly over the surface of a very shallow parabolic dish. The object has been released in such a way that it follows an elliptical trajectory.<br>''Left'': The inertial point of view.<br>''Right'': The co-rotating point of view.]]
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| {{anchor|parabolic turntable}}To demonstrate the Coriolis effect, a parabolic turntable can be used. On a flat turntable, the inertia of a co-rotating object would force it off the edge. But if the surface of the turntable has the correct [[paraboloid]] (parabolic bowl) shape (see the figure) and is rotated at the corresponding rate, the force components shown in the figure are such that the component of gravity tangential to the bowl surface will exactly equal the centripetal force necessary to keep the object rotating at its velocity and radius of curvature (assuming no friction). (See [[Centripetal force#Example: The banked turn|banked turn]].) This carefully contoured surface allows the Coriolis force to be displayed in isolation.<ref name=parabola>When a container of fluid is rotating on a turntable, the surface of the fluid naturally assumes the correct [[parabola|parabolic]] shape. This fact may be exploited to make a parabolic turntable by using a fluid that sets after several hours, such as a synthetic [[resin]]. For a video of the Coriolis effect on such a parabolic surface, see [http://www-paoc.mit.edu/labweb/lab5/gfd_v.htm Geophysical fluid dynamics lab demonstration] John Marshall, Massachusetts Institute of Technology.</ref><ref name=animation>For a java applet of the Coriolis effect on such a parabolic surface, see [http://mensch.org/physlets/inosc.html Brian Fiedler] School of Meteorology at the University of Oklahoma.</ref>
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| Discs cut from cylinders of [[dry ice]] can be used as pucks, moving around almost frictionlessly over the surface of the parabolic turntable, allowing effects of Coriolis on dynamic phenomena to show themselves. To get a view of the motions as seen from the reference frame rotating with the turntable, a video camera is attached to the turntable so as to co-rotate with the turntable, with results as shown in the figure. In the left panel of the figure, which is the viewpoint of a stationary observer, the gravitational force in the inertial frame pulling the object toward the center (bottom ) of the dish is proportional to the distance of the object from the center. A centripetal force of this form causes the elliptical motion. In the right panel, which shows the viewpoint of the rotating frame, the inward gravitational force in the rotating frame (the same force as in the inertial frame) is balanced by the outward centrifugal force (present only in the rotating frame). With these two forces balanced, in the rotating frame the only unbalanced force is Coriolis (also present only in the rotating frame), and the motion is an ''[[#Inertial circles|inertial circle]]''. Analysis and observation of circular motion in the rotating frame is a simplification compared to analysis or observation of elliptical motion in the inertial frame.
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| Because this reference frame rotates several times a minute rather than only once a day like the Earth, the Coriolis acceleration produced is many times larger and so easier to observe on small time and spatial scales than is the Coriolis acceleration caused by the rotation of the Earth.
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| In a manner of speaking, the Earth is analogous to such a turntable.<ref name=Marshall>{{Cite book|title=Atmosphere, Ocean, and Climate Dynamics: An Introductory Text|author=John Marshall & R. Alan Plumb |page=101 |url=http://books.google.com/?id=aTGYbmVaA_gC&pg=PA101|isbn=0-12-558691-4|year=2007|publisher=Academic Press}}</ref> The rotation has caused the planet to settle on a spheroid shape, such that the normal force, the gravitational force and the centrifugal force exactly balance each other on a "horizontal" surface. (See [[equatorial bulge]].)
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| The Coriolis effect caused by the rotation of the Earth can be seen indirectly through the motion of a [[Foucault pendulum]].
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| {{clear}}
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| ==Coriolis effects in other areas==
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| ===Coriolis flow meter===
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| A practical application of the Coriolis effect is the [[mass flow meter]], an instrument that measures the [[mass flow rate]] and [[density]] of a fluid flowing through a tube. The operating principle involves inducing a vibration of the tube through which the fluid passes. The vibration, though it is not completely circular, provides the rotating reference frame which gives rise to the Coriolis effect. While specific methods vary according to the design of the flow meter, sensors monitor and analyze changes in frequency, phase shift, and amplitude of the vibrating flow tubes. The changes observed represent the mass flow rate and density of the fluid.<ref>{{Cite journal|author = Omega Engineering|url = http://www.omega.com/literature/transactions/volume4/t9904-10-mass.html|title = Mass Flowmeters}}</ref>
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| ===Molecular physics===
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| In polyatomic molecules, the molecule motion can be described by a rigid body rotation and internal vibration of atoms about their equilibrium position. As a result of the vibrations of the atoms, the atoms are in motion relative to the rotating coordinate system of the molecule. Coriolis effects will therefore be present and will cause the atoms to move in a direction perpendicular to the original oscillations. This leads to a mixing in molecular spectra between the rotational and vibrational [[energy level|levels]] from which Coriolis coupling constants can be determined.<ref>{{cite book |last1=califano |first1=S |title=Vibrational states |year=1976|publisher= Wiley |isbn=0471129968|pages=226–227}}</ref>
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| ===Insect flight===
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| Flies ([[Diptera]]) and moths ([[Lepidoptera]]) utilize the Coriolis effect when flying: their [[halteres]], or antennae in the case of moths, oscillate rapidly and are used as vibrational gyroscopes.<ref>"Antennae as Gyroscopes", Science, Vol. 315, 9 February 2007, p. 771</ref> See ''Coriolis effect in insect stability''.<ref>{{cite journal|title=Halteres for the micromechanical flying insect |doi=10.1109/ROBOT.2002.1013339 |author=Wu, W.C.; Wood, R.J.; Fearing, R.S. |journal=IEEE International Conference on Robotics and Automation, 2002. Proceedings. ICRA '02. |year=2002 |volume=1 |pages=60–65 |isbn=0-7803-7272-7 }}</ref> In this context, the Coriolis effect has nothing to do with the rotation of the Earth.
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| {{clear}}
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| ==See also==
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| {{Portal|Physics}}
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| {{div col|colwidth=30em}}
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| *[[Analytical mechanics]]
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| *[[Applied mechanics]]
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| *[[Centrifugal force]]
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| *[[Centrifugal force (rotating reference frame)]]
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| *[[Centripetal force]]
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| *[[Classical mechanics]]
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| *[[Dynamics (physics)]]
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| *[[Earth's rotation]]
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| *[[Equatorial Rossby wave]]
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| *[[Frenet–Serret formulas]]
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| *[[Geostrophic wind]]
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| *[[Gyroscope]]
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| *[[Kinetics (physics)]]
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| *[[Mass flow meter]]
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| *[[Mechanics of planar particle motion]]
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| *[[Reactive centrifugal force]]
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| *[[Secondary flow]]
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| *[[Statics]]
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| *[[Uniform circular motion]]
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| {{div col end}}
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| ==References==
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| {{Reflist|colwidth=30em}}
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| ===Further reading===
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| ====Physics and meteorology====
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| {{div col|colwidth=30em}}
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| *Riccioli, G.B., 1651: ''Almagestum Novum'', Bologna, pp. 425–427<br> ([http://www.e-rara.ch/zut/content/pageview/141486 Original book] [in Latin], scanned images of complete pages.)
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| *Coriolis, G.G., 1832: ''Mémoire sur le principe des forces vives dans les mouvements relatifs des machines.'' Journal de l'école Polytechnique, Vol 13, 268–302.<br> ([http://www.aos.princeton.edu/WWWPUBLIC/gkv/history/Coriolis-1831.pdf Original article] [in French], PDF-file, 1.6 MB, scanned images of complete pages.)
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| *Coriolis, G.G., 1835: ''Mémoire sur les équations du mouvement relatif des systèmes de corps.'' Journal de l'école Polytechnique, Vol 15, 142–154 <br> ([http://www.aos.princeton.edu/WWWPUBLIC/gkv/history/Coriolis-1835.pdf Original article] [in French] PDF-file, 400 KB, scanned images of complete pages.)
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| *Gill, AE ''Atmosphere-Ocean dynamics'', Academic Press, 1982.
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| *{{Cite book|title=Turning the World Inside Out and 174 Other Simple Physics Demonstrations |author=Robert Ehrlich |publisher=Princeton University Press |year=1990 |isbn=0-691-02395-6 |page= ''Rolling a ball on a rotating turntable''; p. 80 ff |url=http://books.google.com/?id=ehSTsNS9qB4C&pg=PA80}}
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| *[http://www.atmos.washington.edu/~durrand/ Durran, D. R.], 1993: ''[http://www.atmos.washington.edu/~durrand/pdfs/Coriolis_BAMS.pdf Is the Coriolis force really responsible for the inertial oscillation?]'', Bull. Amer. Meteor. Soc., 74, 2179–2184; Corrigenda. Bulletin of the American Meteorological Society, 75, 261
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| *Durran, D. R., and S. K. Domonkos, 1996: ''[http://www.atmos.washington.edu/~durrand/pdfs/inertial_osc.pdf An apparatus for demonstrating the inertial oscillation]'', Bulletin of the American Meteorological Society, 77, 557–559.
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| *Marion, Jerry B. 1970, ''Classical Dynamics of Particles and Systems'', Academic Press.
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| *Persson, A., 1998 ''[http://www.aos.princeton.edu/WWWPUBLIC/gkv/history/Persson98.pdf] How do we Understand the Coriolis Force?'' Bulletin of the American Meteorological Society 79, 1373–1385.
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| *Symon, Keith. 1971, ''Mechanics'', Addison–Wesley
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| *[http://arxiv.org/abs/physics/0509004v2 Akira Kageyama & Mamoru Hyodo: ''Eulerian derivation of the Coriolis force'']
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| *[http://www.whoi.edu/science/PO/people/jprice/class/aCt.pdf James F. Price: ''A Coriolis tutorial''] Woods Hole Oceanographic Institute (2003)
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| * '''McDONALD''', James E.''':''' "'''''The Coriolis Effect'''''," bullet''':''' ''Everything that moves over the surface of the Earth -- water, air, animals, machines and projectiles -- sidles to the right in the Northern Hemisphere and to the left, in the Southern'', '''''Scientific American''''', pp. 72–78, May 1952. Elementary, non-mathematical; but well written.
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| {{div col end}}
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| ====Historical====
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| {{div col|colwidth=30em}}
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| *Grattan-Guinness, I., Ed., 1994: ''Companion Encyclopedia of the History and Philosophy of the Mathematical Sciences''. Vols. I and II. Routledge, 1840 pp. <br> 1997: ''The Fontana History of the Mathematical Sciences''. Fontana, 817 pp. 710 pp.
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| *Khrgian, A., 1970: ''Meteorology — A Historical Survey''. Vol. 1. Keter Press, 387 pp.
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| *Kuhn, T. S., 1977: Energy conservation as an example of simultaneous discovery. ''The Essential Tension, Selected Studies in Scientific Tradition and Change'', University of Chicago Press, 66–104.
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| *Kutzbach, G., 1979: ''The Thermal Theory of Cyclones. A History of Meteorological Thought in the Nineteenth Century''. Amer. Meteor. Soc., 254 pp.
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| {{div col end}}
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| ==External links==
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| {{Commons|Coriolis effect}}
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| *[http://amsglossary.allenpress.com/glossary/search?id=coriolis-force1 The definition of the Coriolis effect from the Glossary of Meteorology]
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| <!-- Dead note "2": [http://www-paoc.mit.edu/labweb/lab5/gfd_v.htm Rotating turntable setup] The concave turntable used at MIT for educational purposes. -->
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| <!-- Dead note "3": [http://www-paoc.mit.edu/labweb/lab1/gfd_1.htm Taylor columns] The counterintuitive behavior of a rotating fluid. Demonstration at MIT for educational purposes -->
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| *[http://web.archive.org/web/20050906101226/http://met.no/english/topics/nomek_2005/coriolis.pdf The Coriolis Effect — a conflict between common sense and mathematics] PDF-file. 20 pages. A general discussion by Anders Persson of various aspects of the coriolis effect, including Foucault's Pendulum and Taylor columns.
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| *Anders Persson [http://www.meteohistory.org/2005historyofmeteorology2/01persson.pdf The Coriolis Effect: Four centuries of conflict between common sense and mathematics, Part I: A history to 1885] History of Meteorology 2 (2005)
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| *[http://weather.about.com/od/weathertutorials/tp/coriolisvideos.htm 10 Coriolis Effect Videos and Games]- from the About.com Weather Page
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| *[http://scienceworld.wolfram.com/physics/CoriolisForce.html Coriolis Force] – from [[ScienceWorld]]
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| *[http://www.newton.dep.anl.gov/askasci/phy00/phy00733.htm ''Coriolis Effect and Drains''] An article from the NEWTON web site hosted by the [[Argonne National Laboratory]].
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| *[http://www.imaginascience.com/articles/sciencesphysiques/mecanique/coriolis/coriolis4.php Catalog of Coriolis videos]
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| *[http://www.youtube.com/watch?v=mcPs_OdQOYU ''Coriolis Effect: A graphical animation''], a visual earth animation with precise explanation
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| *[http://www.youtube.com/watch?v=khRil3tWYEA ''An introduction to fluid dynamics''] SPINLab Educational Film explains the Coriolis effect with the aid of lab experiments
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| *[http://www.straightdope.com/classics/a1_161.html ''Do bathtubs drain counterclockwise in the Northern Hemisphere?''] by Cecil Adams.
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| *[http://www.ems.psu.edu/~fraser/Bad/BadCoriolis.html ''Bad Coriolis.''] An article uncovering misinformation about the Coriolis effect. By Alistair B. Fraser, Emeritus Professor of Meteorology at [[Pennsylvania State University]]
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| *[http://stratus.ssec.wisc.edu/courses/gg101/coriolis/coriolis.html ''The Coriolis Effect: A (Fairly) Simple Explanation''], an explanation for the layperson
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| *[http://www.classzone.com/books/earth_science/terc/content/visualizations/es1904/es1904page01.cfm?chapter_no=visualization Observe an animation of the Coriolis effect over Earth's surface]
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| *[http://www.youtube.com/watch?v=49JwbrXcPjc Animation clip] showing scenes as viewed from both an inertial frame and a rotating frame of reference, visualizing the Coriolis and centrifugal forces.
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| *[http://www.inwit.com/inwit/writings/coriolisforce.html Vincent Mallette ''The Coriolis Force'' @ INWIT]
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| *[http://pwg.gsfc.nasa.gov/stargaze/Srotfram.htm NASA notes]
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| *[http://andygiger.com/science/e-coriolis/index.html Interactive Coriolis Fountain] lets you control rotation speed, droplet speed and frame of reference to explore the Coriolis effect.
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| *[http://nigerianschoolsonline.com.ng/index.php/article/1419-the-coriolis-effect Nigeianschoolsonline- The Coriolis Effect]
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