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A rotating frame of reference is a special case of a non-inertial reference frame that is rotating relative to an inertial reference frame. An everyday example of a rotating reference frame is the surface of the Earth. (This article considers only frames rotating about a fixed axis. For more general rotations, see Euler angles.)
[edit] Fictitious forcesMain article: Fictitious forces All non-inertial reference frames exhibit fictitious forces. Rotating reference frames are characterized by three fictitious forces[1]
and, for non-uniformly rotating reference frames,
Scientists living in a rotating box can measure the speed and direction of their rotation by measuring these fictitious forces. For example, Léon Foucault was able to show the Coriolis force that results from the Earth's rotation using the Foucault pendulum. If the Earth were to rotate a thousand-fold faster (making each day only ~86 seconds long), these fictitious forces could be felt as easily by humans, as they are when on a spinning carousel. [edit] Relating rotating frames to stationary framesThe following is a derivation of the formulas for accelerations as well as fictitious forces in a rotating frame. It begins with the relation between coordinates of the position of a particle in a rotating frame and the coordinates in an inertial (stationary) frame. Then, by taking time derivatives, formulas are derived that relate the velocity of the particle as seen in the two frames, and the acceleration relative to each frame. Using these accelerations a comparison of Newton's second law as formulated in the frames identifies the fictitious forces. [edit] Relation between positions in the two framesTo derive these fictitious forces, it's helpful to be able to convert between the coordinates whereas the reverse transformation is This result can be obtained from a rotation matrix. Introduce the unit vectors where the (x, y) components are expressed in the stationary frame. Likewise, Thus the time derivative of these vectors, which rotate without changing magnitude, is This result is the same as found using a vector cross product with the rotation vector where [edit] Time derivatives in the two framesIntroduce the unit vectors Then if we have a vector function and we want to examine its first dervative we have (using the product rule of differentiation):[2][3] where This result is also known as the Transport Theorem in analytical dynamics. [edit] Relation between velocities in the two framesA velocity of an object is the time-derivative of the object's position, or The time derivative of a position where subscript i means the inertial frame of reference, and r means the rotating frame of reference. [edit] Relation between accelerations in the two framesAcceleration is the second time derivative of position, or the first time derivative of velocity where subscript i means the inertial frame of reference. Carrying out the differentiations and re-arranging some terms yields the acceleration in the rotating reference frame where [edit] Newton's second law in the two framesWhen the expression for acceleration is multiplied by the mass of the particle, the three extra terms on the right-hand side result in fictitious forces in the rotating reference frame, that is, apparent forces that result from being in a non-inertial reference frame, rather than from any physical interaction between bodies. Using Newton's second law of motion F = m a, we obtain:[1][2][3][4][5]
where m is the mass of the object being acted upon by these fictitious forces. Notice that all three forces vanish when the frame is not rotating, that is, when For completeness, the inertial acceleration Newton's law in the the rotating frame then becomes In other words, to handle the laws of motion in a rotating reference frame:[5][6][7]
[edit] Centrifugal forceMain article: Centrifugal force (rotating reference frame) In classical mechanics, centrifugal force is an outward force associated with rotation. Centrifugal force is one of several so-called pseudo-forces (also known as inertial forces), so named because, unlike real forces, they do not originate in interactions with other bodies situated in the environment of the particle upon which they act. Instead, centrifugal force originates in the rotation of the frame of reference within which observations are made.[8][9][10][11][12][13] [edit] Coriolis effectMain article: Coriolis effect The mathematical expression for the Coriolis force appeared in an 1835 paper by a French scientist Gaspard-Gustave Coriolis in connection with hydrodynamics, and also in the tidal equations of Pierre-Simon Laplace in 1778. Early in the 20th century, the term Coriolis force began to be used in connection with meteorology. Perhaps the most commonly encountered rotating reference frame is the Earth. Moving objects on the surface of the Earth experience a Coriolis force, and appear to veer to the right in the northern hemisphere, and to the left in the southern. Movements of air in the atmosphere and water in the ocean are notable examples of this behavior: rather than flowing directly from areas of high pressure to low pressure, as they would on a non-rotating planet, 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 the equator. This effect is responsible for the rotation of large cyclones (see Coriolis effects in meteorology). [edit] Euler forceMain article: Euler force In classical mechanics, the Euler acceleration (named for Leonhard Euler), also known as azimuthal acceleration[14] or transverse acceleration[15] is an acceleration that appears when a non-uniformly rotating reference frame is used for analysis of motion and there is variation in the angular velocity of the reference frame's axis. This article is restricted to a frame of reference that rotates about a fixed axis. The Euler force is a fictitious force on a body that is related to the Euler acceleration by F = m a , where a is the Euler acceleration and m is the mass of the body.[16] [17] [edit] References and notes
[edit] See also
[edit] External links
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