# Centrifugal force: Wikis

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# Encyclopedia

Centrifugal force (from Latin centrum "center" and fugere "to flee") represents the effects of inertia that arise in connection with rotation and which are experienced as an outward force away from the center of rotation. In Newtonian mechanics, the term centrifugal force is used to refer to one of two distinct concepts: an inertial force (also called a "fictitious" force) observed in a non-inertial reference frame or a reaction force corresponding to a centripetal force. The term is also sometimes used in Lagrangian mechanics to describe certain terms in the generalized force that depend on the choice of generalized coordinates.

## Fictitious centrifugal force

Centrifugal force is most commonly introduced as a force associated with describing motion in a non-inertial reference frame, and referred to as a fictitious or inertial force (a description that must be understood as a technical usage of these words that means only that the force is not present in a stationary or inertial frame).[1][2] There are three contexts in which the concept of the fictitious force arises when describing motion using classical mechanics.[3] In the first context, the motion is described relative to a rotating reference frame about a fixed axis at the origin of the coordinate system. For observations made in the rotating frame, all objects appear to be under the influence of a radially outward force that is proportional to the distance from the axis of rotation and to the rate of rotation of the frame. The second context is similar, and describes the motion using an accelerated local reference frame attached to a moving body, for example, the frame of passengers in a car as it rounds a corner.[3] In this case, rotation is again involved, this time about the center of curvature of the path of the moving body. In both these contexts, the centrifugal force is zero when the rate of rotation of the reference frame is zero, independent of the motions of objects in the frame.[4]

The third context is related to the use of generalized coordinates as is done in the Lagrangian formulation of mechanics, discussed below. Here the term "centrifugal force" is an abbreviated substitute for "generalized centrifugal force", which in general has little connection with the Newtonian concept of centrifugal force.

If objects are seen as moving from a rotating frame, this movement results in another fictitious force, the Coriolis force; and if the rate of rotation of the frame is changing, a third fictitious force, the Euler force is experienced. Together, these three fictitious forces allow for the creation of correct equations of motion in a rotating reference frame.[4]

## Reactive centrifugal force

A reactive centrifugal force is the reaction force to a centripetal force. A mass undergoing curved motion, such as circular motion, constantly accelerates toward the axis of rotation. This centripetal acceleration is provided by a centripetal force, which is exerted on the mass by some other object. In accordance with Newton's Third Law of Motion, the mass exerts an equal and opposite force on the object. This is the "real" or "reactive" centrifugal force: it is directed away from the center of rotation, and is exerted by the rotating mass on the object that originates the centripetal acceleration.[5][6][7]

The concept of the reactive centrifugal force is used often in mechanical engineering sources that deal with internal stresses in rotating solid bodies.[8] Newton's reactive centrifugal force still appears in some sources, and often is referred to as the centrifugal force rather than as the reactive centrifugal force.[9][10][11][12][13][14][15][16][17]

## Fictitious vs. reactive force

The table below compares various facets of the "fictitious force" and "reactive force" concepts of centrifugal force

Fictitious centrifugal force Reactive centrifugal force Non-inertial frames Any Acts as if emanating from the rotation axis, but no real source Bodies moving in circular paths All bodies, moving or not; if moving, Coriolis force also is present The object(s) causing the curved motion, not upon the body in curved motion Away from rotation axis, regardless of path of body Opposite to the centripetal force causing curved path Kinetic: included as force in Newton's laws of motion Kinematic: related to centripetal force

## Lagrangian formulation of centrifugal force

Lagrangian mechanics formulates mechanics in terms of generalized coordinates {qk}, which can be as simple as the usual polar coordinates $(r,\ \theta)$ or a much more extensive list of variables.[18][19] Within this formulation the motion is described in terms of generalized forces, using in place of Newton's laws the Euler–Lagrange equations. Among the generalized forces, those involving the square of the time derivatives {(dqk / dt)2} are sometimes called centrifugal forces.[20][21][22][23]

The Lagrangian approach to polar coordinates that treats $(r,\ \theta)$ as generalized coordinates, $(\dot{r},\ \dot{\theta})$ as generalized velocities and $(\ddot{r},\ \ddot{\theta})$ as generalized accelerations, is outlined in another article, and found in many sources.[24][25][26] For the particular case of single-body motion found using the generalized coordinates $(\dot{r},\ \dot{\theta})$ in a central force, the Euler–Lagrange equations are the same equations found using Newton's second law in a co-rotating frame. For example, the radial equation is:

$\mu\ddot{r} = \mu r\dot\theta^2 - \frac{\mathrm{d}U}{\mathrm{d}r}$

where U(r) is the central force potential. The left side is a "generalized force" and the first term on the right is the "generalized centrifugal force". However, the left side is not comparable to a Newtonian force, as it does not contain the complete radial acceleration, and likewise, therefore, the terms on the right-hand side are "generalized forces" and cannot be interpreted as Newtonian forces.[27]

The Lagrangian centrifugal force is derived without explicit use of a rotating frame of reference,[28] but in the case of motion in a central potential the result is the same as the fictitious centrifugal force derived in a co-rotating frame.[3] The Lagrangian use of "centrifugal force" in other, more general cases, however, has only a limited connection to the Newtonian definition.

## Centrifugal force and absolute rotation

The consideration of centrifugal force and absolute rotation is a topic of debate about relativity, cosmology, and the nature of physical laws.

Can absolute rotation be detected? In other words, can one decide whether an observed object is rotating or if it is you, the observer that is rotating? Newton suggested two experiments to resolve this problem. One is the effect of centrifugal force upon the shape of the surface of water rotating in a bucket. The second is the effect of centrifugal force upon the tension in a string joining two spheres rotating about their center of mass. A related third suggestion was that rotation of a sphere (such as a planet) could be detected from its shape (or "figure"), which is formed as a balance between containment by gravitational attraction and dispersal by centrifugal force.

## History of conceptions of centrifugal and centripetal forces

The conception of centrifugal force has evolved since the time of Huygens, Newton, Leibniz, and Hooke who expressed early conceptions of it. The modern conception as a fictitious force or pseudo force due to a rotating reference frame as described above evolved in the eighteenth and nineteenth centuries.

The concept of centrifugal force in its more technical aspects introduces several additional topics:

• Reference frames, which compare observations by observers in different states of motion. Among the many possible reference frames the inertial frame of reference are singled out as the frames where physical laws take their simplest form. In this context, physical forces are divided into two groups: real forces that originate in real sources, like electrical force originates in charges, and
• Fictitious forces that do not so originate, but originate instead in the motion of the observer. Naturally, forces that originate in the motion of the observer vary with the motion of the observer, and in particular vanish for some observers, namely those in inertial frames of reference.

The analogy between centrifugal force (sometimes used to create artificial gravity) and gravitational forces led to the equivalence principle of general relativity.[29][30]

## References

1. ^ Takwale & Puranik 1980, p. 248.
2. ^ Jacobson 1980, p. 80.
3. ^ a b c See p. 5 in Donato Bini, Paolo Carini, Robert T Jantzen (1997). "The intrinsic derivative and centrifugal forces in general relativity: I. Theoretical foundations". International Journal of Modern Physics D 6 (1).  . The companion paper is Donato Bini, Paolo Carini, Robert T Jantzen (1997). "The intrinsic derivative and centrifugal forces in general relativity: II. Applications to circular orbits in some stationary axisymmetric spacetimes". International Journal of Modern Physics D 6 (1).
4. ^ a b Fetter & Walecka 2003, pp. 38-39.
5. ^ Mook & Vargish 1987, p. 47.
6. ^ Signell 2002, "Acceleration and force in circular motion", §5b, p. 7.
7. ^ Mohanty 2004, p. 121.
8. ^ Roche 2001, "Introducing motion in a circle". Retrieved 2009-05-07.
9. ^ Edward Albert Bowser (1920). An elementary treatise on analytic mechanics: with numerous examples (25th ed.). D. Van Nostrand Company. p. 357.
10. ^ Gerald James Holton and Stephen G. Brush (2001). Physics, the human adventure: from Copernicus to Einstein and beyond. Rutgers University Press. p. 126. ISBN 9780813529080.
11. ^ Ervin Sidney Ferry (2008). A Brief Course in Elementary Dynamics. BiblioBazaar. pp. 87–88. ISBN 9780554609843.
12. ^ Willis Ernest Johnson (2009). Mathematical Geography. BiblioBazaar. p. 15–16. ISBN 9781103199587.
13. ^ Eugene A. Avallone, Theodore Baumeister, Ali Sadegh, Lionel Simeon Marks (2006). Marks' standard handbook for mechanical engineers (11 ed.). McGraw-Hill Professional. p. 15. ISBN 0071428674.
14. ^ Richard Cammack, Anthony Donald Smith, Teresa K. Attwood, Peter Campbell (2006). Oxford dictionary of biochemistry and molecular biology (2 ed.). Oxford University Press. p. 109. ISBN 0198529171.
15. ^ Joseph A. Angelo (2007). Robotics: a reference guide to the new technology. Greenwood Press. p. 267. ISBN 1573563374.
16. ^ P. Grimshaw, A. Lees, N. Fowler, A. Burden (2006). Sport and exercise biomechanics. Routledge. p. 176. ISBN 185996284X.
17. ^ Joel Dorman Steele (2008). Popular Physics (Reprint ed.). READ books. p. 31. ISBN 1408691345.
18. ^ For an introduction, see for example Cornelius Lanczos (1986). The variational principles of mechanics (Reprint of 1970 University of Toronto ed.). Dover. p. 1. ISBN 0486650677.
19. ^ For a description of generalized coordinates, see Ahmed A. Shabana (2003). "Generalized coordinates and kinematic constraints". Dynamics of Multibody Systems (2 ed.). Cambridge University Press. p. 90 ff. ISBN 0521544114.
20. ^ Christian Ott (2008). Cartesian Impedance Control of Redundant and Flexible-Joint Robots. Springer. p. 23. ISBN 3540692533.
21. ^ Shuzhi S. Ge, Tong Heng Lee, Christopher John Harris (1998). Adaptive Neural Network Control of Robotic Manipulators. World Scientific. p. 47–48. ISBN 981023452X. "In the above Euler–Lagrange equations, there are three types of terms. The first involves the second derivative of the generalized co-ordinates. The second is quadratic in $\boldsymbol{\dot q}$ where the coefficients may depend on $\boldsymbol{q}$. These are further classified into two types. Terms involving a product of the type ${\dot q_i}^2$ are called centrifugal forces while those involving a product of the type $\dot q_i \dot q_j$ for i ≠ j are called Coriolis forces. The third type is functions of $\boldsymbol{q}$ only and are called gravitational forces."
22. ^ R. K. Mittal, I. J. Nagrath (2003). Robotics and Control. Tata McGraw-Hill. p. 202. ISBN 0070482934.
23. ^ T Yanao & K Takatsuka (2005). "Effects of an intrinsic metric of molecular internal space". in Mikito Toda, Tamiki Komatsuzaki, Stuart A. Rice, Tetsuro Konishi, R. Stephen Berry. Geometrical Structures Of Phase Space In Multi-dimensional Chaos: Applications to chemical reaction dynamics in complex systems. Wiley. p. 98. ISBN 0471711578. "As is evident from the first terms …, which are proportional to the square of $\dot\phi$, a kind of "centrifugal force" arises … We call this force "democratic centrifugal force". Of course, DCF is different from the ordinary centrifugal force, and it arises even in a system of zero angular momentum."
24. ^ See, for example, Eq. 8.20 in John R Taylor (2005). Classical Mechanics. Sausalito, Calif.: Univ. Science Books. pp. 299 ff. ISBN 189138922X.
25. ^ Francis Begnaud Hildebrand (1992). Methods of Applied Mathematics (Reprint of 1965 2nd ed.). Courier Dover Publications. p. 156. ISBN 0486670023.
26. ^ V. B. Bhatia (1997). Classical Mechanics: With Introduction to Nonlinear Oscillations and Chaos. Alpha Science Int'l Ltd.. p. 82. ISBN 8173191050.
27. ^ Henry M. Stommel and Dennis W. Moore (1989). An Introduction to the Coriolis Force Columbia University Press. Pp. 36–38.
28. ^ See Edmond T Whittaker (1988). A treatise on the analytical dynamics of particles and rigid bodies (Reprint of 1917 2nd ed.). Cambridge University Press. pp. 40–41. ISBN 0521358833.   for an explanation of how the fictitious centrifugal force corresponds to a potential term in the Lagrangian.
29. ^ Barbour & Pfister 1995, p. 69.
30. ^ Eriksson 2008, p. 194.

# Simple English

In physics, centrifugal force (from Latin centrum "center" and fugere "to flee") is a fictitious force that appears when describing physics in a rotating reference frame; it acts on anything with mass considered in such a frame. Centrifugal force is fictitious because although it may feel to a person like a certain force is being exerted on them, someone outside the scene will see something different.

Example: If John is in a car that takes a sharp right turn, he will feel as though he is being pushed to his left. This is an imaginary force, called a centrifugal force, or a "running away from the center" force. John feels it because he is inside the car and is affected by it. However, if John's friend, Andy, is on the side of the road facing the front of John's car and watches John's car take a sharp right turn, Andy will see the car push John to the right with the car as it changes direction. This is a real force called centripetal force (or an "aiming towards the center" force) and acts towards the center of the circle of rotation.

In other words, John's body, traveling in a straight line before the car turned, wanted to keep moving in a straight line because it had momentum in that direction, but the car, which was exerting a force on John by turning, is pulling him to the right. John feels like his body is being pushed to the left as the car turns, but in fact, his body is being pulled to the right by the turning car.

This is a problem that appears in rotating situations. There is a difference between what appears to be happening if watched from the outside or if the event was seen from within a rotating object. These differing viewpoints are called frames of reference.

Despite the name, fictitious forces are experienced as very real by anyone whose immediate environment is a non-inertial frame. Even for observers in an inertial frame, fictitious forces provide a natural way to discuss dynamics within rotating environments such as planets, centrifuges, carousels, turning cars, and spinning buckets.