In differential geometry, the **radius of curvature**, R, is the reciprocal of the curvature. For a curve, it equals the radius of the circular arc which best approximates the curve at that point. For surfaces, the radius of curvature is the radius of a circle that best fits a normal section or combinations thereof.^{ [1] }^{ [2] }^{ [3] }

In the case of a space curve, the radius of curvature is the length of the curvature vector.

In the case of a plane curve, then R is the absolute value of^{ [3] }

where s is the arc length from a fixed point on the curve, φ is the tangential angle and κ is the curvature.

If the curve is given in Cartesian coordinates as *y*(*x*), then the radius of curvature is (assuming the curve is differentiable up to order 2):

and |*z*| denotes the absolute value of z.

If the curve is given parametrically by functions *x*(*t*) and *y*(*t*), then the radius of curvature is

Heuristically, this result can be interpreted as^{ [2] }

If **γ** : ℝ → ℝ^{n} is a parametrized curve in ℝ^{n} then the radius of curvature at each point of the curve, *ρ* : ℝ → ℝ, is given by^{ [3] }

- .

As a special case, if *f*(*t*) is a function from ℝ to ℝ, then the radius of curvature of its graph, **γ**(*t*) = (*t*, *f*(*t*)), is

Let **γ** be as above, and fix t. We want to find the radius ρ of a parametrized circle which matches γ in its zeroth, first, and second derivatives at t. Clearly the radius will not depend on the position **γ**(*t*), only on the velocity **γ**′(*t*) and acceleration **γ**″(*t*). There are only three independent scalars that can be obtained from two vectors **v** and **w**, namely **v** · **v**, **v** · **w**, and **w** · **w**. Thus the radius of curvature must be a function of the three scalars |**γ**′(*t*)|^{2}, |**γ**″(*t*)|^{2} and **γ**′(*t*) · **γ**″(*t*).^{ [3] }

The general equation for a parametrized circle in ℝ^{n} is

where **c** ∈ ℝ^{n} is the center of the circle (irrelevant since it disappears in the derivatives), **a**,**b** ∈ ℝ^{n} are perpendicular vectors of length ρ (that is, **a** · **a** = **b** · **b** = *ρ*^{2}^{} and **a** · **b** = 0), and *h* : ℝ → ℝ is an arbitrary function which is twice differentiable at t.

The relevant derivatives of **g** work out to be

If we now equate these derivatives of **g** to the corresponding derivatives of **γ** at t we obtain

These three equations in three unknowns (ρ, *h*′(*t*) and *h*″(*t*)) can be solved for ρ, giving the formula for the radius of curvature:

or, omitting the parameter t for readability,

For a semi-circle of radius a in the upper half-plane

For a semi-circle of radius a in the lower half-plane

The circle of radius a has a radius of curvature equal to a.

In an ellipse with major axis 2*a* and minor axis 2*b*, the vertices on the major axis have the smallest radius of curvature of any points, *R* = *b*^{2}/*a*; and the vertices on the minor axis have the largest radius of curvature of any points, *R* = *a*^{2}/*b*.

- For the use in differential geometry, see Cesàro equation.
- For the radius of curvature of the earth (approximated by an oblate ellipsoid); see also: arc measurement
- Radius of curvature is also used in a three part equation for bending of beams.
- Radius of curvature (optics)
- Thin films technologies
- Printed electronics
- Minimum railway curve radius

Stress in the semiconductor structure involving evaporated thin films usually results from the thermal expansion (thermal stress) during the manufacturing process. Thermal stress occurs because film depositions are usually made above room temperature. Upon cooling from the deposition temperature to room temperature, the difference in the thermal expansion coefficients of the substrate and the film cause thermal stress.^{ [4] }

Intrinsic stress results from the microstructure created in the film as atoms are deposited on the substrate. Tensile stress results from microvoids (small holes, considered to be defects) in the thin film, because of the attractive interaction of atoms across the voids.

The stress in thin film semiconductor structures results in the buckling of the wafers. The radius of the curvature of the stressed structure is related to stress tensor in the structure, and can be described by modified Stoney formula.^{ [5] } The topography of the stressed structure including radii of curvature can be measured using optical scanner methods. The modern scanner tools have capability to measure full topography of the substrate and to measure both principal radii of curvature, while providing the accuracy of the order of 0.1% for radii of curvature of 90 meters and more.^{ [6] }

A **centripetal force** is a force that makes a body follow a curved path. Its direction is always orthogonal to the motion of the body and towards the fixed point of the instantaneous center of curvature of the path. Isaac Newton described it as "a force by which bodies are drawn or impelled, or in any way tend, towards a point as to a centre". In Newtonian mechanics, gravity provides the centripetal force causing astronomical orbits.

In physics, the **Lorentz transformations** are a six-parameter family of linear transformations from a coordinate frame in spacetime to another frame that moves at a constant velocity relative to the former. The respective inverse transformation is then parameterized by the negative of this velocity. The transformations are named after the Dutch physicist Hendrik Lorentz.

In physics the **Lorentz force** is the combination of electric and magnetic force on a point charge due to electromagnetic fields. A particle of charge q moving with a velocity **v** in an electric field **E** and a magnetic field **B** experiences a force of

In mathematics, the **polar coordinate system** is a two-dimensional coordinate system in which each point on a plane is determined by a distance from a reference point and an angle from a reference direction. The reference point is called the *pole*, and the ray from the pole in the reference direction is the *polar axis*. The distance from the pole is called the *radial coordinate*, *radial distance* or simply *radius*, and the angle is called the *angular coordinate*, *polar angle*, or *azimuth*. The radial coordinate is often denoted by *r* or *ρ*, and the angular coordinate by *φ*, *θ*, or *t*. Angles in polar notation are generally expressed in either degrees or radians.

In mathematics, a **spherical coordinate system** is a coordinate system for three-dimensional space where the position of a point is specified by three numbers: the *radial distance* of that point from a fixed origin, its *polar angle* measured from a fixed zenith direction, and the *azimuthal angle* of its orthogonal projection on a reference plane that passes through the origin and is orthogonal to the zenith, measured from a fixed reference direction on that plane. It can be seen as the three-dimensional version of the polar coordinate system.

In physics, the **Navier–Stokes equations** are a set of partial differential equations which describe the motion of viscous fluid substances, named after French engineer and physicist Claude-Louis Navier and Anglo-Irish physicist and mathematician George Gabriel Stokes.

In mathematics, **curvature** is any of several strongly related concepts in geometry. Intuitively, the curvature is the amount by which a curve deviates from being a straight line, or a surface deviates from being a plane.

The **Friedmann–Lemaître–Robertson–Walker****metric** is an exact solution of Einstein's field equations of general relativity; it describes a homogeneous, isotropic, expanding universe that is path-connected, but not necessarily simply connected. The general form of the metric follows from the geometric properties of homogeneity and isotropy; Einstein's field equations are only needed to derive the scale factor of the universe as a function of time. Depending on geographical or historical preferences, the set of the four scientists – Alexander Friedmann, Georges Lemaître, Howard P. Robertson and Arthur Geoffrey Walker – are customarily grouped as **Friedmann** or **Friedmann–Robertson–Walker** (**FRW**) or **Robertson–Walker** (**RW**) or **Friedmann–Lemaître** (**FL**). This model is sometimes called the *Standard Model* of modern cosmology, although such a description is also associated with the further developed Lambda-CDM model. The FLRW model was developed independently by the named authors in the 1920s and 1930s.

In fluid dynamics, the **Euler equations** are a set of quasilinear hyperbolic equations governing adiabatic and inviscid flow. They are named after Leonhard Euler. The equations represent Cauchy equations of conservation of mass (continuity), and balance of momentum and energy, and can be seen as particular Navier–Stokes equations with zero viscosity and zero thermal conductivity. In fact, Euler equations can be obtained by linearization of some more precise continuity equations like Navier–Stokes equations in a local equilibrium state given by a Maxwellian. The Euler equations can be applied to incompressible and to compressible flow – assuming the flow velocity is a solenoidal field, or using another appropriate energy equation respectively. Historically, only the incompressible equations have been derived by Euler. However, fluid dynamics literature often refers to the full set – including the energy equation – of the more general compressible equations together as "the Euler equations".

**Differential geometry of curves** is the branch of geometry that deals with smooth curves in the plane and the Euclidean space by methods of differential and integral calculus.

Note: This page uses common physics notation for spherical coordinates, in which is the angle between the *z* axis and the radius vector connecting the origin to the point in question, while is the angle between the projection of the radius vector onto the *x-y* plane and the *x* axis. Several other definitions are in use, and so care must be taken in comparing different sources.

In continuum mechanics, the **finite strain theory**—also called **large strain theory**, or **large deformation theory**—deals with deformations in which strains and/or rotations are large enough to invalidate assumptions inherent in infinitesimal strain theory. In this case, the undeformed and deformed configurations of the continuum are significantly different, requiring a clear distinction between them. This is commonly the case with elastomers, plastically-deforming materials and other fluids and biological soft tissue.

In physics, **Maxwell's equations in curved spacetime** govern the dynamics of the electromagnetic field in curved spacetime or where one uses an arbitrary coordinate system. These equations can be viewed as a generalization of the vacuum Maxwell's equations which are normally formulated in the local coordinates of flat spacetime. But because general relativity dictates that the presence of electromagnetic fields induce curvature in spacetime, Maxwell's equations in flat spacetime should be viewed as a convenient approximation.

There are various **mathematical descriptions of the electromagnetic field** that are used in the study of electromagnetism, one of the four fundamental interactions of nature. In this article, several approaches are discussed, although the equations are in terms of electric and magnetic fields, potentials, and charges with currents, generally speaking.

In mathematics, the **Möbius energy** of a knot is a particular knot energy, i.e., a functional on the space of knots. It was discovered by Jun O'Hara, who demonstrated that the energy blows up as the knot's strands get close to one another. This is a useful property because it prevents self-intersection and ensures the result under gradient descent is of the same knot type.

This article describes a **particle in planar motion** when observed from non-inertial reference frames. The most famous examples of planar motion are related to the motion of two spheres that are gravitationally attracted to one another, and the generalization of this problem to planetary motion. See centrifugal force, two-body problem, orbit and Kepler's laws of planetary motion. Those problems fall in the general field of analytical dynamics, the determination of orbits from given laws of force. This article is focused more on the kinematical issues surrounding planar motion, that is, determination of the forces necessary to result in a certain trajectory *given* the particle trajectory. General results presented in fictitious forces here are applied to observations of a moving particle as seen from several specific non-inertial frames, for example, a *local* frame, and a *co-rotating* frame. The Lagrangian approach to fictitious forces is introduced.

In continuum mechanics, a **compatible** deformation **tensor field** in a body is that *unique* tensor field that is obtained when the body is subjected to a continuous, single-valued, displacement field. **Compatibility** is the study of the conditions under which such a displacement field can be guaranteed. Compatibility conditions are particular cases of integrability conditions and were first derived for linear elasticity by Barré de Saint-Venant in 1864 and proved rigorously by Beltrami in 1886.

The **Kirchhoff–Love theory of plates** is a two-dimensional mathematical model that is used to determine the stresses and deformations in thin plates subjected to forces and moments. This theory is an extension of Euler-Bernoulli beam theory and was developed in 1888 by Love using assumptions proposed by Kirchhoff. The theory assumes that a mid-surface plane can be used to represent a three-dimensional plate in two-dimensional form.

In probability theory and statistics, the **generalized multivariate log-gamma (G-MVLG) distribution** is a multivariate distribution introduced by Demirhan and Hamurkaroglu in 2011. The G-MVLG is a flexible distribution. Skewness and kurtosis are well controlled by the parameters of the distribution. This enables one to control dispersion of the distribution. Because of this property, the distribution is effectively used as a joint prior distribution in Bayesian analysis, especially when the likelihood is not from the location-scale family of distributions such as normal distribution.

**Accelerations in special relativity** (SR) follow, as in Newtonian Mechanics, by differentiation of velocity with respect to time. Because of the Lorentz transformation and time dilation, the concepts of time and distance become more complex, which also leads to more complex definitions of "acceleration". SR as the theory of flat Minkowski spacetime remains valid in the presence of accelerations, because general relativity (GR) is only required when there is curvature of spacetime caused by the energy–momentum tensor. However, since the amount of spacetime curvature is not particularly high on Earth or its vicinity, SR remains valid for most practical purposes, such as experiments in particle accelerators.

- ↑ Weisstien, Eric. "Radius of Curvature".
*Wolfram Mathworld*. Retrieved 15 August 2016. - 1 2 Kishan, Hari (2007).
*Differential Calculus*. Atlantic Publishers & Dist. ISBN 9788126908202. - 1 2 3 4 Love, Clyde E.; Rainville, Earl D. (1962).
*Differential and Integral Calculus*(Sixth ed.). New York: MacMillan. - ↑ "Controlling Stress in Thin Films".
*Flipchips.com*. Retrieved 2016-04-22. - ↑ "On the determination of film stress from substrate bending : Stoney's formula and its limits" (PDF).
*Qucosa.de*. Retrieved 2016-04-22. - ↑ Peter Walecki. "Model X". Zebraoptical.com. Retrieved 2016-04-22.

- do Carmo, Manfredo (1976).
*Differential Geometry of Curves and Surfaces*. ISBN 0-13-212589-7.

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