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In mathematics, **Watt's curve** is a tricircular plane algebraic curve of degree six. It is generated by two circles of radius *b* with centers distance 2*a* apart (taken to be at (±*a*, 0). A line segment of length 2*c* attaches to a point on each of the circles, and the midpoint of the line segment traces out the Watt curve as the circles rotate. It arose in connection with James Watt's pioneering work on the steam engine.

In geometry, a **circular algebraic curve** is a type of plane algebraic curve determined by an equation *F*(*x*, *y*) = 0, where *F* is a polynomial with real coefficients and the highest-order terms of *F* form a polynomial divisible by *x*^{2} + *y*^{2}. More precisely, if *F* = *F*_{n} + *F*_{n−1} + ... + *F*_{1} + *F*_{0}, where each *F*_{i} is homogeneous of degree *i*, then the curve *F*(*x*, *y*) = 0 is circular if and only if *F*_{n} is divisible by *x*^{2} + *y*^{2}.

In mathematics, a **affine algebraic plane curve** is the zero set of a polynomial in two variables. A **projective algebraic plane curve** is the zero set in a projective plane of a homogeneous polynomial in three variables. An affine algebraic plane curve can be completed in a projective algebraic plane curve by homogenizing its defining polynomial. Conversely, a projective algebraic plane curve can be restricted to an affine algebraic plane curve by replacing by one some indeterminate of the defining homogeneous polynomial. As these two operations are each inverse to the other, the phrase **algebraic plane curve** is often used without specifying explicitly whether it is the affine or the projective case that is considered.

**James Watt** was a Scottish inventor, mechanical engineer, and chemist who improved on Thomas Newcomen's 1712 Newcomen steam engine with his Watt steam engine in 1776, which was fundamental to the changes brought by the Industrial Revolution in both his native Great Britain and the rest of the world.

- Derivation
- Polar coordinates
- Cartesian coordinates
- Form of the curve
- Watt's linkage
- See also
- References
- External links

The equation of the curve can be given in polar coordinates as

The polar equation for the curve can be derived as follows:^{ [1] } Working in the complex plane, let the centers of the circles be at *a* and *−a*, and the connecting segment have endpoints at *−a*+*be*^{i λ} and *a*+*be*^{i ρ}. Let the angle of inclination of the segment be ψ with its midpoint at *re*^{i θ}. Then the endpoints are also given by *re*^{i θ} ± *ce*^{i ψ}. Setting expressions for the same points equal to each other gives

In mathematics, the **complex plane** or ** z-plane** is a geometric representation of the complex numbers established by the

Add these and divide by two to get

Comparing radii and arguments gives

Similarly, subtracting the first two equations and dividing by 2 gives

Write

Then

Expanding the polar equation gives

Letting *d*^{2}=*a*^{2}+*b*^{2}–*c*^{2} simplifies this to

The construction requires a quadrilateral with sides 2*a*, *b*, 2*c*, *b*. Any side must be less than the sum of the remaining sides, so the curve is empty (at least in the real plane) unless *a*<*b*+*c* and *c*<*b*+*a*.

The has a crossing point at the origin if there is a triangle with sides *a*, *b* and *c*. Given the previous conditions, this means that the curve crosses the origin if and only if *b*<*a*+*c*. If *b*=*a*+*c* then two branches of the curve meet at the origin with a common vertical tangent, making it a quadruple point.

Given *b*<*a*+*c*, the shape of the curve is determined by the relative magnitude of *b* and *d*. If *d* is imaginary, that is if *a*^{2}+*b*^{2} <*c*^{2} then the curve has the form of a figure eight. If *d* is 0 then the curve is a figure eight with two branches of the curve having a common horizontal tangent at the origin. If 0<*d*<*b* then the curve has two additional double points at ±*d* and the curve crosses itself at these points. The overall shape of the curve is pretzel-like in this case. If *d*=*b* then *a*=*c* and the curve decomposes into a circle of radius *b* and a lemniscate of Booth, a figure eight shaped curve. A special case of this is *a*=*c*, *b*=√2*c* which produces the lemniscate of Bernoulli. Finally, if *d*>*b* then the points ±*d* are still solutions to the Cartesian equation of the curve, but the curve does not cross these points and they are acnodes. The curve again has a figure eight shape though the shape is distorted if *d* is close to *b*.

In geometry, a **hippopede** is a plane curve determined by an equation of the form

In geometry, the **lemniscate of Bernoulli** is a plane curve defined from two given points *F*_{1} and *F*_{2}, known as **foci**, at distance 2*a* from each other as the locus of points *P* so that *PF*_{1}·*PF*_{2} = *a*^{2}. The curve has a shape similar to the numeral 8 and to the ∞ symbol. Its name is from *lemniscatus*, which is Latin for "decorated with hanging ribbons". It is a special case of the Cassini oval and is a rational algebraic curve of degree 4.

An **acnode** is an isolated point in the solution set of a polynomial equation in two real variables. Equivalent terms are "isolated point or hermit point".

Given *b*>*a*+*c*, the shape of the curve is determined by the relative sizes of *a* and *c*. If *a*<*c* then the curve has the form of two loops that cross each other at ±*d*. If *a*=*c* then the curve decomposes into a circle of radius *b* and an oval of Booth. If *a*>*c* then the curve does not cross the *x*-axis at all and consists of two flattened ovals.^{ [2] }

When the curve crosses the origin, the origin is a point of inflection and therefore has contact of order 3 with a tangent. However, if *a*^{2}=*b*^{2}+<*c*^{2}^{[ clarification needed ]} then tangent has contact of order 5 with the tangent, in other words the curve is a close approximation of a straight line. This is the basis for Watt's linkage.

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 responsible for astronomical orbits.

A **cardioid** is a plane curve traced by a point on the perimeter of a circle that is rolling around a fixed circle of the same radius. It can also be defined as an epicycloid having a single cusp. It is also a type of sinusoidal spiral, and an inverse curve of the parabola with the focus as the center of inversion.

The **nephroid** is a plane curve whose name means 'kidney-shaped'. Although the term *nephroid* was used to describe other curves, it was applied to the curve in this article by Proctor in 1878.

The **pedal curve** results from the orthogonal projection of a fixed point on the tangent lines of a given curve. More precisely, for a plane curve *C* and a given fixed *pedal point**P*, the **pedal curve** of *C* is the locus of points *X* so that the line *PX* is perpendicular to a tangent *T* to the curve passing through the point *X*. Conversely, at any point *R* on the curve *C*, let *T* be the tangent line at that point *R*; then there is a unique point *X* on the tangent *T* which forms with the pedal point *P* a line perpendicular to the tangent *T* – the pedal curve is the set of such points *X*, called the *foot* of the perpendicular to the tangent *T* from the fixed point *P*, as the variable point *R* ranges over the curve *C*.

In trigonometry, **tangent half-angle formulas** relate the tangent of half of an angle to trigonometric functions of the entire angle. Among these are the following

In geometry, a **strophoid** is a curve generated from a given curve *C* and points *A* and *O* as follows: Let *L* be a variable line passing through *O* and intersecting *C* at *K*. Now let *P*_{1} and *P*_{2} be the two points on *L* whose distance from *K* is the same as the distance from *A* to *K*. The locus of such points *P*_{1} and *P*_{2} is then the strophoid of C with respect to the pole *O* and fixed point *A*. Note that *AP*_{1} and *AP*_{2} are at right angles in this construction.

The **Albers equal-area conic projection**, or **Albers projection**, is a conic, equal area map projection that uses two standard parallels. Although scale and shape are not preserved, distortion is minimal between the standard parallels.

**Cylindrical multipole moments** are the coefficients in a series expansion of a potential that varies logarithmically with the distance to a source, i.e., as . Such potentials arise in the electric potential of long line charges, and the analogous sources for the magnetic potential and gravitational potential.

The **Newman–Penrose** (**NP**) **formalism** is a set of notation developed by Ezra T. Newman and Roger Penrose for general relativity (GR). Their notation is an effort to treat general relativity in terms of spinor notation, which introduces complex forms of the usual variables used in GR. The NP formalism is itself a special case of the tetrad formalism, where the tensors of the theory are projected onto a complete vector basis at each point in spacetime. Usually this vector basis is chosen to reflect some symmetry of the space-time, leading to simplified expressions for physical observables. In the case of the NP formalism, the vector basis chosen is a null tetrad: a set of four null vectors—two real, and a complex-conjugate pair. The two real members asymptotically point radially inward and radially outward, and the formalism is well adapted to treatment of the propagation of radiation in curved spacetime. The most often-used variables in the formalism are the Weyl scalars, derived from the Weyl tensor. In particular, it can be shown that one of these scalars-- in the appropriate frame—encodes the outgoing gravitational radiation of an asymptotically flat system.

The **history of Lorentz transformations** comprises the development of linear transformations forming the Lorentz group or Poincaré group preserving the Lorentz interval and the Minkowski inner product .

The main **trigonometric identities** between trigonometric functions are proved, using mainly the geometry of the right triangle. For greater and negative angles, see Trigonometric functions.

In mathematics, the **spectral theory of ordinary differential equations** is the part of spectral theory concerned with the determination of the spectrum and eigenfunction expansion associated with a linear ordinary differential equation. In his dissertation Hermann Weyl generalized the classical Sturm–Liouville theory on a finite closed interval to second order differential operators with singularities at the endpoints of the interval, possibly semi-infinite or infinite. Unlike the classical case, the spectrum may no longer consist of just a countable set of eigenvalues, but may also contain a continuous part. In this case the eigenfunction expansion involves an integral over the continuous part with respect to a spectral measure, given by the Titchmarsh–Kodaira formula. The theory was put in its final simplified form for singular differential equations of even degree by Kodaira and others, using von Neumann's spectral theorem. It has had important applications in quantum mechanics, operator theory and harmonic analysis on semisimple Lie groups.

A two-dimensional elastic membrane under tension can support transverse vibrations. The properties of an idealized drumhead can be modeled by the **vibrations of a circular membrane** of uniform thickness, attached to a rigid frame. Due to the phenomenon of resonance, at certain vibration frequencies, its resonant frequencies, the membrane can store vibrational energy, the surface moving in a characteristic pattern of standing waves. This is called a normal mode. A membrane has an infinite number of these normal modes, starting with a lowest frequency one called the fundamental mode.

In general relativity, a point mass deflects a light ray with impact parameter by an angle approximately equal to

In fluid dynamics, the **Oseen equations** describe the flow of a viscous and incompressible fluid at small Reynolds numbers, as formulated by Carl Wilhelm Oseen in 1910. Oseen flow is an improved description of these flows, as compared to Stokes flow, with the (partial) inclusion of convective acceleration.

For a plane curve *C* and a given fixed point *O*, the **pedal equation** of the curve is a relation between *r* and *p* where *r* is the distance from *O* to a point on *C* and *p* is the perpendicular distance from *O* to the tangent line to *C* at the point. The point *O* is called the *pedal point* and the values *r* and *p* are sometimes called the *pedal coordinates* of a point relative to the curve and the pedal point. It is also useful to measure the distance of *O* to the normal even though it is not an independent quantity and it relates to as .

In optics, the **Fraunhofer diffraction equation** is used to model the diffraction of waves when the diffraction pattern is viewed at a long distance from the diffracting object, and also when it is viewed at the focal plane of an imaging lens.

In general relativity, the **Weyl metrics** are a class of *static* and *axisymmetric* solutions to Einstein's field equation. Three members in the renowned Kerr–Newman family solutions, namely the Schwarzschild, nonextremal Reissner–Nordström and extremal Reissner–Nordström metrics, can be identified as Weyl-type metrics.

**Isentropic expansion** waves are created when a supersonic flow is redirected along a curved surface. These waves are studied to obtain a relation between deflection angle and Mach number. Each wave in this case is a Mach wave, so it is at an angle , where M is the Mach number immediately before the wave. Expansion waves are divergent because as the flow expands the value of Mach number increases, thereby decreasing the Mach angle.

In physics and engineering, the radiative heat transfer from one surface to another is the equal to the difference of incoming and outgoing radiation from the first surface. In general, the heat transfer between surfaces is governed by temperature, surface emissivity properties and the geometry of the surfaces. The relation for heat transfer can be written as an integral equation with boundary conditions based upon surface conditions. Kernel functions can be useful in approximating and solving this integral equation.

- Weisstein, Eric W. "Watt's Curve".
*MathWorld*. - O'Connor, John J.; Robertson, Edmund F., "Watt's Curve",
*MacTutor History of Mathematics archive*, University of St Andrews . - Catalan, E. (1885). "Sur la Courbe de Watt".
*Mathesis*.**V**: 154. - Rutter, John W. (2000).
*Geometry of Curves*. CRC Press. pp. 73ff. ISBN 1-58488-166-6.

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