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In physics and geometry, the catenary is the curve that an idealized hanging chain or cable assumes under its own weight when supported only at its ends. The curve has a U-like shape, superficially similar in appearance to a parabola (though mathematically quite different). It also appears in the design of certain types of arches and as a cross section of the catenoid -- the shape assumed by a soap film bounded by two parallel circular rings.
Mathematically, the catenary curve is the graph of the hyperbolic cosine function. The surface of revolution of the catenary curve, the catenoid, is a minimal surface, and is the only minimal surface of revolution other than the plane. The mathematical properties of the catenary curve were first studied by Robert Hooke in the 1670s, and its equation was derived by Leibniz, Huygens and Johann Bernoulli in 1691.
Catenaries and related curves appear in architecture and engineering, in the design of bridges and arches. A sufficiently heavy anchor chain will form a catenary curve.
The word catenary is derived from the Latin word catena, which means "chain". The English word catenary is usually attributed to Thomas Jefferson, who wrote in a letter to Thomas Paine on the construction of an arch for a bridge:
I have lately received from Italy a treatise on the equilibrium of arches, by the Abbé Mascheroni. It appears to be a very scientifical work. I have not yet had time to engage in it; but I find that the conclusions of his demonstrations are, that every part of the catenary is in perfect equilibrium.
It is often said  that Galileo thought the curve of a hanging chain was parabolic. In his Two New Sciences (1638), Galileo says that a hanging cord is an approximate parabola, and he correctly observes that this approximation improves as the curvature gets smaller and is almost exact when the elevation is less than 45°. That the curve followed by a chain is not a parabola was proven by Joachim Jungius (1587–1657); this result was published posthumously in 1669.
The application of the catenary to the construction of arches is attributed to Robert Hooke, whose "true mathematical and mechanical form" in the context of the rebuilding of St Paul's Cathedral alluded to a catenary. Some much older arches approximate catenaries, an example of which is the Arch of Taq-i Kisra in Ctesiphon.
In 1671, Hooke announced to the Royal Society that he had solved the problem of the optimal shape of an arch, and in 1675 published an encrypted solution as a Latin anagram in an appendix to his Description of Helioscopes, where he wrote that he had found "a true mathematical and mechanical form of all manner of Arches for Building." He did not publish the solution to this anagram in his lifetime, but in 1705 his executor provided it as Ut pendet continuum flexile, sic stabit contiguum rigidum inversum, meaning "As hangs a flexible cable so, inverted, stand the touching pieces of an arch."
Euler proved in 1744 that the catenary is the curve which, when rotated about the x-axis, gives the surface of minimum surface area (the catenoid) for the given bounding circles. Nicolas Fuss gave equations describing the equilibrium of a chain under any force in 1796.
Catenary arches are often used in the construction of kilns. To create the desired curve, the shape of a hanging chain of the desired dimensions is transferred to a form which is then used as a guide for the placement of bricks or other building material.
The Gateway Arch in St. Louis, Missouri, United States is sometimes said to be an (inverted) catenary, but this is incorrect. It is close to a more general curve called a flattened catenary, with equation y = A cosh(B x ), which is a catenary if A B = 1 . While a catenary is the ideal shape for a freestanding arch of constant thickness, the Gateway Arch is narrower near the top. According to the U.S. National Historic Landmark nomination for the arch, it is a "weighted catenary" instead. Its shape corresponds to the shape that a weighted chain, having lighter links in the middle, would form.
|Inverted catenary arches|
In free-hanging chains the force exerted is uniform with respect to length of the chain and so the chain follows the catenary curve. The same is true of a simple suspension bridge or "catenary bridge," where the roadway follows the cable.
However in a suspension bridge with a suspended roadway, the chains or cables support the weight of the bridge, and so do not hang freely. In most cases the roadway is flat, so when the weight of the cable is negligible compared with the weight being supported, the force exerted is uniform with respect to horizontal distance, and the result is a parabola, as discussed below (although the term "catenary" is often still used, in an informal sense). If the cable is heavy then the resulting curve is between a catenary and a parabola.
The catenary produced by gravity provides an advantage to heavy anchor rodes. An anchor rode (or anchor line) usually consists of chain or cable or both. Anchor rodes are used by ships, oilrigs, docks, floating wind turbines, and other marine equipment which must be anchored to the seabed.
When the rode is slack, the catenary curve presents a lower angle of pull on the anchor or mooring device than would be the case if it were nearly straight. This enhances the performance of the anchor and raises the level of force it will resist before dragging. To maintain the catenary shape in the presence of wind, a heavy chain is needed, so that only larger ships in deeper water can rely on this effect. Smaller boats must rely on the performance of the anchor itself.
The radius of curvature is then
When a parabola is rolled along a straight line, the roulette curve traced by its focus is a catenary. The envelope of the directrix of the parabola is also a catenary. The involute from the vertex, that is the roulette formed traced by a point starting at the vertex when a line is rolled on a catenary, is the tractrix.
Another roulette, formed by rolling a line on a catenary, is another line. This implies that square wheels can roll perfectly smoothly if the road has evenly spaced bumps in the shape of a series of inverted catenary curves. The wheels can be any regular polygon except a triangle, but the catenary must have parameters corresponding to the shape and dimensions of the wheels.
Over any horizontal interval, the ratio of the area under the catenary to its length equals a, independent of the interval selected. The catenary is the only plane curve other than a horizontal line with this property. Also, the geometric centroid of the area under a stretch of catenary is the midpoint of the perpendicular segment connecting the centroid of the curve itself and the x-axis.
In the mathematical model. the chain (or cord, cable, rope, string, etc.) is idealized by assuming that it is so thin that it can be regarded as a curve and that it is so flexible any force of tension exerted by the chain is parallel to the chain. The analysis of the curve for an optimal arch is similar except that the forces of tension become forces of compression and everything is inverted. An underlying principle is that the chain may be considered a rigid body once it has attained equilibrium. Equations which define the shape of the curve and the tension of the chain at each point may be derived by a careful inspection of the various forces acting on a segment using the fact that these forces must be in balance if the chain is in static equilibrium.
Let the path followed by the chain be given parametrically by r = (x, y) = (x(s), y(s)) where s represents arc length and r is the position vector. This is the natural parameterization and has the property that
where u is a unit tangent vector.
A differential equation for the curve may be derived as follows. Let c be the lowest point on the chain, called the vertex of the catenary,  and measure the parameter s from c. Assume r is to the right of c since the other case is implied by symmetry. The forces acting on section of the chain from c to r are the tension of the chain at c, the tension of the chain at r and the weight of the chain. The tension at c is tangent to the curve at c and is therefore horizontal, and it pulls the section to the left so it may be written (−T0, 0) where T0 is the magnitude of the force. The tension at r is parallel to the curve at r and pulls the section to the right, so it may be written Tu=(Tcos φ, Tsin φ), where T is the magnitude of the force and φ is the is the angle between the curve at r and the x-axis (see tangential angle). Finally, weight of the chain is represented by (0, −λgs) where λ is the mass per unit length, g is the acceleration of gravity and s is the length of chain between c and r.
The chain is in equilibrium so the sum of three forces is 0, therefore
and dividing these gives
It is convenient to write
which is the length of chain whose weight is equal in magnitude to the tension at c. Then
is an equation defining the curve.
The horizontal component of the tension, Tcos φ = T0 is constant and the vertical component of the tension, Tsin φ = λgs is proportional to the length of chain between the r and the vertex.
The differential equation given above can be solved to produce equations for the curve.
the formula for arc length gives
The second of these equations can be integrated to give
and by shifting the position of the x-axis, β can be taken to be 0. Then
The x-axis thus chosen is called the directrix of the catenary.
It follows that the magnitude of the tension at a point T = λgy which is proportional to the distance between the point and the directrix.
and, again, by shifting the position of the y-axis, α can be taken to be 0. Then
The y-axis thus chosen passes though the vertex and is called the axis of the catenary.
These results can be used to eliminate s giving
The differential equation can be solved using a different approach.
it follows that
As before, the x and y-axes can be shifted so α and β can be taken to be 0. Then
and taking the reciprocal of both sides
Adding and subtracting the last two equations then gives the solution
In general the parameter a and the position of the axis and directrix are not given but must be determined from other information. Typically, the information given is that the catenary is suspended at given points P1 and P2 and with given length s. The equation can be determined in this case as follows: Relabel if necessary so that P1 is to the left of P2 and let h be the horizontal and v be the vertical distance from P1 to P2. Translate the axes so that the vertex of the catenary lies on the y-axis and its height a is adjusted so the catenary satisfies the standard equation of the curve
and let the coordinates of P1 and P2 be (x1, y1) and (x2, y2) respectively. The curve passes through these points, so the difference is height is
and the length of the curve from P1 to P2 is
When s2−v2 is expanded using these expressions the result is
This is a transcendental equation in a and must be solved numerically. It can be shown with the methods of calculus that there is at most one solution with a>0 and so there is at most one position of equilibrium.
If the density of the chain is variable then the analysis above can be adapted to produce equations for the curve given the density, or given the curve to find the density.
Let w denote the weight per unit length of the chain, then the weight of the chain has magnitude
where the limits of integration are c and r. Balancing forces as in the uniform chain produces
Differentiation then gives
In terms of φ and the radius of curvature ρ this becomes
A similar analysis can be done to find the curve followed by the cable supporting a suspension bridge with a horizontal roadway. If the weight of the roadway per unit length is w and the weight of the cable and the wire supporting the bridge is negligible in comparison, then the weight on the cable from c to r is wx where x is the horizontal distance between c to r. Proceeding as before gives the differential equation
This is solved by simple integration to get
and so the cable follows a parabola. If the weight of the cable and supporting wires are not negligible then the analysis is more complex.
In a catenary of equal strength, cable is strengthened according to the magnitude of the tension at each point, so its resistance to breaking is constant along its length. Assuming that the strength of the cable is proportional to its density per unit length, the weight, w, per unit length of the chain can be written T/c, where c is constant, and the analysis for nonuniform chains can be applied.
In this case the equations for tension are
and by differentiation
where ρ is the radius of curvature.
The solution to this is
In this case, the curve has vertical asymptotes and this limits the span to πc. Other relations are
In an elastic catenary, the chain is replaced by a spring which can stretch in response to tension. The spring is assumed to stretch in accordance with Hooke's Law. Specifically, if p is the natural length of a section of spring, then the length of the spring with tension T applied has length
The curve followed by an elastic spring can now be derived following a similar method as for the inelastic spring.
The equations for tension of the spring are
where p is the natural length of the segment from c to r and λ0 is the mass per unit length of the spring with no tension and g is the acceleration of gravity. Write
Integrating gives the parametric equations
Again, the x and y-axes can be shifted so α and β can be taken to be 0. So
are parametric equations for the curve.
With no assumptions have been made regarding the force G acting on the chain, the following analysis can be made.
First, let T=T(s) be the force of tension as a function of s. The chain is flexible so it can only exert a force parallel to itself. Since tension is defined as the force that the chain exerts on itself, T must be parallel to the chain. In other words,
where T is the magnitude of T and u is the unit tangent vector.
Second, let G=G(s) be the external force per unit length acting on a small segment of a chain as a function of s. The forces acting on the segment of the chain between s and s+Δs are the force of tension T(s+Δs) at one end of the segment, the nearly opposite force −T(s) at the other end, and the external force acting on the segment which is approximately GΔs. These forces must balance so
Divide by Δs and take the limit as Δs → 0 to obtain
These equations can be used as the starting point in the analysis of a flexible chain acting under any external force. In the case of the standard catenary, G = (0, −λg) where the chain has mass λ per unit length and g is the acceleration of gravity.
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