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Solid of revolution

A solid of revolution is a three-dimensional geometric figure generated by rotating a two-dimensional region in the plane about a fixed axis, typically a line in the plane that does not intersect the interior of the region.^[1] These solids are fundamental in geometry and calculus, as they model shapes formed by rotational symmetry around an axis of revolution.^[2] Common examples of solids of revolution include the sphere, formed by rotating a semicircle about its diameter; the right circular cone, obtained by rotating a right triangle about one of its legs; and the cylinder, generated by rotating a rectangle about one of its sides.^[3] More complex shapes like the torus arise from rotating a circle about an axis in its plane that lies outside the circle itself.^[4] Historically, ancient Greek mathematicians such as Euclid recognized certain solids of revolution, defining the sphere, cone, and cylinder as figures produced by rotating plane sections around a straight line axis in his Elements.^[5] In modern mathematics, particularly in integral calculus, the volumes of solids of revolution are computed using techniques such as the disk method, which approximates the solid with thin cylindrical disks perpendicular to the axis of rotation; the washer method, an extension for regions between curves forming annular washers; and the cylindrical shell method, which uses concentric cylindrical shells parallel to the axis.^[2]^[6] These methods rely on definite integrals to sum infinitesimal volumes, providing exact formulas for arbitrary generating regions.^[7] Solids of revolution also appear in applications like engineering for modeling lathe-turned parts and in physics for rotational dynamics.^[8]^[9]

Definition and Properties

Formal Definition

A solid of revolution is a three-dimensional figure generated by rotating a plane region about a fixed axis lying in the same plane, with the solid comprising the entire volume swept out by the rotating region.^[10] The plane region, known as the generating region, is typically a two-dimensional area bounded by a curve and one or more straight lines.^[11] The key components of this construction include the generating curve, which defines the boundary of the region—often represented as the graph of a function y = f(x) for x in an interval [a, b] where f(x) \geq 0—and the axis of rotation, which may be the x-axis, y-axis, or an arbitrary line in the plane.^[11] The resulting solid is the union of all points obtained by rotating every point in the generating region around the axis, forming a figure with rotational symmetry about that axis.^[11] In mathematical notation, for rotation about the x-axis, the solid consists of all points (x, y, z) in three-dimensional space such that a \leq x \leq b and the distance from (x, y, z) to the x-axis is at most f(x), or equivalently, points of the form (x, r \cos \theta, r \sin \theta) where $0 \leq r \leq f(x) and $0 \leq \theta \leq 2\pi.^[11] This setup presupposes familiarity with plane regions in the coordinate plane and the concept of rotational symmetry, foundational elements in multivariable calculus.^[12]

Geometric Properties

Solids of revolution exhibit rotational symmetry about the axis of rotation, remaining invariant under any angle of rotation around that axis. This symmetry is a direct consequence of the generative process, where a plane curve or region is rotated continuously to form the three-dimensional figure. Perpendicular to the axis, every cross-section is a circular disk or annulus, ensuring that the solid appears identical from any rotational viewpoint.^[13]^[2] Cross-sections perpendicular to the axis of rotation are either disks or annuli (washers), depending on whether the generating region includes points on the axis or forms a gap. A disk arises when the rotation fills the area completely up to the axis, while an annulus forms if there is an inner boundary away from the axis, creating a ring-shaped slice. Cross-sections parallel to the axis are more intricate, typically consisting of rotated profiles of the generating curve, but they maintain the overall rotational symmetry of the solid.^[14] The boundedness of a solid of revolution depends on the domain of the generating curve: finite intervals produce bounded solids, such as a sphere from a semicircle rotated about its diameter, whereas infinite domains yield unbounded solids, like a paraboloid from rotating a parabola extending to infinity.^[13]^[15]^[16] The boundary of a solid of revolution is a surface of revolution, generated by the path of the curve's points during rotation, enclosing the interior volume. A key interior property is the location of the centroid, which, due to rotational symmetry, lies on the axis of rotation; its position along the axis reflects the distribution of the generating region's mass and can be found using theorems like Pappus's centroid theorem, which relates volume to the centroid's path length.^[17]^[18]

Examples of Solids of Revolution

Basic Shapes

Basic solids of revolution are formed by rotating simple curves around an axis, yielding familiar three-dimensional figures that illustrate the foundational concept of rotational generation. A sphere of radius r arises from rotating the semicircle defined by the equation y = \sqrt{r^2 - x^2} for -r \leq x \leq r about the x-axis.^[19] The volume of this sphere is given by \frac{4}{3} \pi r^3./06:_Geometry/6.03:_Volume_of_Geometric_Solids) A cylinder with radius r and finite height h results from rotating the horizontal line segment y = r from x = 0 to x = h about the x-axis; an infinite cylinder extends indefinitely along the axis.^[19] Its volume is \pi r^2 h./06:_Geometry/6.03:_Volume_of_Geometric_Solids) A cone with height h and base radius r is produced by rotating a straight line from the apex, such as y = mx + b connecting (0, 0) to (h, r), about the x-axis.^[19] The volume of the cone is \frac{1}{3} \pi r^2 h./06:_Geometry/6.03:_Volume_of_Geometric_Solids) These shapes—spheres, cylinders, and cones—represent classical solids extensively analyzed by Archimedes in his treatise On the Sphere and Cylinder around 250 BCE, where he established key relationships among their volumes and surfaces.^[20]

Advanced Solids

A torus is a solid of revolution formed by rotating a circle of radius r, offset from the axis of rotation by a distance R > r, about that axis, resulting in a ring-shaped solid with rotational symmetry.^[21] The volume of this solid is given by V = 2\pi^2 R r^2.^[21] A paraboloid arises from rotating the parabola y = x^2 (or a scaled variant) about the x-axis, generating an unbounded bowl-shaped solid that extends infinitely in the positive x-direction.^[22] When capped at a finite height h with base radius a, the enclosed volume is V = \frac{1}{2} \pi a^2 h, half that of the circumscribing cylinder, underscoring the paraboloid's tapering form.^[22] The uncapped solid has infinite volume due to its asymptotic expansion.^[22] More intricate solids emerge from rotating regions bounded by multiple curves, such as the area between two functions, yielding vase-like or irregular profiles. For instance, revolving y = \sin x over one period about the x-axis produces a wavy, undulating solid with periodic constrictions and expansions along the axis.^[23] Similarly, rotating the region between an outer curve like y = e^{-x^2} and an inner curve like y = 0 can form ornate, symmetric vessels with varying cross-sections. Pathological solids occur when the generating curve intersects the axis of rotation, leading to self-intersections in the resulting figure.^[24] A prominent example is the spindle torus, generated by rotating a circle where the offset distance c < r, causing the solid to overlap itself along the axis and form a lemon- or apple-like shape with intersecting volumes.^[24]

Volume Calculation Methods

Disk and Washer Method

The disk and washer methods are integration techniques used to compute the volume of a solid of revolution by considering cross-sections perpendicular to the axis of rotation, which form disks or washers (annular regions). These methods rely on the principle that the volume can be found by integrating the area of these cross-sections along the axis, where each cross-section is a circle or ring with radius equal to the distance from the axis to the curve being rotated. This approach stems from Cavalieri's principle, which states that solids with equal cross-sectional areas at every height have the same volume, combined with the area of a circle formula A = \pi r^2.^[25] For the disk method, consider a region under the curve y = f(x) from x = a to x = b, rotated about the x-axis, where f(x) \geq 0 is continuous. The solid formed has cross-sections that are disks with radius f(x), so the volume is given by

V = \pi \int_{a}^{b} [f(x)]^2 \, dx.

This formula arises by approximating the volume with thin cylindrical disks of thickness \Delta x and radius f(x_i), each with volume \pi [f(x_i)]^2 \Delta x, and taking the limit as \Delta x \to 0 to obtain the integral; the \pi factor accounts for the circular area. For rotation about the y-axis, express the radius as a function of y, yielding

V = \pi \int_{c}^{d} [g(y)]^2 \, dy,

where the curve is rewritten as x = g(y) over y = c to y = d.^[25]^[26] The washer method extends the disk method to solids formed by rotating the region between two curves, y = f(x) and y = g(x) with f(x) \geq g(x) \geq 0, about the x-axis. The cross-sections are washers with outer radius f(x) and inner radius g(x), so the volume is

V = \pi \int_{a}^{b} \left( [f(x)]^2 - [g(x)]^2 \right) \, dx.

This subtracts the volume of the inner solid from the outer one, reflecting the annular shape. For rotation about the y-axis, solve for x in terms of y to define the outer and inner radii as functions of y, integrating with respect to y over the appropriate bounds. The method assumes the axis of rotation is external to the region or lies on its boundary to avoid overlapping or negative volumes.^[25]^[27] To apply these methods, first identify the axis of rotation and the bounding curves, then express the radius (or radii) as a function of the integration variable, ensuring the functions are continuous and nonnegative over the interval. Determine the limits of integration from the region's extent along that axis. For example, revolving the semicircle y = \sqrt{r^2 - x^2} from x = -r to x = r about the x-axis using the disk method gives

V = \pi \int_{-r}^{r} (r^2 - x^2) \, dx = \frac{4}{3} \pi r^3,

deriving the volume of a sphere and verifying the formula against known geometry. These steps ensure the integral accurately captures the solid's volume under the assumptions of continuity and proper bounding.^[27]^[26]

Cylindrical Shell Method

The cylindrical shell method calculates the volume of a solid of revolution by approximating it as a collection of thin cylindrical shells concentric with the axis of rotation. Each shell has a radius equal to the distance from the axis, a height given by the function describing the curve, and a negligible thickness, allowing the volume to be found by integrating the lateral surface area of these shells times their thickness. This approach is particularly effective when integrating with respect to the variable perpendicular to the axis of rotation.^[28] For a region bounded by the curve y = f(x), the x-axis, and vertical lines at x = a and x = b (with f(x) \geq 0), rotated about the y-axis, the volume V is given by

V = 2\pi \int_{a}^{b} x f(x) \, dx.

This formula arises from the volume of a single shell, which is its circumference $2\pi x times height f(x) times thickness dx, summed via integration over the interval [a, b]. The derivation relies on the surface area of a cylinder, $2\pi r h, where r = x and h = f(x), confirming the method's foundation in basic geometry.^[28]^[29] To adapt the method for rotation about the x-axis, express the region in terms of y, with the curve x = g(y) (where g(y) \geq 0), bounded horizontally from y = c to y = d. The volume becomes

V = 2\pi \int_{c}^{d} y g(y) \, dy,

with each shell now having radius y, height g(y), and thickness dy. This mirrors the y-axis case but shifts the integration variable to align with the perpendicular direction.^[28] A key advantage of the shell method is its ability to avoid solving for inverse functions, which can complicate the disk method. Consider the region bounded by y = \sqrt{x}, the x-axis, and x = 1, rotated about the y-axis. Using shells, V = 2\pi \int_{0}^{1} x \sqrt{x} \, dx = 2\pi \int_{0}^{1} x^{3/2} \, dx = 2\pi \left[ \frac{2}{5} x^{5/2} \right]_{0}^{1} = \frac{4\pi}{5}. In contrast, the disk method requires rewriting as x = y^2, recognizing washers with outer radius 1 and inner radius y^2, leading to V = \pi \int_{0}^{1} (1 - y^4) \, dy = \frac{4\pi}{5}, but demands more setup for the inverse and washer identification. Thus, shells simplify integration when the original function is readily available.^[28]^[30] Despite its utility, the shell method has limitations, including reduced intuitiveness for rotations parallel to the integration axis and the assumption of non-negative heights to ensure positive volumes without absolute values. Careful selection of limits is also essential to avoid over- or under-counting the solid's extent.^[28]

Representations in Different Coordinate Systems

Parametric Representation

A parametric representation provides a powerful way to describe and compute the volumes of solids of revolution formed by rotating curves that cannot be easily expressed as explicit functions, such as circles or ellipses. Consider a plane curve defined parametrically by the equations x = x(t) and y = y(t), where the parameter t varies from \alpha to \beta. Rotating the region bounded by this curve and the relevant axis about the x-axis generates a solid whose volume is obtained via the parametric adaptation of the disk method. The volume V is given by the integral

V = \pi \int_{\alpha}^{\beta} [y(t)]^2 x'(t) \, dt,

assuming x'(t) > 0 to ensure the orientation yields a positive volume. This formula derives from the standard disk method V = \pi \int y^2 \, dx, where the substitution dx = x'(t) \, dt incorporates the parametric description; the arc length element ds = \sqrt{[x'(t)]^2 + [y'(t)]^2} \, dt appears in surface area calculations but simplifies here to the x-directed thickness for volume disks. These formulas assume the curve is a graph over the axis (e.g., y as a function of x) and the region does not cross the axis; for closed or non-monotonic curves, split the integral over intervals where the derivative is positive. For rotation about the y-axis, a shell-like parametric integral provides the volume

V = 2\pi \int_{\alpha}^{\beta} x(t) \, y(t) \, x'(t) \, dt,

assuming x'(t) > 0. This expression adapts the cylindrical shell method, with shells of radius x(t), height y(t), and thickness dx = x'(t) \, dt.^[31] A representative example is the solid torus formed by rotating a circle of radius r whose center lies at a distance R > r from the y-axis. The parametric equations for the generating circle are x(t) = R + r \cos t, y(t) = r \sin t, with t from 0 to $2\pi. For such closed curves, the simple integral requires splitting the parameter (e.g., where x'(t) > 0); using appropriate methods like Pappus's centroid theorem yields the known volume $2 \pi^2 r^2 R. This parametric framework excels at handling non-functional curves like circles or ellipses directly, avoiding the need to invert or segment the relations between x and y, and thus simplifies computations for symmetric or closed generating curves.

Polar Representation

In polar coordinates, a curve is described by the equation r = f(\theta), where r is the distance from the origin (pole) and \theta is the polar angle measured from the polar axis (corresponding to the positive x-axis in Cartesian coordinates). Rotating the region bounded by this curve and the rays \theta = \alpha to \theta = \beta about the polar axis generates a solid of revolution. This setup is particularly suited for curves exhibiting radial or angular symmetry, such as roses and limaçons, where the polar form simplifies the integration limits and expressions compared to Cartesian coordinates.^[32] The volume of this solid can be computed using a formula derived from Pappus's centroid theorem or direct integration via infinitesimal conical elements. Specifically,

V = \frac{2\pi}{3} \int_{\alpha}^{\beta} [f(\theta)]^3 \sin \theta \, d\theta,

where the integrand accounts for the contribution of each angular sector, with \sin \theta representing the perpendicular distance factor from the axis of rotation. For curves symmetric about the polar axis, the integral is typically evaluated over the interval where \sin \theta \geq 0 (e.g., $0 to \pi) to ensure positive volume elements, as the full $0 to $2\pi range may yield cancellation due to the sign of \sin \theta. This approach traces back to early applications of polar coordinates in calculus, as seen in 19th-century texts on integral methods for symmetric figures.^[32]^[33] For rotation about the y-axis (perpendicular to the polar axis), the formula adjusts to reflect the distance to that axis, yielding

V = \frac{2\pi}{3} \int_{\alpha}^{\beta} [f(\theta)]^3 \cos \theta \, d\theta.

This variant uses \cos \theta as the relevant projection, derived analogously via Pappus's theorem applied to the x-coordinate of the centroid. The computation is more complex for non-symmetric curves, often requiring careful handling of the \cos \theta sign across quadrants.^[33] A representative example is the cardioid r = a(1 + \cos \theta), rotated about the polar axis from \theta = 0 to \theta = \pi (covering the full symmetric region). Substitute into the formula:

V = \frac{2\pi a^3}{3} \int_0^\pi (1 + \cos \theta)^3 \sin \theta \, d\theta.

Use the substitution u = 1 + \cos \theta, so du = -\sin \theta \, d\theta. The limits change from \theta = 0 (u = 2) to \theta = \pi (u = 0):

V = \frac{2\pi a^3}{3} \int_2^0 u^3 (-du) = \frac{2\pi a^3}{3} \int_0^2 u^3 \, du = \frac{2\pi a^3}{3} \left[ \frac{u^4}{4} \right]_0^2 = \frac{2\pi a^3}{3} \cdot \frac{16}{4} = \frac{8\pi a^3}{3}.

This result confirms the volume of the apple-shaped solid formed by the revolution. The polar method excels here, as the cardioid's natural polar equation avoids cumbersome Cartesian conversion, highlighting its utility for such symmetric shapes in historical and modern computations.^[32]^[34]

References

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A solid of revolution is generated by revolving a region in the -plane about a vertical or horizontal axis of revolution.
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Nov 16, 2022 · We should first define just what a solid of revolution is. To get a solid of revolution we start out with a function, y=f(x) y = f ( x ) , on ...
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The sphere and right circular cone are familiar examples of such solids. ... FIGURE 1 The right circular cone and the sphere are solids of revolution. In ...
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Washer area nf - ng2. SOLIDS OF REVOLUTION. Cones and spheres and circular cylinders are "solids of revolution. ... circle revolved to give torus. EXAMPLE ...
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If a region in the plane is revolved about a given line, the resulting solid is a solid of revolution, and the line is called the axis of revolution.
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revolution is complete. Solids of revolution are common in mechanical applications, such as machine parts produced by a lathe. We spend the rest of this ...
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Solids of Revolution. If a region in a plane is revolved around a line in that plane, the resulting solid is called a solid of revolution, as shown in the ...<|control11|><|separator|>
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... Solid of Revolution, Sphere, Spheroid, Surface of Revolution Parallel, Toroid, Torus, Unduloid Explore this topic in the MathWorld classroom. Explore with ...
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Truncated cones of equal thickness are stacked to simulate the slicing of a solid of revolution made from part of a sine curve. As you increase the number of ...
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A spindle torus is one of the three standard tori given by the parametric equations x = (c+acosv)cosu (1) y = (c+acosv)sinu (2) z = asinv (3) with c.Missing: revolution | Show results with:revolution
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As shown in Figure 5.25, a solid of revolution is formed by revolving a plane region about a line. The line is called the axis of revolution. To develop a ...
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Volume of Revolution for Parametric Equations Let 𝑥 = 𝑓 𝑡 , 𝑦 = 𝑔(𝑡) be the equations of the parametric form, where '𝑡′ is a parameter. There are two cases ...
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Mar 20, 2024 · The volume generated by the revolution of the cardioid r = a ( 1 − cos ⁡ θ ) r = a(1-\cosθ) r=a(1−cosθ) about its axis is: · 8 3 π a 3 \frac{8}{3 ...<|control11|><|separator|>