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In mathematics, engineering, and manufacturing, a solid of revolution is a solid figure obtained by rotating a plane figure around some straight line (the axis) that lies on the same plane.
Assuming that the figure lies entirely on one side of the axis, the solid's volume is equal to the length of the circle described by the figure's barycenter, times the figure's area.
A representative disk is three-dimensional volume element of a solid of revolution. The element is created by rotating a line segment (of length "w") around some axis (located "r" units away); such that, a cylindrical volume, of π∫r2w units, is enclosed.
See also: surface of revolution
Formulas for solids of revolution
Rotations about the y-axis The volume of the solid formed by rotating the area between the curves of f(x) and g(x) and the lines x = a and x = b about the y-axis is given by ![V = 2\pi \int_a^b x[f(x) - g(x)] dx](http://en.wikipedia.org/math/aac08e5b35d83fcfd38af31e266f9b67.png) If one of the bounding curves is actually the x-axis, then we can let g(x) = 0 in the formula above, and we have: 
Rotations about the x-axis The volume of the solid formed by rotating the area between the curves of f(x) and g(x) and the lines x = a and x = b about the x-axis is given by ![V = \pi \int_a^b [f(x)]^2 - [g(x)]^2 dx](http://en.wikipedia.org/math/f7e8e20af20b657b3ed12d9be895e5f7.png) As above, we can use ![V = \pi \int_a^b [f(x)]^2 dx](http://en.wikipedia.org/math/e21201884291d87797138cd7b4d603bf.png) if one of the bounding curves is actually the x-axis.
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