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A proof of the recursion formula relating the volume of the n-ball and an (n − 2)-ball can be given using the proportionality formula above and integration in cylindrical coordinates. Fix a plane through the center of the ball. Let r denote the distance between a point in the plane and the center of the sphere, and let θ denote the azimuth.
A closed n-ball of radius r is the set of all points of distance less than or equal to r away from x. In Euclidean n-space, every ball is bounded by a hypersphere. The ball is a bounded interval when n = 1, is a disk bounded by a circle when n = 2, and is bounded by a sphere when n = 3.
2 cm – approximate width of an adult human finger; 2.54 cm – 1 inch; 3.08568 cm – 1 attoparsec; 3.4 cm – length of a quail egg [112] 3.5 cm – width of film commonly used in motion pictures and still photography; 3.78 cm – amount of distance the Moon moves away from Earth each year [113] 4.3 cm – minimum diameter of a golf ball [114]
The covering number quantifies the size of a set and can be applied to general metric spaces. Two related concepts are the packing number , the number of disjoint balls that fit in a space, and the metric entropy , the number of points that fit in a space when constrained to lie at some fixed minimum distance apart.
The ratio of the volume of a sphere to the volume of its circumscribed cylinder is 2:3, as was determined by Archimedes. The principal formulae derived in On the Sphere and Cylinder are those mentioned above: the surface area of the sphere, the volume of the contained ball, and surface area and volume of the cylinder.
This corresponds to a well-stacked woodpile, 4 feet deep by 4 feet high by 8 feet wide (122 cm × 122 cm × 244 cm), or any other arrangement of linear measurements that yields the same volume. A more unusual measurement for firewood is the "rick" or face cord .
Regulation size and weight for a football is a circumference of 68–70 cm (27–28 in) and a weight of 410–450 g (14–16 oz). The ball is inflated to a pressure of 0.6–1.1 bars (8.7–16.0 psi) at sea level. [20] This is known as "Size 5". Smaller balls, Sizes 1, 3, and 4, are also produced for younger players or as training tools. [20]
[2] [3] Another equivalent way to define the width of a compact curve or of a convex set is by looking at its orthogonal projection onto a line. In both cases, the projection is a line segment, whose length equals the distance between support lines that are perpendicular to the line. So, a curve or a convex set has constant width when all of ...