A ball is a round, usually spherical but sometimes ovoid, object with various uses. It is used in ball games, where the play of the game follows the state of the ball as it is hit, kicked or thrown by players. In the context of sports, "ball" need not refer to a spherical object, as is the case in American football. Balls can also be used for simpler activities, such as catch, marbles and juggling. Balls made from hard-wearing materials are used in engineering applications to provide very low friction bearings, known as ball bearings. Black powder weapons use stone and metal balls as projectiles.
Although many types of balls are today made from rubber, this form was unknown outside the Americas until after the voyages of Columbus. The Spanish were the first Europeans to see bouncing rubber balls (albeit solid and not inflated) which were employed most notably in the Mesoamerican ballgame. Balls used in various sports in other parts of the world prior to Columbus were made from other materials such as animal bladders or skins, stuffed with various materials. As balls are one of the most familiar spherical objects to humans, the word "ball" is used to refer to, or to describe, anything spherical or near-spherical. A sphere (from Greek — sphaira, "globe, ball") is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle, which is in two dimensions, a sphere is the set of points which are all the same distance r from a given point in space. This dista
ce r is known as the radius of the sphere, and the given point is known as the center of the sphere. The maximum straight distance through the sphere is known as the diameter. It passes through the center and is thus twice the radius. In mathematics, a careful distinction is made between the sphere (a two-dimensional surface embedded in three-dimensional Euclidean space) and the ball (the interior of the three-dimensional sphere).
American football, known in the United States simply as football, is a sport played between two teams of eleven with the objective of scoring points by advancing the ball into the opposing team's end zone by running with it or throwing it to a teammate. Points can be scored by carrying the ball over the opponent's goal line or catching a pass thrown over that goal line (6), kicking the ball through the opponent's goal posts (3), or tackling an opposing ball carrier (2) in his own end zone. In the United States, the major forms are high school football, college football and professional football. Each of these are played under slightly different rules. High school football is governed by the National Federation of State High School Associations and college football by the National Collegiate Athletic Association. The highest level league for professional football is the National Football League. American football is closely related to Canadian football but with some differences in rules and the field. Both sports can be traced to early versions of association and rugby football.