Disk (mathematics)

Disk (mathematics)

In geometry, a disk (also spelled disc) is the region in a plane bounded by a circle.

A disk is said to be closed or open according to whether or not it contains the circle that constitutes its boundary. In Cartesian coordinates, the open disk of center and radius R is given by the formula

while the closed disk of the same center and radius is given by

The area of a closed or open disk of radius R is πR2 (see π).

The ball is the disk generalised to metric spaces. In context, the term ball may be used instead of disk.

In theoretical physics a disk is a rigid body which is capable of participating in collisions in a two-dimensional gas. Usually the disk is considered rigid so that collisions are deemed elastic.