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A mobile is a type of kinetic sculpture constructed to take advantage of the principle of equilibrium. It consists of a number of rods, from which weighted objects or further rods hang. The objects hanging from the rods balance each other, so that the rods remain more or less horizontal. Each rod hangs from only one string, which gives it freedom to rotate about the string. Mobiles are popular in the nursery, where they hang over cribs to give infants something to entertain them and give them visual stimulation. Mobiles have inspired many composers, including Morton Feldman and Earle Brown. Frank Zappa also claimed that his compositions were modelled on the concept of the mobile.
The meaning of the term "mobile" as applied to sculpture has evolved since it was first suggested by Marcel Duchamp in 1931. At this point, "mobile" was synonymous with the term "kinetic art", describing sculptural works in which motion is a defining property. By the sequential attachment of additional objects, the final creation consists of many balanced parts joined by lengths of wire whose individual elements are capable of moving independently or as a whole when prompted by air movement or direct contact. Thus, "mobile" has become a more well-defined term referring to the many such hanging construct. A succinct definition of the term "mobile" in a visual art sense could be a type of kinetic sculpture in which an ensemble of balanced parts capable of motion are hung freely in space but which never come into contact with each other.
Man Ray is credited with inventing the mobile as an artwork, with his 1920 piece "Obstruction".  Artist Bruno Munari further developed the technique with his "Useless Machines" in 1933, made in cardboard and playful colors.
- Tomkins, Calvin: Duchamp: A Biography, pages 294. Henry Holt and Company, Inc., 1996. ISBN 0-8050-5789-7
- Munari, Bruno: Air Made Visible, 288 pages, Lars Müller Publishers, 2001. ISBN 978-3-907044-89-6