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 external visual stimulation. Mobiles have inspired many composers, including Morton Feldman and Earle Brown who were inspired by Alexander Calder's mobiles to create mobile-like indeterminate pieces. Frank Zappa also claimed that his compositions were modelled on Calder mobiles.
The meaning of the term "mobile" as applied to sculpture has evolved since it was first suggested by Marcel Duchamp in 1931 to describe the early, mechanized creations of Alexander Calder.[1] At this point, "mobile" was synonymous with the term "kinetic art", describing sculptural works in which motion is a defining property. While motor or crank-driven moving sculptures may have initially prompted it, the word "mobile" later came to refer more specifically to Calder’s free-moving creations. Influenced by the abstract work of Piet Mondrian, Joan Miró and Sophie Taeuber-Arp, Calder in many respects invented an art form where objects (typically brightly coloured, abstract shapes fashioned from sheet metal) are connected by wire much like a balance scale. 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.
This information is taken from Wikipedia - http://en.wikipedia.org