A wheel is a circular device that is capable of rotating on its axis, facilitating movement or transportation or performing labour in machines. A wheel together with an axle overcomes friction by facilitating motion by rolling. In order for wheels to rotate a moment needs to be applied to the wheel about its axis, either by way of gravity or by application of another external force. Common examples are found in transport applications. More generally the term is also used for other circular objects that rotate or turn, such as a Ship's wheel and flywheel. Etymology The English word wheel comes from the Proto-Indo-European *k^wek^wlo-, which was an extended form of the root *k^wel- meaning "to revolve, move around". This is also the root of the Greek ?????? kuklos, the Sanskrit chakra, and Persian charkh, all meaning "circle" or "wheel",