A wheel is a circular device that is capable of rotating on its axis, facilitating movement or transportation whilst supporting a load (mass), 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, steering 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 κυκλος kuklos, the Sanskrit chakra, and Persian charkh, all meaning "circle" or "wheel", and also in Lithuanian, sukti means "to rotate". The Latin word rota is from the Proto-Indo-European *rotā-, the extended o-grade form of the root *ret- meaning "to roll, revolve".

History

Most authorities regard the wheel as one of the oldest and most important inventions, which originated in ancient Mesopotamia in the 5th millennium BC (Ubaid period), originally in the function of potter's wheels. Near the northern side of the Caucasus several graves were found, in which since 3700 BC people had been buried on wagons or carts (both types). The earliest depiction of what may be a wheeled vehicle (here a wagon—four wheels, two axles), is on the Bronocice pot, a ca. 3500 BC clay pot excavated in southern Poland.

The wheel reached Europe and Western Asia in the 4th millennium BC, and the Indus Valley by the 3rd millennium BC. In China, the wheel is certainly present with the adoption of the chariot in ca. 1200 BC, although Barbieri-Low (2000) argues for earlier Chinese wheeled vehicles, circa 2000 BC. Whether there was an independent "invention of the wheel" in East Asia or whether the concept made its way there after jumping the Himalayan barrier remains an open question.

Although they did not develop the wheel proper, the Olmec and certain other western hemisphere cultures seem to have approached it, as wheel-like worked stones have been found on objects identified as children's toys dating to about 1500 BC.