Wheel
A wheel is a disc- or torus-shaped mechanical device, the fundamental operation of which is to transfer linear motion (going along) into rotary motion (going around). It is one of the simple machines. It is believed that the wheel was invented around 3200 - 3500 B.C.
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Wheels can be fastened on an axle, which when fitted to a third object such as a vehicle body, will efficiently impart the energy of the converted linear motion to that body. Since the strongest motions available in nature to pre-industrial humans (exertion of their own muscle, exertion of animal muscle, flowing water, etc.) were linear and horizontal, the wheel/axle/object combination enabled them to leverage these sources in any direction by tilting the wheel (vertically in a horse driven mill wheel, for instance). Expended linear force, instead of being pushed ahead and scattered in unlimited forward space, was now conserved along the compact 360 degree surface of the turning wheel. Compared to a vehicle without wheels, such as a travois, barrow or sled, this compaction of momentum, along with a related reduction in surface friction, allows wheeled vehicles to travel farther even while less push or pull force is applied.
Since a wheel is a rigid object, it will only be non-rotating when all the torques on it are balanced. Since forces produce greater torques when they are closer to the axis, a wheel can be used to transform between large and small forces applied by friction with belts or other wheels. The combination of the wheel with the wedge produced the toothed gear, which was fundamental to the advent of industrial class complex machines. Other variations on wheels produce the pulley and the windlass.
With its cycling/compaction of momentum, reduction of friction and resulting lower amount of work necessary to move items, the wheel is essential in transport. On many wheeled vehicles (for instance, automobiles and motorcycles), a rigid wheel does not come directly in contact with the surface, but instead serves as a mounting point for a flexible tire, usually filled with compressed air. For rail vehicles this is not the case. A development of wheels to allow them to travel through rough terrain is the caterpillar track found on some tractors and armored vehicles. Land vehicles without wheels are the hovercraft, the electro-magnetic monorail, and relying on surface conditions the sled, the snowboard and/or skis.
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In Buddhism, the wheel is a central conceptual metaphor. Life is said to be an endless cycle of births and rebirths, which one can only escape by achieving enlightenment. Accordingly the wheel to a Buddhist has a meaning akin to that of a cross for a Christian or a crescent for a Muslim, although it is not a symbol of a deity as such. The mandala is a common representation which is based on the wheel's image.
The Breaking wheel is a form of torture.