Wheel

A wheel is a disc or circle-shaped mechanical device. Its main purpose is to allow things to roll; in other words, the wheel spins, and object on the wheels moves more easily along the ground.  It is a simple machine.Prater, Edward L. 1994. Basic Machines, Naval Education and Training Professional Development and Technology Center, NAVEDTRA 14037 The principle behind the wheel is that of mechanical advantage.  

Most land vehicles roll on wheels. Wheels are often used in pairs, connected by a rod of wood or metal known as an axle. The wheel and axle turn together. The part of the wheel that attaches to the axle is called the hub. 

The wheel with an axle is the basis of many machines, not just vehicles.  The potter's wheel, the lathe and the windlass are examples.Bowser, Edward Albert, 1890,  An elementary treatise on analytic mechanics: with numerous examples.   (Originally from the University of Michigan) D. Van Nostrand Company, pp. 190 Many machines have wheels with teeth, known as gears.

 History 
Most experts believe that  the ancient Mesopotamians invented the wheel about 4000 BC.True potter's wheels, which are freely-spinning and have a wheel-and-axle mechanism, were developed in Mesopotamia (Iraq) by 4200–4000 BC. The oldest surviving example, which was found in Ur (modern day Iraq), dates to about 3100 BC.

People in Asia also discovered it on their own around 3500 BC.  The Inca and Maya had wheels on children's toys around 1500 BC, but they did not use wheels for work.  Africa south of the Sahara desert and Australia did not have the wheel until people there met people from Europe.  Early wheels were solid disks; the spoked wheel was invented around 2000 BC. The earliest documentation of a vehicle with wheels is a painting dated between 33503500 BC.

 Uses 
Today, wheels are used in cars, carts, airplanes, wheelchairs, bicycles, trains, and skateboards, in addition to many more devices.

 References 

Category:Basic English 850 words
Category:Tools