Wheel and Axle
Wheeled wagon carrying prisoners of war (Nineveh, Iraq, 700 BC)
Wheels do not exist in nature, and no animals use them except people. Even people didn't use wheels until the early Bronze Age, around 3500 BC, much later than the other simple machines (the lever and the inclined plane).
The earliest evidence for people using wheels comes from Sumeria (modern Iraq), about 3500 BC, just about the time when people first began to live in cities. The earliest wheels were probably not wagon wheels, but pottery wheels. Only about 300 years later did people get the idea to use wheels on wagons. You can see from the pictures that spokes developed later than solid wheels.
A cart on the Standard of Ur (Iraq, 2600 BC)
Wheels help people do work in two ways. First, like levers or inclined planes, wheels allow you to do something easy for a longer time, instead of doing something hard for a shorter time. If you turn a large wheel fixed to an axle, the axle will also turn. You can turn the large wheel easily (but it takes a lot of turning to go all the way around). The axle will go around a much shorter distance, but with more force. So you can use a wheel to create a mechanical advantage - you can turn something heavy, by spinning a large wheel attached to an axle that is attached to the heavy thing. That's how a pencil sharpener works. Or, you can do it the other way around - use a lot of force to turn the axle, and that will spin the wheels really fast. That's what cars do.
Also, wheels on a wagon only touch the ground at one spot at a time, keeping the rest of the wagon off the ground. This makes less friction, so that the wagon is easier to move than if you were pulling it along like a sled.
Wheels are the most important part of pottery wheels, wagons and cars, but also of wheelbarrows, spinning wheels, water wheels, and pulleys.
To find out more about machines, check out these books from Amazon.com or from your library:
