There are six simple machines, including the lever, the wheel and axle, the inclined plane, the wedge, the screw and the pulley. Together, these six form the basis of all mechanized systems that have ever been built.
Machines are devices that make it easier to do work. Lifting, pulling, carrying and dividing objects requires the expenditure of energy, and simple machines direct that energy in efficient ways to accomplish tasks with minimal force applied. A wheel and axle, for example, rolls smoothly over terrain without losing undue energy to friction, as would a sledge. Levers and pulleys redirect force in such a way as to make lifting and pulling far more efficient than a brute-force application of energy. A wedge, such as an ax head, concentrates a driving force on a very thin edge that allows it to drive deeply into objects and pry them apart. A screw confers a mechanical advantage by converting rotational motion into linear motion, a principle adopted as early as the first century in the water screw, which lifts water against gravity. Inclined planes can be seen in every ramp. These redirect lifting forces into simple linear motion that gradually rises over obstacles without a vertical hoist.