# 6 Kinds of Simple Machines

Work is performed by applying a force over a distance. These six simple machines create a greater output force than the input force; the ratio of these forces is the mechanical advantage of the machine. All six of the simple machines listed here have been used for thousands of years, and the physics behind several of them were quantified by the Greek philosopher Archimedes (ca. 287–212 BCE). When combined, these machines can be used together to create an even greater mechanical advantage, as in the case of a bicycle.

## Lever

A lever is a simple machine that consists of a rigid object (often a bar of some kind) and a fulcrum (or pivot). Applying a force to one end of the rigid object causes it to pivot about the fulcrum, causing a magnification of the force at another point along the rigid object. There are three classes of levers, depending on where the input force, output force, and fulcrum are in relation to each other. The earliest lever was in use as a balance scale by 5000 BCE; Archimedes is credited with saying "Give me a place to stand and I will move the earth." Baseball bats, seesaws, wheelbarrows, and crowbars are all types of levers.

## Wheel & Axle

A wheel is a circular device that is attached to a rigid bar in its center. A force applied to the wheel causes the axle to rotate, which can be used to magnify the force (by, for example, having a rope wind around the axle). Alternately, a force applied to provide rotation on the axle translates into rotation of the wheel. It can be viewed as a type of lever that rotates around a center fulcrum. The earliest wheel and axle combination known was a toy model of a four-wheeled cart made in Mesopotamia about 3500 BCE. Ferris wheels, tires, and rolling pins are examples of wheels and axles.

## Inclined Plane

An inclined plane is a plane surface set at an angle to another surface. This results in doing the same amount of work by applying the force over a longer distance. The most basic inclined plane is a ramp; it requires less force to move up a ramp to a higher elevation than to climb to that height vertically. No one invented the inclined plane since it occurs naturally in nature, but people used ramps to build large buildings (monumental architecture) as early as 10,000–8,500 BCE. Archimedes's "On Plane Equilibrium" describes the centers of gravity for various geometrical plane figures.

## Wedge

The wedge is often considered a double inclined plane—both sides are inclined—that moves to exert a force along the lengths of the sides. The force is perpendicular to the inclined surfaces, so it pushes two objects (or portions of a single object) apart. Axes, knives, and chisels are all wedges. The common "door wedge" uses the force on the surfaces to provide friction, rather than separate things, but it's still fundamentally a wedge. The wedge is the oldest simple machine, made by our ancestors Homo erectus at least as long ago as 1.2 million years to make stone tools.

## Screw

A screw is a shaft that has an inclined groove along its surface. By rotating the screw (applying a torque), the force is applied perpendicular to the groove, thus translating a rotational force into a linear one. It is frequently used to fasten objects together (as the hardware screw and bolt does). The Babylonians in Mesopotamia developed the screw in the 7th century BCE, to elevate water from a low-lying body to a higher one (irrigate a garden from a river). This machine would later to be known as Archimedes' screw.

## Pulley

A pulley is a wheel with a groove along its edge, where a rope or cable can be placed. It uses the principle of applying force over a longer distance, and also the tension in the rope or cable, to reduce the magnitude of the necessary force. Complex systems of pulleys can be used to greatly reduce the force that must be applied initially to move an object. Simple pulleys were used by the Babylonians in the 7th century BCE; the first complex one (with several wheels) was invented by the Greeks about 400 BCE. Archimedes perfected the existing technology, making the first fully-realized block and tackle.

## What's a Machine?

The first use of the word "machine" ("machina") in Greek was by the ancient Greek poet Homer in the 8th century BCE, who used it to refer to political manipulation. The Greek playwright Aeschylus (523–426 BCE) is credited with using the word in reference to theatrical machines such as the "deus ex machina" or "god from a machine." This machine was a crane that brought actors playing gods onto the stage.