Simple machines

Amachine is a contrivance designed to accomplish a certain task or tasks when given mechanical power. Machines are categorized as either complex (also called compound) or simple. Simple machines use few if any moving parts, they change the degree of force or the direction of an object, and they require some form of energy in order to work. Simple machines can also be combined to create complex machines, which are able to do more difficult tasks than simple machines can do alone.

There are six types of simple machines: lever, wheel and axle, inclined plane, wedge, screw, and pulley. The fundamental idea behind these simple machines remains the same: each simple machine was invented to make a very difficult task possible or at least much easier to accomplish.

Background

The six types of simple machines were defined during the Renaissance but the physical laws behind them were defined by Archimedes (ca. 287–212 BCE). At its most basic, the definition of a machine involves a device or an object that does work resulting in output greater than the force applied. This phenomenon is known as "mechanical advantage" or "leverage." In physics, the energy expended is called "work" and is expressed by the equation W = F × d, where W is work, F is force, and d is distance or displacement.

Machines make work easier by transmitting a force from one location to another, by altering the direction of a force, by increasing the degree of a force, or by enlarging the speed or distance that a force displaces in an object. However, no machine can simultaneously increase a force and increase the distance that force displaces. With every increase in force, there is an equal decrease in the distance that the object is displaced. Likewise, any increase in the distance an object is displaced is met by a proportional decrease in the force applied. Although a machine makes work easier, the quantity of work a machine produces is equal to the amount of work applied, with the exception of some energy being lost to friction or gravity. In other words, simple machines do not produce energy—they convert it. Galileo Galilei (1564–1642) was the first to understand this law, which is known as the Law of Conservation of Energy.

The six simple machines fall into two categories: machines that utilize two rotational forces, or torques, acting against one another, namely the lever, wheel, and pulley; and machines that operate on a vector (magnitude and direction of a force applied to an object), namely the inclined plane, wedge, and screw.

Overview

A lever is a straight beam, rod, or stick that pivots on an object called a fulcrum. With a classic lever, the fulcrum is closer to one end of the beam. A heavy object can be moved if it is placed on the shorter end of the beam and pressure (the effort) is exerted on the longer end. By increasing the distance (length) of the side opposite the object, less work (effort) is needed to create the force necessary to move the heavy object (the load or resistance). With other levers, the load (resistance) is located between the fulcrum and the effort; examples include a bottle opener, wheelbarrow, and nutcracker. A third type of lever has the effort placed between the fulcrum and load.

A wheel rotates on an axis or axle. Force applied to the circular device magnifies the output of the axle. It can also be used to carry and move an object, which overcomes the friction that would make it difficult to push or pull it along the ground. The first such wheels were logs; lacking axles, several logs were required, and each time the object passed over a roller, it had to be moved to the back of the line. When the fixed axle was invented, the combined wheel and axle became extraordinarily useful. Examples of the wheel in non-load-bearing applications include the potter’s wheel, doorknob, steering wheel, and rolling pin. The external force applied to a wheel is called torque.

A pulley guides a rope or cable, the length of which gains a mechanical advantage by applying the force over a greater distance, reducing the effort needed to pull it. A block and tackle is a set of two or more pulleys acted upon by a single rope; a block consists of more than one pulley on a single axle, and a tackle is a second pulley supported by the rope of the first. The more ropes and pulleys used, the greater the weight can be lifted.

An inclined plane is a fixed object that allows for an object to be raised or lowered, which uses less force than what would be needed to lift or lower an object alone. Aside from the friction between the object and the surface of the plane, the force needed to move the object depends on the slope of the inclined plane. The slope is the ratio of the rise (height) and run (length) of the inclined plane, which is measured in degrees. An increase in height or decrease in length will produce a steeper slope; a decrease in height or an increase in length will lead to a gentler slope. The less the slope, the less work is required to move the object, assuming there is no change in friction. Modern-day uses include ramps for wheelchairs and motor vehicles.

A screw consists of a shaft with a helical, or spiral, groove. When force is applied by turning the screw (torque), the shaft moves in the direction of the treads, which is in most cases clockwise. The screw creates a mechanical advantage by transmitting a relatively small rotational force to the length of the threads. The length of the threads can be increased by placing them closer together, which enhances the mechanical advantage and enables the screw to hold or lift heavier objects. The Archimedes screw is a device that uses the inclined planes of the threads to lift quantities of water. Likewise, a corkscrew uses the helical motion to a cork.

The wedge was defined sometime after Galileo, and it comprises two inclined planes that exert force on both sides. Since the forces are perpendicular to each of the angled sides, the surfaces of the object are pushed away when effort is applied to the back of the wedge, such as when a wedge axe splits wood. Other examples of the wedge are knives and chisels. Although a door wedge does not split another object, the outward forces produced by the sloped sides create the friction necessary to keep a door open.

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