In physics and mechanics a simple machine is a mechanical device that changes the direction or magnitude of a force. In general, they can be defined as the simplest mechanisms that use mechanical advantage (also called leverage) to multiply force. A simple machine uses a single applied force to do work against a single load force. Usually the term refers to the six classical simple machines which were defined by Renaissance scientists:
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Lever
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Wheel and axle
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Pulley
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Inclined plane
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Wedge
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Screw
These are the elementary "building blocks" of which all complicated machines are composed. For example, wheels, levers, and pulleys are all used in the mechanism of a bicycle. In the 20th century, a realization that at least one simple machine, the hydraulic press, had been left out, and arguments that some of the six classical devices can be considered as modifications of others, has led some modern sources to avoid specifying any list of simple machines as "basic". Nevertheless, the above six are what is usually meant by "simple machine" and are still regarded as the foundation of mechanical technology. Simple machines fall into two classes: those dependent on the vector resolution of forces (inclined plane, wedge, screw) and those in which there is an equilibrium of torques (lever, pulley, wheel).
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