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In physics, a simple machine is any device that only requires the application of a single force to work. Work is done when a force is applied and results in movement over a set distance. The work done is the product of the force and the distance. The amount of work required to achieve a set objective is constant, however the force required to do this can be reduced provided the lesser force is applied over a longer distance. Increase the distance and reduce the force. The ratio between the two is the mechanical advantage.

The traditional list of simple machines is:

• The inclined plane (N)
• The wheel and axle (O)
• The lever (T)
• The pulley (U)
• The wedge (V [single wedge]; X [double wedge])
• The screw (Y)

(The letters are a mnemonic, because their shapes resemble the shapes of the simple machines.)

The ratio of the output force to the input force is the mechanical advantage. For example, the mechanical advantage of a lever is equal to the ratio of its two arms. The mechanical advantage of an inclined plane (with the force acting parallel to the plane) is the cosecant of the angle of inclination.

Note: Real machines are also affected by factors such as friction and elasticity, so the actual mechanical advantage of a simple machine will usually differ from its theoretical value.

These simple machines fall into two general 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). Simple machines are often used in combination as components of more complex machines. For example: wheels, axles, levers, and pulleys are all used in the formation of modern bicycles.

Variations to the list of simple machines:

• Some say there are only five simple machines, arguing that the wedge is a moving inclined plane.
• Others further simplify the list to four saying that the screw is a helical inclined plane. This position is less accepted because a screw simultaneously converts a rotational force (torque) to a linear force.
• Some go even further to insist that only two simple machines exist, as a pulley and wheel and axle can be viewed as unique forms of levers, leaving only the lever and the inclined plane.^{[citation needed]}
• Hydraulic and hydrostatic systems are sometimes included.[1]

• Machine
• Invention
• Archimedes' screw