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What are Simple Machines ?
Gears gears cogs bits n pieces by Elsie esq. on Flickr

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. 
  • Ignoring friction losses, the work done on the load is equal to the work done by the applied force. 
  • They can be used to increase the amount of the output force, at the cost of a proportional decrease in the distance moved by the load. 
  • The ratio of the output to the input force is called the mechanical advantage.

Usually the term refers to the six classical simple machines which were defined by Renaissance scientists:

  • Lever
  • Wheel and axle
  • Pulley
  • Inclined plane
  • Wedge
  • Screw
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motion gears -team force by ralphbijker on Flickr

A Gear is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque.

  • Two or more gears working in tandem are called transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine.
  • Geared devices can change the speed, magnitude and direction of a power source. 
  • The most common situation is for a gear to mesh with another gear, however a gear can also mesh a non-rotating toothed part, called a rack, thereby producing translation instead of rotation.
  • The gears in a transmission are analogous to the wheels in a pulley.
  • An advantage of gears is that the teeth of a gear prevent slipping.
  • When two gears of unequal number of teeth are combined a mechanical advantage is produced, with both the rotational speeds and the torques of the two gears differing in a simple relationship

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old wagon by Rennett Stowe on Flickr

Wheel and Axle is a simple machine. 

  • It is a modified lever of the first class  that rotates in a circle around a center point or fulcrum. 
  • The larger wheel (or outside) rotates around the smaller wheel (axle). 
  • Bicycle wheels, ferris wheels, and gears are all examples of a wheel and axle. 
  • Wheels can also have a solid shaft with the center core as the axle such as a screwdriver or drill bit or the log in a log rolling contest.
  • The traditional form as recognized in 19th century textbooks is as shown in the image. 
  • This also shows the most widely recognized application, i.e., lifting water from a well. 
  • The form consists of a wheel that turns an axle and in turn a rope converts the rotational motion to linear motion for the purpose of lifting.

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Gizri 34 by HovaHe on Flickr

A Pulley, also called a sheave or a drum, is a mechanism composed of a wheel on an axle or shaft that may have a groove between two flanges around its circumference.

  • A rope, cable, belt, or chain usually runs over the wheel and inside the groove, if present.
  • Pulleys are used to change the direction of an applied force, transmit rotational motion, or realize a mechanical advantage in either a linear or rotational system of motion. 
  • It is one of the six simple machines. 
  • Two or more pulleys together are called a block and tackle.
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