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Four Pulley Assembly
A pulley
(a.k.a. a block and tackle) is a simple machine consisting of a rope and a grooved rotating wheel. The wheel pivots around a fixed point with the rope snugly entrenched in the disc’s groove. Pulleys are used to reduce the force required to lift heavy objects.

A one-pulley assembly does not reduce the amount of force required to lift an object, it only changes the direction of the force. Instead of having to pull a rope upwards to lift something, you are able to pull it downward; or vica versa.[1]

A two-pulley assembly cuts the force required to lift an object in half. Although the distance which must be pulled becomes double. The tradeoff for less force is always more distance. Twice as many pulleys means the object is half as difficult to lift, but you have to pull-in twice as much rope.[2]

Pulleys are used frequently in all types of industry; from cranes, to conveyor belts, to crawler tractor tracks, to combines, to oil drills.

[edit] References

  1. How a Block and Tackle Works. HowStuffWorks.com [July 24, 2009].
  2. How a Block and Tackle Works. HowStuffWorks.com [July 24, 2009].