About Pulleys
A pulley is a simple mechanical device. Use of a pulley system can result in a significant decrease in the force or energy required to move or lift an object. Pulley systems are used in cranes, sailboat rigging, winches and engine hoists, to name a few applications. While pulley systems can be quite elaborate, there are basic rules of force that apply and can be best demonstrated in reference to the simple block-and-tackle pulley system.
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History
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The development of the pulley is generally credited to Archimedes, although there is no written history specifically announcing its introduction. Archimedes was an ancient Greek scientist and mathematician. He is famous for many advances concerning the use of basic machines, most notably the Archimedes Screw, which was developed as an efficient way to lift water to a higher elevation.
Features
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A pulley is, essentially, a wheel. It rotates freely and has a groove or track along its circumference. A rope or cable can be passed along this groove in order to change direction of force and, when applied properly, to decrease force required to lift or move an object.
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Effects
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In the simplest design, a single pulley can be used to lift an item. This does not reduce the force needed to lift the item. It only changes the direction of the force, which can be a significant advantage.
Adding a second pulley will decrease the force by half. Two pulleys, properly aligned in the system, will allow a 100 lb. weight to be lifted with 50 lbs of force.
Adding a third pulley will, again, reduce the force required by half. In the example, 25 lbs. of force would be required with three pulleys.
Considerations
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There is a sacrifice to be made by using the pulley system. For each pulley added there is an increase in the amount of rope (or cable) required to be pulled in order to move the object. In a single-pulley system the amount of rope pulled is equal to the distance the object moves. In a two-pulley system the amount of rope pulled is twice that of the distance the object moves, and so on.
Tips
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There is a limit to the benefit of the pulley system. The friction caused by the rope moving through the pulley is added to the force required to move the object. In the above examples, friction was not calculated. For each pulley added, the amount of friction increases. Eventually, in any pulley system, the addition of too many pulleys becomes a hindrance rather than an advantage.
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