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What is a Pulley?

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  • Written By: V. Wagner
  • Edited By: Lucy Oppenheimer
  • Last Modified Date: 06 April 2014
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A pulley is a simple machine that is the basis of much modern machinery. This machine changes the direction of an applied force, allowing a heavy load to be moved. A person can pull on the cable part of the device, and it will pull a weight. There are six simple machines: an inclined plane, a lever, a screw, a wedge, a wheel and axle, and a pulley.

At its most simple, a pulley is a wheel with a groove. A rope, belt, or cable runs inside the groove. That mechanism can be used alone or connected with others in a pulley system. The greater the number of pulleys in the system, the less force it will take to lift an object.

A block and tackle system, which is a basic system, can illustrate how this machine provides this kind of lifting advantage. A 10-pound (4.5-kg) weight has a rope attached to it. To lift the weight, a person will need to exert 10 pounds of force (4.5 kilogram-force) straight up. If the weight is attached to one pulley, the person still needs to exert 10 pounds of force (4.5 kgf), but the direction it needs to be exerted in is down, instead of up, making use of the individual's body weight more efficiently. If the weight is attached to two pulleys, the person will need to exert only 5 pounds of force (2.26 kgf). Four pulleys will require 2.5 pounds (1.133 kgf) of force, and so on.

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There are three basic types of pulley systems: (1) fixed, with a fixed axle, (2) movable, with a free axle, and (3) compound, with a combination of fixed and movable axles. Movable and compound axles, by multiplying the force, are more efficient than fixed systems.

Another type of system is the belt and pulley system, which can often be seen under the hood of a car. These systems are closed systems in which several pulleys are connected by one belt. They allow for the transmission and amplification of energy from one pulley to another.

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Discuss this Article

croydon
Post 6

@pastanaga - I think it's an excellent idea to get kids thinking about that kind of thing, particularly if you think they might want to go into engineering or construction or something similar one day. There's nothing like being able to demonstrate an explanation.

pastanaga
Post 5

@SkittisH - That's a really good point. I hadn't thought of the fact that pulleys are still used in elevators. In fact that's probably the place I've seen them most often (in movies at least, when a character is leaping around on top of the elevator).

I saw some at a science museum a week ago and they were quite an interesting exhibit. They had four different rope pulley set ups to demonstrate how much easier it is to lift a weight when you are using the right gear.

It's really quite extraordinary, when you think about it, because it's not the kind of thing that would ever occur to me. I would just tie a rope on and try to lift the whole weight.

seekinfo2
Post 3

My uncle works in construction and they are still using pulleys. Even though there are many high-tech machines, sometimes a old-fashioned pulley is what is needed. They are low cost and very effective.

SkittisH
Post 2

Life for people who live or work in tall buildings would be full of a lot more stairs if not for pulleys, because this invention includes the elevator pulley. Elevators basically are just cars that are moved up and down by a big pulley; they're pulled by motors now, but they were invented before motors were, and that means that the original design was a bit different. Imagine back then...instead of your legs getting tired from climbing stairs, your arms might get tired from the hoisting yourself around in the hand-powered elevators!

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