What is a Pulley?
A pulley is a simple machine that was designed in ancient times, but is still the basis of much modern machinery. It is essentially a wheel through which a rope or other cord is wound. One end of the rope is usually anchored to a weight or other load, and when the free end is pulled — usually by a person, but possibly also by a machine — the wheel changes the direction of the applied force and makes it much simpler to lift the attached weight. This type of tool comes in a number of different sizes and specific varieties, but in almost all cases the goal is the same, namely, to make lifting heavy objects easier and to more efficiently channel energy.
Engineers generally talk about “simple machines” in six different categories: inclined planes, levers, screws, wedges, wheel and axels, and pulleys. All are made of basic, everyday components and all make humans’ work simpler in one way or another. Pulleys are usually most concerned with lifting and moving large objects.
In its most basic form, this tool consists of little more than 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 the object.
Types of System
There are three basic types of system: (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, tend to be more efficient than fixed systems.
Wheel-and-rope tools can also be used in combination with other tools for more specific purposes. The so-called “belt and pulley system” is one example. These tend to be “closed” systems in which several grooved wheels are connected by one circular, connected belt, and are designed for the transmission and amplification of energy rather than straight lifting power.
One of the biggest benefits of this type of machine is the ability to move heavy objects and channel lifting energy without exerting much force. In general, the heavier the object the more force a person must exert to move it. Channeling that energy up a rope and around a wheel — or series of wheels, depending on the weight of the object &madash; can make the overall exertion, which is commonly known as “lifting advantage,” much more efficient.
For example, a person will typically need to exert about 10 pounds (4.5 kilograms) of force to lift a 10-pound (4.5-kg) weight, and in most cases that force will need to be applied in a straight upwards motion. If the weight is attached to a pulley, the person will still need to exert the same amount of measured force, but the direction it needs to be exerted in changes. Once attached to the rope system, the person must pull down rather than lift up. This makes more efficient use of the individual’s body weight and exertion.
The more wheels there are in a given system, the more efficient it’s likely to be. If the weight is attached to two systems, for instance, the person will need to exert only 5 pounds (2.26 kg) of force. Four will require 2.5 (1.13 kg) pounds of force, and so on.
These tools can be found in many modern day settings. Elevators are perhaps one of the most common: though they aren’t seen on the outside, large systems of spinning wheels and cables help the cars move up and down. In most cases these are controlled by computers and are lifted through mechanical means rather than manpower, but the efficiency is the same. Many car engines also depend on belted systems to help shuttle energy from the engine to the wheel shafts and other moving parts. Sailboats frequently use them to control tension in the sails, and they're common in cranes and other machinery, too.
@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.
@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.
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.
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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