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How does a pully work?

Question asked by rick in #Coffee Room on Aug 2, 2006
rick · Aug 2, 2006
Rank C2 - EXPERT
This might be a basic question for the Mechanical Engineers. Can anyone explain how does a pully reduce mechanical effort?

Rick Posted in: #Coffee Room
aashima · Aug 3, 2006
Rank B3 - LEADER
let me try.

This might be a basic question for the Mechanical Engineers. Can anyone explain how does a pully reduce mechanical effort?

i guess its somewhere related to dynamic physics.
it could be explained as : rolling friction is less in magnitude than the sliding friction i.e. moving a body from one place to another is easier if somehow we convert its motion into rolling one rather than sliding horizontally.
pulley is based on the same principle... it works in rolling motion hence effort reduced.
that was an idea i could recollect from the basics of physics i studied in my 11th std. do correct me if i am wrong.
instruite · Oct 3, 2006
Rank D2 - MASTER
krishnpr · Dec 20, 2006
Jerry · Dec 22, 2006
Rank C2 - EXPERT

Please read the above link
That's a nice link, very informative. Thanks!

harddisk_techie · Aug 16, 2007
Imagine that you have the arrangement of a 100 pound (45.4 kilogram) weight suspended from a rope, as shown below:

In the above figure, if you are going to suspend the weight in the air then you have to apply an upward force of 100 pounds to the rope. If the rope is 100 feet (30.5 meters) long and you want to lift the weight up 100 feet, you have to pull in 100 feet of rope to do it. This is simple and obvious.
Now imagine that you add a pulley to the mix, as shown below:

Does this change anything? Not really. The only thing that changes is the direction of the force you have to apply to lift the weight. You still have to apply 100 pounds of force to keep the weight suspended, and you still have to reel in 100 feet of rope in order to lift the weight 100 feet.
The following figure shows the arrangement after adding a second pulley:

This arrangement actually does change things in an important way. You can see that the weight is now suspended by two pulleys rather than one. That means the weight is split equally between the two pulleys, so each one holds only half the weight, or 50 pounds (22.7 kilograms). That means that if you want to hold the weight suspended in the air, you only have to apply 50 pounds of force (the ceiling exerts the other 50 pounds of force on the other end of the rope). If you want to lift the weight 100 feet higher, then you have to reel in twice as much rope 0- 200 feet of rope must be pulled in. This demonstrates a force-distance tradeoff. The force has been cut in half but the distance the rope must be pulled has doubled.
The following diagram adds a third and fourth pulley to the arrangement:

In this diagram, the pulley attached to the weight actually consists of two separate pulleys on the same shaft, as shown on the right. This arrangement cuts the force in half and doubles the distance again. To hold the weight in the air you must apply only 25 pounds of force, but to lift the weight 100 feet higher in the air you must now reel in 400 feet of rope. A block and tackle can contain as many pulleys as you like, although at some point the amount of friction in the pulley shafts begins to become a significant source of resistance.

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Gopal Dinesh
Gopal Dinesh · Nov 18, 2008
Rank D2 - MASTER
How does a simple pulley work....?
A rope winds around the pulley so that two ropes hand downwards. Using one side of the rope to apply force causes the pulley to rotate and redirect the force, moving the load upwards even though the pull is applied downwards......
vaitheeswaran · Nov 22, 2008
Rank D3 - MASTER
Can anyone explain how does a pully reduce mechanical effort?

Hi ,

A pulley is a mechanism composed of a wheel with a groove between two flanges around the wheel's circumference. A rope, cable or belt usually runs inside the groove. 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.
A belt and pulley system is characterized by two or more pulleys in common to a belt. This allows for mechanical power, torque, and speed to be transmitted across axes and, if the pulleys are of differing diameters, a mechanical advantage to be realized.
gohm · Nov 23, 2008
Rank A3 - PRO
Lots of good replies and answers. Nothing really new to add here, it works by converting force into several directions thus reducing the initial force.

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