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Pulley

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This rusty iron wheel has a rope wrapped around it. The wheel and rope hang over an old water well. Together they make up a type of machine called a pulley.  

What Is a Pulley?

A pulley is a simple machine that consists of a rope and grooved wheel. The rope fits into the groove in the wheel, and pulling on the rope turns the wheel. Pulleys are generally used to lift objects, especially heavy objects. The object lifted by a pulley is called the load. The force applied to the pulley is called the effort.

Q : Can you guess what the pulley pictured above is used for?

A : The pulley is used to lift heavy buckets full of water out of the well.

Types of Pulleys

Some pulleys are attached to a beam or other secure surface and remain fixed in place. They are called fixed pulleys. Other pulleys are attached to the object being moved and are moveable themselves. They are called moveable pulleys. Sometimes, fixed and moveable pulleys are used together. They make up a compound pulley. The three types of pulleys are compared in the Table below . You can experiment with an interactive animation of compound pulleys with various numbers of pulleys at this link:

http://www.walter-fendt.de/ph14e/pulleysystem.htm

Q : Which type of pulley is the old well pulley above?

A : The well pulley is a single fixed pulley. It is securely attached to the wooden beam above it.

Pulleys
Type of Pulley How It Works Example No. of Rope Segments Pulling Up Ideal Mechanical Advantage Change in Direction of Force?
Single fixed pulley

Flagpole pulley

1 1 yes
Single moveable pulley

Zip-line pulley

2 2 no
Compound pulley (fixed & moveable pulleys)

Crane pulley

≥ 2 ≥ 2 varies

Mechanical Advantage of Pulleys

The mechanical advantage of a simple machine such as a pulley is the factor by which the machine changes the force applied to it. The ideal mechanical advantage of a machine is its mechanical advantage in the absence of friction. All machines must overcome friction, so the ideal mechanical advantage is always somewhat greater than the actual mechanical advantage of the machine as it is used in the real world.

In a pulley, the ideal mechanical advantage is equal to the number of rope segments pulling up on the object. The more rope segments that are helping to do the lifting work, the less force that is needed for the job. Look at the table of types of pulleys. It gives the ideal mechanical advantage of each type.

  • In the single fixed pulley, only one rope segment pulls up on the load, so the ideal mechanical advantage is 1. In other words, this type of pulley doesn’t increase the force that is applied to it. However, it does change the direction of the force. This allows you to use your weight to pull on one end of the rope and more easily raise the load attached to the other end.
  • In the single moveable pulley, two rope segments pull up on the load, so the ideal mechanical advantage is 2. This type of pulley doesn’t change the direction of the force applied to it, but it increases the force by a factor of 2.
  • In a compound pulley, two or more rope segments pull up on the load, so the ideal mechanical advantage is 2 or greater than 2. This type of pulley may or may not change the direction of the force applied to it—it depends on the number and arrangement of pulleys—but the increase in force may be great.

To learn more about the mechanical advantage of different types of pulleys, watch the video at this link:

http://www.youtube.com/watch?v=yfAdmRJDKIc

Q : If a compound pulley has four rope segments pulling up on the load, by what factor does it multiply the force applied to the pulley?

A : With four rope segments, the ideal mechanical advantage is 4. This means that the compound pulley multiplies the force applied to it by a factor of 4. For example if 400 Newtons of force were applied to the pulley, the pulley would apply 1600 Newtons of force to the load.

Summary

  • A pulley is a simple machine that consists of a rope wrapped around a grooved wheel. Pulleys are generally used to lift a load.
  • There are three types of pulleys: fixed pulleys, moveable pulleys, and compound pulleys. Compound pulleys consist of two or more fixed and moveable pulleys.
  • The ideal mechanical advantage of a pulley equals the number of rope segments pulling up on the load.

Vocabulary

  • pulley : Simple machine that consist of a rope and grooved wheel.

Practice

Watch video number 15 at the following URL, and then answer the questions below.

http://www.animatedscience.co.uk/flv/

  1. If you apply 600 N of input force to a single fixed pulley, the output force is __________, and the mechanical advantage of the pulley is __________.
  2. To get an output force of 600 N with a single moveable pulley, you must apply an input force to the pulley of __________, and the pulley’s mechanical advantage is __________.
  3. If you apply an input force of 200 N to a pulley system has three rope segments lifting the load, then the output force is __________, and the mechanical advantage is __________.

Review

  1. What is a pulley? What work do pulleys do?
  2. Compare and contrast pulleys A and B in the Figure below .

  1. In the compound pulley pictured in the Figure below , how many Newtons of force are applied to the load if 200 Newtons of effort force are applied to the pulley?

  1. How would you construct a compound pulley with an ideal mechanical advantage of 6?

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