CK-12 Physical Science Concepts - For Middle School

4.47. Pulley http://www.ck12.org

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

MEDIA

Click image to the left for use the URL below.

URL: http://www.ck12.org/flx/render/embeddedobject/82427

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.