Pulleys

Now that you have learned about mechanical advantage, let's discuss our last example of a simple Pulley Wheelmachine.

In some types of work, pulling is easier than pushing. Sometimes the direction in which you are pulling has to be a little different than the direction that the load needs to be moved. In these cases, wheels and ropes can be used to change the direction of the effort that is used to move the load. These wheels and ropes used together are called a pulley. A pulley is a wheel that turns on an axle. The wheel has a groove in it. A belt or rope is placed around the wheel and sits in the groove.

Fixed Pulley on Flagpole

There are two basic types of pulleys - fixed and movable. A fixed pulley is fixed in place, attached to a stable structure. A flag pole usually uses a fixed pulley to move the flag up or down. Fixed pulleys may be used in warehouses to lift loads off of trucks or trains. A fixed pulley works by changing the direction of the force and allowing you to raise things above your head (such as a flag up a flagpole). However, fixed pulleys do not have any mechanical advantage. You must exert the same effort to lift an object on a fixed pulley as you would if you were to carry the object up yourself.

A moveable pulley does provide mechanical advantage. With a moveable pulley, one end of the rope is tied to something stable and the pulley and the load both move. There are many times when a fixed pulley is used to anchor a moveable pulley. Fixed and moveable pulleys are used together to move very heavy loads, such as pianos. An example of a moveable pulley is a block and tackle that is used to remove engines from cars. Next time you go to a car mechanic shop with your parents, take note of the pulley system that is used to lift engines out of cars!

Block and Tackle Pulley


A moveable pulley gives us mechanical advantage because each rope used in a moveable pulley shares the load. If there are two ropes (as in a simple moveable pulley setup), each rope carries half the load. A simple moveable pulley gives you a mechanical advantage of 2. Used together, there can be many ropes and several wheels. The mechanical advantage of the setup will be the number of sections of rope that you can count. In a multiple movable pulley with three sections of rope, the mechanical advantage would be 3.

You can visit the following website to see an illustration of how a moveable pulley compares to a fixed pulley:
www.howstuffworks.com/pulley.htm.

You may also use search terms to locate other examples, such as "pulley & movement" or "pulley & move" or "pulley & example". Notice how the amount of force required to move a heavy object decreases as you add more pulleys to the system.