A Pulley Is Made up of a Rope (Or Something Similar) Around a Wheel. There Are a Couple

Pulleys

A pulley is made up of a rope (or something similar) around a wheel. There are a couple of terms you will need to understand before we get started. A fixed pulley is a pulley that does not change locations- or move up or down. A moveable pulley is a pulley that does move up and down. Pulleys can be single (just one) or in combinations. Pulleys in combinations are called pulley systems or a block and tackle.

The formulas we have used so far still apply. But there is another way to determine IMA. IMA can be determined by counting the number of sections of rope that help to support the weight of the object. These are called supporting ropes. Since pulleys are lifting objects up, supporting ropes are sections that are moving up. That would include sections wrapped around the pulley(s) and the section that is being pulled up by the person using the pulley. Study each image that follows to help learn what a supporting rope is.

A single fixed pulley has one pulley that is attached and cannot move. The section of rope that attaches to the load (Fr) is a supporting rope. The other section, attached to the effort (Fe), is not. You can tell this for 2 reasons. First of all this section is being pulled down. Secondly, it is not helping to support the load. Therefore, there is only one supporting rope. Since there is only one supporting rope, the IMA is 1. With an IMA of 1, no mechanical advantage is gained. We can assume Fr = Fe. But in an actual test, there will be friction so Fr Fe. AMA will be less than one. You may be wondering what the advantage is to a single fixed pulley. The advantage is in changing the direction of the load. Typically this would not be used to lift a heavy object. You will see these used in window blinds and flag poles.

At right is a single moveable pulley. Both of the sections of rope are supporting ropes so the IMA is 2. It may not look like it, but this arrangement will help to lift a heavy object more than the previous one. The effort force will be roughly ½ the Fr. Remember friction will have some affect. Since Fe will be going down, de will be going up.

The image to the left is a single fixed, single moveable pulley.There are three sections of rope, two of which are supporting. The IMA is 2. The effort force will be roughly 1/2 the resistance force and the amount of rope pulled through the pulleys is 2 times greater than the distance the resistance moves.

What are 2 advantages to using pulleys?

What does it mean for a pulley to be moveable?

How is IMA determined for a pulley? See diagrams for examples.

What is the advantage to using a pulley with an IMA of 1?

What type of pulley has an IMA of 1?

What is the relationship between Fe and Fr if the IMA = 1?

In a single fixed pulley, what will be the relationship between de and dr?

What would you guess AMA to be for a pulley with an IMA of 1?

What do pulley systems (combination of fixed and moveable pulley) do to IMA?

What happens to Fe when using a pulley system?

What happens to de when using a pulley system?

In a single pulley moveable pulley, if the Fe is ½ the Fr, how much will de change by?

Draw a single fixed, single moveable pulley in which the rope attaches to the top of the moveable pulley is pulled the opposite way. What will the IMA be for this pulley arrangement?

Find images of pulleys with different number of supporting ropes (pulley arrangements). Be able to recognize and count supporting ropes.