Pulleys

Pulleys use mechanical advantage, similar to levers, to lift up loads. Pulleys are wheel shaped with a groove that allows a cord to sit inside the groove. They can be used by hand or attached to a motorised winch to increase the amount of weight that can be lifted.

Pulleys can either be fixed (stationary) or moving. The mechanical advantage in a pulley system is equal to the number of ropes leading to or coming from moving pulleys. In the simplest case, a single pulley is used to re-direct the force needed to move a load:

A single pulley changes the direction of force, making pulling down easier than lifting up. Single pulley systems are demonstrated in cranes, lifting a bucket from a well, raising a flag or adjusting window blinds. Even though there is no actual mechanical advantage with one pulley, it is referred to as having a mechanical advantage of one.

One pulley doesn’t make a mechanical advantage, as the same amount of force is needed. However, if additional pulleys are added, a mechanical advantage is created. Using two pulleys together means you need half the force to lift. This is called a block and tackle, and is used to lift large, difficult-shaped objects, such as furniture. Adding more wheels to the block and tackle increases the load it can lift.

In a 2:1 pulley system, shown below, a second pulley is attached to the load, and so moves along with it. In this system, there are two ropes attached to the moving pulley, giving a mechanical advantage of 2. So, the force required to lift the crate will be half of the weight of the load, but for every meter of lift, we'll have to pull 2 m of rope through the system.

The mechanical advantage is equal to the number of sections of rope pulling up on the object.

A 4:1 pulley system is shown below. It contains two moving pulleys, each with two ropes going to or coming from them. The four ropes attached to moving pulleys gives this system a 4:1 mechanical advantage.

The force needed to lift the load is ¼ of the load, but four times as much rope will have to be pulled through the system for each unit of lift of the load.

The mechanical advantage of a movable pulley (one where the pulley can move freely along the rope) is two.

Activity

1. Click on the link below:

https://www.compassproject.net/html5sims/pulleysim/pulley_en.html

2.  On the right-hand side, change the pulley system to a single system, and pull down on the rope using 10 newtons of effort. Make a note of how many newtons were used to lift the load and the distance it rose. 

3.  Reset the simulation and change the pulley system to a double fixed.  

Again, pull on the rope with 10 newtons of effort. Make a note of the distance the load rose. Compare it to the distance the load rose using a single pulley. What do you notice?

 The 10 N load below would still require 10 N of force to lift as the extra pulley is not taking any additional strain in weight - the weight is still taken by only one section of rope.

4. Reset the simulation. Change the pulley to a single compound. Pull down on the rope until the effort arrow goes green. Note the amount of effort in newtons and the height the load was lifted.

Compare this to the two previous attempts. What do you notice? 

The 10 N load below would require half of the force to lift. There are two sections of rope taking the strain, so 5 N of force would be needed to lift it. The mechanical advantage would be 2.

5.  Reset the simulation and change the pully system to a double compound. Again, pull on the rope until the arrow goes green. Note the effort in newtons. 

6. Reset the simulation and change the pully system to a triple compound. Again, pull on the rope until the arrow goes green. Note the effort in newtons. 

You should notice that every time you add a pulley, the amount of effort required is reduced by half.