The third simple machine introduced to the Year 3 children was the pulley. As with gears, pulleys are used to make our lives easier – small effort in: big effort out. Pulleys have been used for thousands of years to help life heavy objects – the first records of pulley uses date back to Mesopotamia, where people used rope pulleys for hoisting water as early a 1500 BC.
We learned that a pulley is simply a wheel, with a groove on the outside for a rope or a cable, which may be attached to a load. The more pulleys that are used in a system, the easier it is to lift the load.
The children were asked if they could think of examples of where pulleys are used. They found this more difficult than thinking of levers and gears. As well as cranes, we came up with the action of lifting water from a well; rock climbing and pulling fish in with a rod.
We looked at the different parts of the pulley and noticed that they were similar to the other simple machines that we had looked at.
This time the effort refers to a pulling force.
The children remembered the terms very well and had no problem identifying the various components.
There are three basic kinds of pulleys: fixed pulleys, moveable pulleys and combination pulleys.
This table, taken from the K’nex Levers & Pulleys manual, gives an excellent summary:
In all the simple machines that we have looked at in Year 3, mechanical advantage has been the key. This is the amount of help you can get from using a machine to do a job, instead of just using purely your own strength.
Mechanical advantage is a ratio between the number of pulleys and the amount of effort needed. If you have a single pulley then there is no mechanical advantage: you are having to put in the same effort; although being able to pull down, in the same direction as gravity, does make it easier.
If you have a two pulley system, then you have to put half the effort in. It’s as though you are now twice as strong. The mechanical advantage is 2:1.
The children found this maths quite easy to understand and were soon able to work out the mechanical advantage for any number of pulleys: 150 pulleys would require 1/150 of the effort – MA 150:1. This is a really useful way to introduce ratio to younger children.
There are however, some disadvantages to using pulleys, which applied to levers and gears too. The more pulleys you have in your system, the longer the rope needs to be. For example, if you have 150 pulleys, then you will have to pull a rope that is 150 times longer than if you had only 1 pulley – this might end up being an awful long distance to pull.
This simple table provides a summary:
|Levers||Lift heavy objects||Long levers|
|Gears||Go up hills||Slow|
|Pulleys||Pull heavy objects||Lots of rope|
The children were then tasked with constructing their own pulley systems, which they did extremely well. They had to remember that one fixed pulley didn’t offer any mechanical advantage.