Simple machines make our lives easier by allowing us to do tasks with less force. The mechanical advantage is the factor of which a machine multiplies the force put into it. For example, if a simple machine has a mechanical advantage of 3 it means that it made the work you did 3 times easier or you only had to put in 1/3 the force you would have if you didn’t have the simple machine.
A pulley is an example of a simple machine that uses ropes around wheels that have a groove in them.
To calculate the mechanical advantage of a pulley you simply have to count the number of rope sections that support whatever object you are lifting (not counting the rope that is attached to the effort). For example, in a one pulley system the MA is 1. In a two pulley system the MA is 2. The more compound the pulley is the easier the load is to lift. If you were lifting a 600kg object with 2 pulleys you would have to use enough effort to pull 300kg to lift the 600kg object! Therefore your mechanical advantage would be 2 (600/300).
MA= load/effort OR number of rope sections
Source: Prentice Hall Science Explorer. Motion and Forces. Copywright 2007 Pearson Education Inc.
I have the same question as Mary below. Did I misunderstand? Because I thought that with the 4 pulley system it would be 500/4= 125.