How do pulleys provide mechanical advantage?
A pulley increases the distance over which a force does work.
Mechanical advantage comes from changing how work is being done on an object. Work is defined as a force exerted over a distance.
When we use simple machines, such as pulleys, the total work done moving an object is the same. In other words;
Simple machines make work easier by either increasing the force applied to an object, as is the case with a lever, or increasing the distance over which that force is applied, which is the case with a pulley.
Consider a weight hanging at the end of a rope. If you want to lift the weight to a height,
The work done in this case is
Now suppose you attach a pulley to the top of the weight. In order to move the weight a distance
But the amount of work done must remain the same.
Now divide both sides by
The amount of force required to lift the weight with a pulley is equal to half the actual weight. That means that a single pulley can provide a 2 to 1 mechanical advantage. You do work over twice the distance you would normally have to in order to decrease the amount of force you put in.