Pulley is one of the earliest devices used by man to hoist or shift heavy loads and raise water from wells. It was developed from the concept of wheel—the most wonderful invention made by primitive man.
Pulley is also a wheel made of wood or metal with a groove cut around its rim. It is fitted with a string or rope around its groove. Pulley turns on axis or an axle and is fixed in a framework called a block.
Do you know how heavy loads are lifted by pulleys? There are several types of pulley systems. For example in a single fixed pulley the load is attached to one end of the string running round the groove of the pulley and the force is applied at the other end of the rope. In this system the force applied is equal to the weight lifted but it has the advantage that force is applied in the downward direction which is easier than pulling up. Single pulleys are used in drawing water from wells etc.
The second system is of a single movable pulley. In this system weight is attached to the movable pulley. A string runs over this pulley whose one end is fixed and the other end passes over a fixed pulley. The second pulley is used to apply force in the downward direction. If both the pulleys are frictionless, then the force applied on the movable pulley becomes just double than the actual force applied. This has a mechanical advantage equal to two. This type of pulley is used in the cranes. Sales in boats are raised and lowered with the help of movable pulleys.
To get more mechanical advantage, several pulleys are used together in many combinations. For instance, in three pulley system, the upper block is fixed and the lower block is movable. A string runs over all the pulleys alternately on movable and fixed pulleys. The weight to be lifted is attached to the lower block. When the number of pulleys is same in both the blocks, the end of the string is tied to the upper block and passed onto the pulleys. If the number of pulleys is not equal in both the blocks, the string starts from the lower block and it is repeatedly turned over the pulleys and is taken up. In fact the weight of the load to be raised with a pulley depends on the number of wheels and how they are connected together.