EngQuest 2010

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Pulleys

A pulley consists of a rope (or a belt or a chain) that passes around a wheel. Sometimes the pulley is fixed, and sometimes it moves with the object that is being shifted.

Fixed pulleys

Fixed pulleys are used to change the direction of a force. An example would be a pulley at the top of a flagpole (see Diagram 9). Because of the pulley at the top, the person raising the flag can stand on the ground and hoist the flag by pulling down on the rope. Imagine how much harder it would be without a pulley - the person would need to climb up the flagpole with the flag!

Fixed pulleys do not give a mechanical advantage. The distance that the load moves is exactly the same as the distance moved by the effort.

Other examples of fixed pulleys include:

• the pulley at the top of a yacht mast. The deckhand can raise the sail up the mast by pulling down on the rope. The pulley has changed the direction of the force to make it much more convenient.

• the pulley at the end of the boom of a crane. The crane works by pulling upwards to lift the load. The pulley changes the direction of the force so that the motor that does the lifting can be on the ground rather than at the top of the crane.

Did you spot the two pulley examples in the Simple Machines Animation? How are they different?

Moving pulleys

Diagram 10 shows one moving pulley attached to the engine (the load), and one fixed pulley attached to the support above. This type of pulley system is called a 'block and tackle', where 'block' refers to the pulleys and 'tackle' is the chain that the person is pulling to lift the engine.

The fixed pulley changes the direction of the force to make it more convenient. This means that the person can lift the load by pulling down on the rope. The moving pulley gives a mechanical advantage and makes it easier to lift the load.

In a pulley system, each moving pulley halves the effort, but means that the effort has to be applied for twice the distance. This is why a person can lift an engine out of a car using only a 'block and tackle'. The mechanical advantage is achieved by pulling the chain over a much longer distance than the distance that the engine is actually lifted.