Have you seen your school flag being raised before? This is possible with a simple pulley. When you pull down on the rope, the pulley turns and the flag goes up. Pulleys change the direction of the force.
A pulley is simply a wheel with a groove in it, and a rope in the groove. It is also called a drum or a sheave. The groove is important because it helps to keep the rope in place. It is used to lift up or lower heavy objects. With a pulley, an object to be lifted can be tied to one end of the rope, and a force is applied to the other end by pulling the rope downwards. The downward force turns the wheel with the rope and pulls the load upwards at the other end.
Types of Pulleys
There are three types of pulleys. Each pulley system depends on how the wheel and rope are put together.
This is the simplest form of pulleys. Simple pulleys have their axles fixed in place and cannot be moved. The rope moved in the groove of the pulley but the wheel is fixed to one spot. The wheel on your school flag pole is a good example.
In a movable pulley, (see diagram on the left) the load is attached to the pulley and both of them can move from place to place. In this type, one end of the rope is attached to a fixed point that does not move. With a movable pulley, you need less effort to lift a load. Can you tell how the movable pulley is similar to the Class Two Lever? Both of them have the load between the effort and the fulcrum!
This is also called a combined pulley. It is a combination of pulleys designed to make the effort less than half of the weight of the load. This kind is very common at construction sites where cranes lift very heavy steel and concrete objects. One good example of a compound pulley is a Block and Tackle. Click here to see a diagram of the Block and Tackle