What is a pulley?
Have you seen your school flag hoisted? That is possible with a simple pulley. When you pull down on the rope, the pulley at the top of the pole turns, and the flag goes up. Pulleys change the direction of the force.
A pulley is simply a wheel with a groove 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 or lower heavy objects. With a pulley, the item to be lifted is tied to one end of the rope. 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 ropes are combined.
It is the simplest form of pulleys. Simple pulleys have their axles fixed in place and cannot be moved. The rope moves 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), a 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 effortless than half of the weight of the load. This kind is 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.
The Block and Tackle Pulley System
A block and tackle is a combination of pulleys and ropes used for lifting heavy loads. It was designed by a scientist called Archimedes, who lived on an island called Sicily in Italy.
The block and tackle pulley system is several pulleys sharing one fixed axle at the top. This part is known as the block. The block is connected by ropes to another set of movable pulleys at the bottom, also connected to one axle. The movable part at the bottom is called the tackle.