As Pulley is a type of simple machine it helps to reverse the direction of lifting force. When one end of the rope is pulled downwards, the load on the other end of the rope is pulled upward. Therefore, the direction of force is changed from downwards to upwards. Pulley can work on one, two and four wheels.
The theory of operation for a pulley system assumes that the pulleys and lines are weightless, and that there is no energy loss due to friction. The more pulleys, the easier it is to pull or lift an object. If pulley involved is more, the greater distance to pull, is still easier to lift an object.
Fixed And Moving Pulleys
→ A Fixed Pulley:
When a pulley is hinged to a fixed support, it is a Fixed Pulley. It does not mean that it cannot move. It changes the direction of the force on a rope or belt that moves along its circumference. It is free to rotate around the fixed pivot.
→ A Moving Pulley:
The pulley which is made free to move up and down along with the load, it is called a Movable Pulley. It can rotate like fixed pulley and is supported by two parts of the same rope and has a mechanical advantage of two.
Hence, we can say that a single fixed pulley can be used to change only the direction of the applied force whereas a movable pulley can be used to decrease the input force required.