Pulley

A pulley is a wheel with a grooved edge where a long, thin object such as a rope, cable, string or chain can be held. Pulleys can be used to change the direction or magnitude of effort. They are often used to lift objects from the ground.

See Image 1

Single pulleys
A single pulley attached to a fixed, or stationary, point changes the direction of the effort, but it does not change the magnitude of the effort. Let's say we have an object with a mass of 10 kilograms (kg). The Earth pulls down on the object with a force of 98 newtons (N). In order to simply lift the object, you would have to exert a force of more than 98 N upwards. If you were to attach it to a single fixed pulley, however, you would have to exert a force of more than 98 N downwards on the rope. In fact, you might have to exert more than 98 N to overcome the force of friction within the pulley.

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Since the effort to lift the object is not increased, it may seem that single fixed pulleys are not very helpful. They can help, however, by simply changing the direction of the effort. Pulling down on a rope is easier than pulling up on an object because you can use the force of gravity and weight of your body to pull the rope.

Multiple pulleys

If a fixed pulley is combined with a movable pulley, or a pulley that is attached to the load and allowed to move freely along with it, the effort is reduced by half. This happens because the distance the rope must be pulled is doubled. As has been discussed in earlier chapters, when distance is increased, force is decreased. To lift our 10 kg mass with one fixed pulley and one movable pulley, you would have to apply more than 49 N of force, but you would have to move the rope twice as far. For each pulley added to the system, the effort is decreased. Each pulley added, however, also causes additional friction, making the system less efficient.

See Image 3

A system in which two or more pulleys are connected is called a block and tackle or chain hoist. These systems are often used in garages, factories and on cranes to lift heavy loads.