A pulley is essentially a wheel (usually a grooved wheel or sheave) mounted on an axle.

A single pulley can redirect a force.

To begin thinking about how pulleys work, remember that the force in the rope is always the same along its length. In the image below, each side of the pulley carries half the weight. This is the same as if the weight were supported by two separate supports.

Each weight is supported by two connection points.

The idea of dividing and redirecting a force can allow a pully to produce a mechanical advantage. In the image below, the weight is divided by two, and redirected. A person could lift the weight (ideally) using a force equal to half the object’s weight.

The force is divided and redirected.

Pulleys can be used (or rigged) in various arrangements to form various tackles. The mechanical advantage can be increased many times over. Each time the mechanical advantage is multiplied, so is the length that the rope is required to be pulled.