Simple Machines

A pulley is a simple machine consisting of a grooved wheel through which a rope runs. The pulley can be thought of as a kind of lever if one thinks of the grooved wheel as the fulcrum of the lever. Then the effort force is the force applied on one end of the pulley rope, and the resistance force is the weight that is lifted at the opposite end of the pulley rope.

In the simplest form of a pulley, the grooved wheel is attached to some immovable object, such as a ceiling or beam. When a person pulls down on one end of the pulley rope, an object at the opposite end of the rope is raised. In a fixed pulley of this design, the mechanical advantage is one. That is, a person can lift a weight equal to the force applied. The advantage of the pulley is one of direction. An object can be made to move upward or downward with such a pulley. Venetian blinds are a simple example of the fixed pulley.

A lever being used. Photograph by Yoav Levy. Phototake NYC. Reproduced by permission.

In a movable pulley, one end of the pulley rope is attached to a stationary object (such as a ceiling or beam), and the grooved wheel is free to move along the rope. When a person lifts on the free end of the rope, the grooved wheel and any attached weight slides upward on the rope. The mechanical advantage of this kind of pulley is two. That is, a person can lift twice as much weight as the force applied on the free end of the pulley rope.

More complex pulley systems can also be designed. For example, one grooved wheel can be attached to a stationary object, and a second movable pulley can be attached to the pulley rope. When a person pulls on the free end of the pulley rope, a weight attached to the movable pulley can be moved upward with a mechanical advantage of two. In general, in more complicated pulley systems, the mechanical advantage of the pulley is equal to the number of ropes that hold up the weight to be lifted. Combinations of fixed and movable pulleys are also known as a block and tackle. Some blocks and tackles have mechanical advantages high enough to allow a single person to lift weights as heavy as that of an automobile.