Pulley - Rope and Pulley Systems

Rope and Pulley Systems

A rope and pulley system, that is a block and tackle, is characterized by the use of a single continuous rope to transmit a tension force around one or more pulleys to lift or move a load—the rope may be a light line or a strong cable. This system is included in the list of simple machines identified by Renaissance scientists.

If the rope and pulley system does not dissipate or store energy, then its mechanical advantage is the number of parts of the rope that act on the load. This can be shown as follows.

Consider the set of pulleys that form the moving block and the parts of the rope that support this block. If there are p of these parts of the rope supporting the load W, then a force balance on the moving block shows that the tension in each of the parts of the rope must be W/p. This means the input force on the rope is T=W/p. Thus, the block and tackle reduces the input force by the factor p.

• A gun tackle has a single pulley in both the fixed and moving blocks with two rope parts supporting the load W.

• Separation of the pulleys in the gun tackle show the force balance that results in a rope tension of W/2.

• A double tackle has two pulleys in both the fixed and moving blocks with four rope parts supporting the load W.

• Separation of the pulleys in the double tackle show the force balance that results in a rope tension of W/4.