A double pulley system, also known as a block and tackle, provides a mechanical advantage that allows for increased lifting force to be applied when pulling a rope or cable. By dividing weight between multiple pulleys, heavier objects may be lifted with less overall effort.
The pulley belongs to a category of devices known as simple machines. These devices are force multipliers which are designed to make work easier. While a single pulley is only able to change the direction in which force is applied, systems that incorporate multiple pulleys make lifting easier by providing greater mechanical advantage. Each additional pulley added to the system serves to further distribute the weight being lifted; this increases both lifting force and the distance a rope or cable is required to travel to produce it.
Simple machines may be used individually to perform certain tasks, or they may be incorporated as components within more sophisticated devices designed to utilize multiple simple machines at the same time. While additional pulleys multiply force more effectively, they also suffer from the increased friction produced at the point of contact between the rope and each pulley. Multiple pulley systems are commonly used in construction cranes and in the rigging of sailing vessels.