# The Mechanical Advantage of Block & Tackle

••• ship's block and tackle image by David Smith from Fotolia.com
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A block and tackle pulley is a machine that greatly reduces the amount of force necessary to move or lift an object such as a heavy crate. A standard pulley is composed of a single wheel on an axle with a rope running over it. On its own, a pulley can only change the direction of a force applied to an object. A system of pulleys working together can form a block and tackle, which multiplies force in addition to changing the force's direction, meaning less force is required to move an object. The degree to which the block and tackle multiplies force is its mechanical advantage.

## Function

Block and tackles are frequently used in areas where heavy machinery is not available and human power must be substituted instead. In antiquity, block and tackles were used in construction projects to move heavy loads. In the modern era, they are used frequently on boats where it would be impractical to have a crane or other heavy lifting machinery.

## Lifting With A Pulley

If we were attempting to lift a 200 lb. crate from the floor into the rafters of a building, we could do it by using a simple pulley. We would put one pulley in the rafters and pass a rope through it, attaching one end of the rope to the crate. By pulling the other end of the rope (the hauling part), we could lift the crate into the rafters. In this system, each time we pull the rope one foot with 200 lbs. of force, we lift the crate one foot. Lifting with anything less than 200 lbs. of force will not move our 200 lb. crate.

## Lifting With A Block and Tackle

If, instead of attaching the rope directly to the crate, we passed it through a new pulley attached to the crate and then attached the end of the rope to the rafters, we would have a block and tackle. Now, each time we pulled the free end of the rope, the rope would have to travel between the rafters and the crate twice. We would have to pull two feet on the rope to lift the crate a single foot. However, we would only have to pull with 100 lbs. of force.