# Block and tackle - effort required to raise a given weight

## Description

A block and tackle is a system of two or more pulleys with a rope or cable threaded between them, usually used to lift or pull heavy loads.The formula used to find the effort required to raise a given weight is:

Related formulas## Variables

Fa | the force applied to the hauling part of the line (the input force) (N) |

L | the weight of the load (the output force) (N) |

N | the ideal mechanical advantage of the system (which is the same as the number of segments of line extending from the moving block) (dimensionless) |

eff | mechanical efficiency of the system (equal to one for an ideal frictionless system; a fraction less than one for real-world systems with energy losses due to friction and other causes) (dimensionless) |