Physics / Engineering

Pulley systems

Count the rope segments and you have the mechanical advantage. The work does not change: a machine buys force with distance, never with energy.

By Mykhailo Makarov

Actual application image

What happens

Count the strands pulling up on the load and you have the mechanical advantage - it is not a number out of a table. Lift the same weight through a fixed block or through a six-fold tackle and the work you put in is identical; the tackle only spreads it over six metres of rope. Two markers rise beside the rig, the load and the rope hauled in, and they separate at exactly the advantage: force is bought with distance and never with energy. Add friction and the work in rises while the work out does not move - efficiency only ever costs.

Equations used in the module

Equations
F = W / n,   n = rope segments holding the load
d_rope = n · d_load   (work is not multiplied)
η = W d_load / (F d_rope)

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