Mechanical Advantage
If frictional losses are neglected, the mechanical advantage of a block and tackle is equal to the number of parts in the line that either attach to or run through the moving block, or the number of supporting ropes. The formula is derived using virtual work in detail in the article mechanical advantage.
An ideal block and tackle with a moving block supported by n rope sections has the mechanical advantage,
where FA is the hauling, or input, force and FB is the load.
The mechanical advantage of a tackle dictates how much easier it is to haul or lift the load. A tackle with a mechanical advantage of 4 (a double tackle) will be able to lift a 100 N load with only 25 N of tension on the hauling part of the line.
Ideal mechanical advantage correlates directly with velocity ratio. The velocity ratio of a tackle refers to the relative velocities of the hauling line to the hauled load. A line with a mechanical advantage of 4 has a velocity ratio of 4:1. In other words, to raise a load at 1 metre per second, the hauling part of the rope must be pulled at 4 metres per second.
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