Editing Block and tackle (section)

===Rove to advantage===
An ideal block and tackle with a moving block supported by ''n'' rope sections has the mechanical advantage (MA),
<math display="block">MA = \frac{F_B}{F_A} = n,\!</math>
where F<sub>A</sub> is the hauling (or input) force and F<sub>B</sub> is the load.

Consider the set of pulleys that form the moving block and the parts of the rope that support this block. If there are ''n'' of these parts of the rope supporting the load ''F<sub>B</sub>,'' then a force balance on the moving block shows that the tension in each of the parts of the rope must be ''F<sub>B</sub>/n.''  This means the input force on the rope is ''F<sub>A</sub>''=''F<sub>B</sub>/n.''  Thus, the block and tackle reduces the input force by the factor ''n.''

[[Image:Polispasto4.jpg|thumb|upright|A double tackle has two pulleys in both the fixed and moving blocks with four rope parts (n) supporting the load (F<sub>B</sub>) of 100&nbsp;N. The mechanical advantage is 4, requiring a force of only 25&nbsp;N to lift the load.]]

<gallery>
Image:Pulley1a.svg|Separation of the pulleys in the gun tackle show the force balance that results in a rope tension of ''W/2.'' 
Image:Pulley3a.svg|Separation of the pulleys in the double tackle show the force balance that results in a rope tension of ''W/4.''
</gallery>

Ideal mechanical advantage correlates directly with [[velocity ratio]]. The velocity ratio of a tackle is the ratio between the velocity of the hauling line to that of 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. Therefore, the mechanical advantage of a double tackle is 4.