Differential pulley load, and an endless rope looped around the pulleys. To avoid slippage, the rope is usually replaced by a chain, and the connected pulleys by sprockets. The two sections of chain carrying the single pulley exert opposing and unequal torques on the connected pulleys, such that only the "difference" of these torques has to be compensated manually by pulling the loose part of the chain. This leads to a mechanical advantage: the force needed to lift a load is only a fraction of the load's weight. At the same time, the distance the load is lifted is smaller than the
Block and tackle each of the parts of the rope must be "F/n." This means the input force on the rope is "F"="F/n." Thus, the block and tackle reduces the input force by the factor "n." 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