The pulleys are machines, ie devices used to modify forces or their effects. Among the machines, in addition to the pulleys may be mentioned the lever, the inclined plane and the lathe. In all machines exerts a force called Power, which serves to overcome a so-called resistance. In reality, the principles that allow lifting levers are the same as the pulleys make lifting. In the case of the pulleys the power arm is identical to the resistance arm, so to lift a load of a given weight must exert a force equal to that performed by the first. In the case of movable pulleys the power arm is longer than that of resistance, making it possible to lift a load with a smaller force to that exerted by it.
It is based on first-class lever in which the fulcrum is in the middle, the power at one end and the resistance in the other. However, the pulley has the advantage of changing the direction of the force on the ground that the weight of the triggers that adds to the tensile stress carried out.
It is based on second-class lever, where the fulcrum is at one end, the resistance in the environment and the power at the other end. With this type of block can be overcome with a low power, highly resistant.