Pulley If there are "p" of these parts of the rope supporting the load "W," then a force balance on the moving block shows that the tension in each of the parts of the rope must be "W/p." This means the input force on the rope is "T"="W/p." Thus, the block and tackle reduces the input force by the factor "p." The simplest theory of operation for a pulley system assumes that the pulleys and lines are weightless, and that there is no energy loss due to friction. It is also assumed that the lines do not stretch. In equilibrium, the
Block and tackle blocks. Frictional losses also mean there is a practical point at which the benefit of adding a further sheave is offset by the incremental increase in friction which would require additional force to be applied in order to lift the load. Too much friction may result in the tackle not allowing the load to be released easily, or by the reduction in force needed to move the load being judged insufficient because undue friction has to be overcome as well. A block which is open or can be opened to engage with a bight, without the necessity to thread the