# Work done by a machine. Efficiency

Work done by a machine. Efficiency

If the pulley system shown in Fig. 8.7 were a “perfect machine”, i.e., composed of weightless and frictionless strings and pulleys, then a load of 40 N would be raised through a distance of 1 m by an effort of ION moving a distance of 4 m. The work done by the machine on the load is then 40 x 1m = 40 J, while the work done by the effort is lOx 4 m = 40 J also. These are equal, as we should expect for a perfect machine. In practice, however, some work is always wasted in overcoming friction and raising moving parts, and therefore the useful work done by a machine is always less than the work done by the effort. The ratio of the useful work done by the machine to the total work put into the machine is called the efficiency of the machine. Usually, this ratio is expressed as a percentage, so we may write This equation will be found very useful for working out problems, but it is not a fundamental definition of efficiency and should not be used as such.