WOKK 179 



The total work out is in this case and in all cases the same 

 as the total work in. Useful work out is less than total 

 work in. 



199. Efficiency. The proportion of wasted work will not 

 always be the same as in the problem above. If the machine 

 is very carefully constructed, the friction may be so reduced 

 that there is very little waste. Such a machine is said to be 

 very efficient, since it does its work with but little waste. 



The efficiency of a machine is measured by the fraction of 

 the total work done upon it which appears in the useful work 

 accomplished by it. 



useful work out 

 Lmciency = - - - - : - 

 total work in 



In the particular case which we have been considering, the 

 efficiency is calculated as follows : 



25,000 



Efficiency = -, or 80 per cent. 

 o 



200. Summary. The principles which have been developed 

 in connection with the study of the pulley apply to all 

 machines. 



Work is equal to force times distance. There is no work 

 without movement. 



Force applied times distance equals work in. 



Resistance times distance traveled equals useful work out. 



Useful work out is always less than work in. 



Total work out equals total work in. 



Machines do not increase work ; they only transmit it. 



