WASTE U'ORK AND EFFICIENT IVORK 173 



168. The Power or the Speed with which Work is Done. A 



man can load a wagon more quickly than a growing boy. 

 The work done* by the one is equal to the work done by the 

 other, but the man is more efficient, because the time required 

 for a given task is very important. 



An engine which hoists a 5O-pound weight in i second is 

 much more efficient than a man who requires 50 seconds for 

 the same task ; hence in estimating the value of a working 

 agent, whether animal or mechanical, we must consider not 

 only the work done, but the speed with which it is done. 



The rate at which a machine is able to accomplish a unit 

 of work is called power, and the unit of power customarily 

 used is the horse power. 



Any power which can do 550 foot pounds of worjc per 

 second is said to be one horse power (H.P.). This unit was 

 chosen by James Watt, the inventor of a steam engine. Al- 

 though called a horse power it is greater than the power of 

 an average horse. 



An ordinary man can do one sixth of a horse power. The 

 average locomotive of a railroad has more than 500 H. P., while 

 the engine of an ocean liner may have as high as 70,000 H.P. 



169. Waste Work and Efficient Work. In our study of ma- 

 chines we omitted a factor which in practical cases cannot be 

 ignored, namely, friction. No surface can be made perfectly 

 smooth, and when a barrel rolls over an incline, or a rope 

 passes over a pulley, or a cogwheel turns its neighbor, there 

 is rubbing and slipping and sliding. Motion is thus hindered, 

 and the effective value of the acting force is lessened. In or- 

 der to secure the desired result it is necessary to apply a 

 force in excess of that calculated. This extra force, which 

 must be supplied if friction is to be counteracted, is in reality 

 waste work ; the greater the friction, the greater the waste 

 work, and the less we actually get out of the machine. 



