DEFINITIONS AND MECHANICAL PRINCIPLES 1 1 



18. Unit of work. It has been stated that the unit of 

 force is the pound. The unit of distance is the foot. The 

 unit of work is unit force acting through unit distance 

 and is named the foot-pound. A foot-pound is then the 

 amount of work performed in raising a mass weighing 

 I pound i foot. It is to be noted that the amount of 

 work done in raising I pound through 10 feet is the same 

 as raising 10 pounds through I foot. It is to be noted 

 further that, in considering the amount of work, time is 

 not taken into account. It is the same regardless of 

 whether I minute or many times I minute was used in 

 performing the operation. The horse-power hour is an- 

 other unit of work commonly used and will be under- 

 stood after power has been defined. 



19. Power is the rate of work. To obtain the power 

 received from any source the number of foot-pounds of 

 work done in a given time must be determined. The unit 

 of power commonly used is the horse power. 



20. A horse power is work at the rate of 33,000 foot- 

 pounds a minute, or 550 pounds a second. That is, if a 

 weight of 33,000 pounds be raised through I foot in I 

 minute, one horse power of work is being done. This unit 

 was arbitrarily chosen by early steam engine manufac- 

 turers to compare their engines with the power of a horse. 



If a horse is walking 2.5 miles an hour and exerting a 

 steady pull on his traces of 150 pounds, the effective 

 energy which he develops is : 



150 X 5280 X 2.5 _ 

 ~ H ' 



60 X 33000 



21. A machine is a device for applying work. By it 

 motion and forces are modified so as to be used to greater 

 advantage. A machine is not a source of work. In fact, 

 the amount of work imparted to a machine always ex- 



