SIMPLE MACHINES 197 



less when we walk up than when we run up the hill. 

 The energy consumed in both cases is the same, but to 

 run up consumes energy faster than to walk up. An 

 engine that can do 100 foot pounds of work in a second 

 has more power than one which can only do 50 foot 

 pounds in a second; that is, the former engine can liberate 

 energy faster than the latter. The former engine can 

 also do more work in a day than the latter one. Since 

 work and time can be measured, the power of a machine 

 can also be measured by .finding how much work it can 

 do in a unit of time. Since the unit of work in the English 

 system is the foot pound and the unit of time is the second, 

 the unit of power is a foot pound of work per second, or 



F Xd W 



P = = = one foot pound per second when 



t t 



W and / are unity. The work accomplished by a machine, 

 divided by the time required to do the work, gives the 

 power of the machine. 



As the inch is too small a unit for measuring long 

 distances, so the foot pound per second is too small a 

 unit with which to measure the power of engines. James 

 Watt, the inventor of the steam engine, chose a larger 

 unit, namely, 550 foot pounds per second, for measuring 

 the power of engines; 550 foot pounds per second is called 

 the horse power (H.P.), because it was thought that an 

 average horse could do 550 foot pounds of work in a 

 second. The power of an average man is about one- 

 seventh of a horse power. Railroad locomotives have 

 several hundred horse power, varying from 500 to 1000, 

 and the combined power of the engines of an ocean liner 

 is many thousand H.P., sometimes as much as 70,000 H.P. 1 



1 In the metric system the erg is the absolute unit of work, so the 

 corresponding unit of power is the erg per second. This unit of power is 



