542 



MACHINES, WORK, AND ENERGY 



order to work at this rate, however, a horse must exert an 

 average pull of 125 Ib. while walking at the rate of 3 miles 

 per hour, or he must exert an average pull of 150 Ib. while 

 walking at the rate of 2% miles per hour. (Calculate.) 

 A strong horse weighing 1400 Ib. can stand it to work at this 

 rate, 10 hours each day (see Art. 657). An average man 

 can stand it to work at the rate of about J^ of a horse-power, 

 eight hours each day. 



IV. MACHINES 



640. Machines and Their Uses. Any device is called a 

 machine if it is used to transfer or transform energy, or if it is 



FIG. 339. A farm gasoline tractor pulling a three-bottom plow and doing 

 the work of 6 or 8 horses or of 50 men. 



used to change the direction, or magnitude of a force doing work 

 (see Art. 85, Definition of Energy). 



Man uses machines for a great variety of purposes. A crow- 

 bar, a set of pulleys, or a jackscrew enables a man to move a 

 body whose weight is so great that he would be unable to move 

 it without the use of the machine. A fishing pole enables the 

 boy to drop his hook quietly into the pool at a point he other- 

 wise could not reach; it also enables him to jerk the hook more 



