62 MECHANICS 



MECHANICS 



WORK AND POWER 



Work is the overcoming of resistance through a distance. 

 The unit of work is the foot-pound; that is, it equals 1 Ib. 

 raised vertically 1 ft. The amount of work done is equal to 

 the resistance in pounds multiplied by the distance in feet 

 through which it is overcome. If a body is lifted, the resist- 

 ance is the weight or the overcoming of the attraction of grav- 

 ity, the work done being the weight in pounds multiplied by 

 the height of the lift in feet. If a body moves in a horizontal 

 direction, the work done is the friction overcome, or the force 

 needed to move a resistant body or combination of bodies, 

 multiplied by the distance moved through. 



It must always be kept in mind that motion in itself is not 

 work and that the mere application of a force also is not work ; 

 a force must act through a distance overcoming resistance in 

 order that work be done. 



Power is the rate of doing work, or the quantity of work 

 done in unit time. The ordinary unit of mechanical power 

 is the horsepower, which is equivalent to 33,000 ft.-lb. per min., 

 or :>:<) ft.-lb. per sec. The term horsepower is commonly 

 abbreviated H. P. 



The work necessary to be done in raising a body weighing 

 W Ib. through a height of h ft. equals W h ft.-lb. The total 

 work that any moving body is capable of doing in being brought 



i iv 



to rest equals its kinetic energy, or - , in which v is the velocity 



of the body, in feet per second, and g = 32.16. 



The kinetic energy of a 200,000-lb.. train running at 40 mi. 

 per hr., or . r >S.7 ft. per sec., is 200,000X58.7^(2X32.16) 

 = 10,714,220 ft.-lb.; the retarding force necessary to stop 

 the train within 2.000 ft. is 10,714.220H-2,000 = 5,357.1 Ib., 

 and the average power required to stop the train in i min. 

 is 10,714.220-5-} = 21. l-'s. tin ft.-lb. per min., or 21,428,440 

 -^ 33,000 = 649.3 H. P. 



