258 MECHANICS 



5XF ft.-lb.; and, if d is the distance, in feet, from center to 

 center of rails, the vertical force on the outer rail due to this 



5F 



overturning moment is Ib. 



WORK 



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 resistance is the 

 weight, or the overcoming of the attraction of gravity, the work 

 done being the weight W, in pounds, multiplied by the height h 

 of the lift, in feet, or Wh ft.-lb. 



Power is the amount of work performed in a unit of time. 

 One H. P. is 550 ft.-lb. of work in 1 sec., 33,000 ft.-lb. in 1 min. 

 or 1,980,000 ft.-lb. in 1 hr. In the metric system, 1 H. P. is 

 75 meter kilograms per second, usually written 75 m. Kg. sec. 



Kinetic energy is the capacity of a moving body to perform 

 work. If the moving body has a weight W and a velocity v , 

 the work that it is capable of doing in being brought to rest 



Wifl 

 is -- . A body falling through a height of h ft. acquires during 



its fall a velocity of v= ^2gh; its kinetic energy is therefore, 



2g 



EXAMPLE 1. What is the horsepower of a stream of water 

 discharging 12 cu. ft. per sec. through a height of 125 ft.? 



SOLUTION. The kinetic energy per second, is 62.5 X 12 X 125 

 ft.-lb., 62.5 being the weight of 1 cu. ft. of water. The horse- 

 power is, therefore, 6 2.5X12X125 



550 



EXAMPLE 2. What is the kinetic energy per second of a jet 

 of water whose area of cross-section is .1 sq. ft. and whose 

 velocity is 10 ft. per sec.? 



