60 PRACTICAL LESSONS IN SCIENCE. 



has been done upon it, so that both work and energy may be 

 measured by the same unit. The unit usually employed is the 

 work done, or energy imparted, in raising one pound to a ver- 

 tical height of one foot; it is called a foot-pound. With this unit 

 we may measure any species of work, and thereby compare work 

 of any kind with that of any other kind. For instance, let us 

 compare the work done by a man in sawing through a stick of 

 wood, whose saw must move 100 ft. against an average re- 

 sistance of 20 Ibs., with that done by a bullet in penetrating 

 a plank to a depth of two inches (1-6 ft.) against an average 

 resistance of 500 Ibs. Moving a saw 100 ft. against a resist- 

 ance of 20 Ibs. is equivalent to raising 20 Ibs. 100 ft., or doing 

 2,000 ft.-lbs. of work. A bullet moving 1-6 of a foot against 

 500 Ibs. resistance does the same amount of work as is required 

 to raise 500 Ibs. 1-6 of a foot high; that is, about 83 1-3 ft.-lbs. 

 of work ; hence the sawyer does about 24 times as much work as 

 is done by the bullet. The work done, the energy exerted by a 

 moving body, is equal to the weight of the body moved, multi- 

 plied by the number of feet it is raised. With a given velocity a 

 body will rise to a definite elevation, which is determined by 

 dividing the square of the velocity by twice the force of gravity ; 

 formula : -^-, the force of gravity being about 32 1-6. Hence, the 

 energy of a moving body, the work it may do, can be estimated, 

 if we know its weight and its velocity, by the following formula : 

 w * ya . Illustration: The energy of a cannon ball weighing 

 40 Ibs., and moving with the velocity of 800 ft. per second, 

 would be about 398,134 foot-pounds. The kinetic energy of two 

 bodies having the same velocity will vary as the weight of the 

 bodies; that is, if one body weighs 50 Ibs. and the other 100 

 Ibs., the energy in the one case will be twice as great as in 

 the other. The kinetic energy of two bodies having the same 

 weight varies as the square of the velocity; that is, doubling 

 the velocity multiplies the energy by 4, or tripling the velocity 

 multiplies the energy by 9. A railroad train, having a velocity 

 of 20 miles an hour, will, if the steam is shut off, run four 

 times as far as it would if its velocity were only 10 miles an 



