CHAPTER III 



ENERGY, THE POWER OF NATURE 



49. Energy is the power of doing work. Work, in the 

 scientific sense, is any change brought about in the position 

 of portions of matter against resistance. Change of position 

 implies motion, and thus work may be spoken of as the 

 moving of matter. Lifting water from a well by means of 

 a bucket and rope is work against the resistance of gravity ; 

 tearing a piece of paper is work against the resistance of 

 cohesion ; pulling a piece of iron from a magnet is work 

 against the resistance of magnetic attraction, and so on. 

 Work is measured by the resistance overcome, and the 

 distance through which it is overcome ; the resistance 

 usually chosen for this purpose is weight or the pull of the 

 Earth on matter in consequence of gravitation ( 38). In 

 English-speaking countries the unit of work usually adopted 

 is the foot-pound, the amount of work necessary to raise I 

 Ib. weight to . the height of I ft. The work of raising I o 

 Ibs. i ft. is 10 foot-pounds, and the work of raising i Ib. 

 10 ft. is 10 foot-pounds also. The work a man of 150 

 Ibs. weight does in climbing to the top of a mountain 10,000 

 ft. high is 1,500,000 foot-pounds, as much work as lifting 

 170 tons of coal from the ground up to carts 4 ft. high. 



50. Newton's first Law of Motion expresses the property 

 of Matter called Inertia, thus : All bodies remain in a state 

 of rest or of uniform motion in a straight line except when 

 compelled by some external power to change that state. On 

 the Earth friction is always at work retarding motion. A 



