32 The Realm of Nature CHAP. 



unlike the case of inertia ( 50). An exhausted mountaineer, 

 on reaching the summit referred to in 49, might ask, 

 " Where are my million and a half foot-pounds of energy ? 

 are they not lost for ever ? " If the mountain were precipitous 

 on one side the climber could answer his question by an 

 experiment, not on his own person, but on a block of stone 

 of equal weight (150 Ibs.) Such a block in virtue of its 

 elevated position has acquired the power of doing work. 

 The attraction of the Earth draws the stone downward, and 

 once allowed to fall it moves faster and faster until it strikes 

 the ground with enough energy of motion to do 1,500,000 

 foot-pounds of work. This energy in a real case would be 

 expended partly in heating the air during descent, and 

 partly in shattering the stone and heating the fragments 

 and the ground. The amount of energy expended and the 

 ultimate form assumed are the same if the stone rolls down 

 a slope as if it falls vertically. 



53. Energy of Motion. The faster a body is moving 

 the more work it can do, i.e. the more energy it contains. 

 A leaden bullet thrown against a man by the hand might 

 inflict a painful blow, projected from a sling at the same 

 distance it would produce a serious bruise, but fired out of 

 a gun it would pass right through the victim. The greater 

 the velocity of the bullet the greater is its power of doing 

 work. But a small bullet striking a steel target is stopped, 

 while a cannon ball, though moving at the same speed, breaks 

 its way through ; hence the greater the mass in motion the 

 greater is its energy. When the mass of a moving body is 

 doubled its energy is doubled, but when the velocity of a 

 moving body is doubled the energy is increased fourfold. 

 For example, a small river flowing at 6 miles an hour could 

 do as much work in turning mills as a river four times the 

 volume flowing at the rate of 3 miles an hour. This is 

 expressed in the form of a Law Energy of motion is pro- 

 portional to the moving mass and to the square of the 

 velocity. 



54. Potential and Kinetic Energy. Energy of position 

 may be termed an expectant, energy of motion an active 

 power of doing work ; or, to use the usual terms, the former 



