THE FORMS OF ENERGY. 109 



forces of matter are made manifest have one common origin ; or, in other 

 words, are so directly related and materially dependent that they are 

 convertible, as it were, one into another, and possess equivalents of 

 power in their action. In modern times the proofs of their converti- 

 bility have been accumulated to a very considerable extent, and a 

 commencement made of the determination of their equivalent forces" 

 (Exp. lies., iii. p. 1). * 



This is a full statement in the language of the time of the principle 

 of the conservation of energy, made just when the principle was 

 struggling into general recognition, and before it was placed on 

 a firm experimental foundation by the work of Joule and others. 

 Faraday's statement is divisible into two parts, the first asserting the 

 existence and mutual convertibility of " forces," the second asserting this 

 convertibility in definite ratios. Our account will naturally divide into 

 two corresponding parts. We shall give 



(1) An account of each of the forms of energy hitherto recognised, 

 and a statement of the evidence which leads to the belief that they are 

 all forms of one " something " which we term energy, t 



(2) An account of the modes of measuring the amount of each form 

 in a system, and an examination of the evidence which leads to the 

 belief that one form changes into another in a definite ratio or at a fixed 

 " rate of exchange." We shall then see in what sense we can hold that 

 the total quantity of energy is constant. 



The Various Forms Of Energy. We say that a man possesses 

 "energy" when he can do work in overcoming obstacles, either mental 

 or physical. By analogy, the same word is used in physics, and we say 

 that a body possesses energy, when by virtue of its motion or condition 

 it can do work in moving either itself or other bodies against resistance. 



When the body can do work by virtue of its motion, it is said to 

 possess 



Kinetic Energy. Tf a body of mass m starts with velocity v, and 

 moves through a distance s against a uniform force which would pro- 

 duce in it acceleration a, and therefore be measured by ma, we know 

 that its velocity v at the end of s is given by 



If we multiply each side of this equation by m we get 



mv 2 - mv' 2 

 - =mas 



= force x distance travelled against it, 

 = work done against the force. 



If v' = the whole motion is exhausted, and 



n 



- = work which the body does against the force in coming to rest. 

 jL 



* An excellent history of the growth of the doctrine of energy is given in Hel tn's 

 Die Energetik. 



+ The footnote on p. 116 may be read here. 



