CHAPTEE III. 



STATIC AND KINETIC ENERGY. 



Primary 

 forces. 



Electric 

 aud mag- 

 netic forces 

 not 

 primary. 



Static and 



kinetic 



energy. 



Strained 

 elasticity. 



I HAVE defined potential energy as " energy due to 

 the possible action of a primary force- that is not 

 actually in action." ^ 



By primary forces I mean forces which do not originate 

 in any other forces, but are to be referred directly to the 

 laws of nature. Gravity aud chemical affinity are primary 

 forces : matter has been created with them. But the 

 attractive and repulsive forces of electricity and magnetism 

 are not primary forces, because tlie electrised and mag- 

 netised states of matter are not parts of its original 

 constitution. Matter may acquire and may lose elec- 

 tricity or magnetism, but it cannot acquire and cannot 

 lose weight. 



Energy is either potential or actual."^ Energy is also 

 either static or kinetic.^ All potential energy is static ; 

 but actual energy may become static. 



The simplest case of static actual energy is tliat of 

 strained elasticity. If work is done, for instance, by 

 bending a spring or stretching a piece of india-rubber, the 

 work done is represented by the static actual energy due 

 to the strained elasticity ; and when the spring or india- 

 rubber starts back to its original shape, it parts with the 

 energy, which then assumes some other form. A spring 



1 See p. 20. 



2 Kinetic, from ki.u4o>, to move. Profe.ssor Eankine uses the terms 

 potential and actual energy where I speak of static and kinetic. He con- 

 sequently classes as a kind of potential energy what I call static actual 

 energy. The difference is only one of words. 



