42 GENERAL SCIENCE 



energy until released is valueless. It is simply because 

 it can readily be transformed into kinetic energy that we 

 think of it as energy. Although there are several distinct 

 forms of energy, all energy is either kinetic or potential. 

 The capacity for doing work can be possessed only by 

 a body already in motion or by one held under some sort 

 of strain. Other forms of energy are chemical energy, 

 light energy, heat energy, and electrical energy. 



Inertia and Force. All matter has a tendency to 

 continue in its state of rest or motion. If it is at rest, 



force is required to 

 put it in motion, 

 and when it is in 

 motion, force is re- 

 quired to bring it 

 to rest again or 

 even to change its 

 rate of motion. 

 This tendency of 

 matter to continue 



m a Stat 



FIG. 35. Illustration of Inertia. The inertia of 

 the coin causes it to remain on the fingers when or motion W6 Call 

 B card is snapped away. inertia (Figure35) 



We have often noticed this tendency, but we have never 

 heard its name. As the street car starts, the people who 

 are standing are thrown backward and as it stops sud- 

 denly they are thrown forward. This is on account of 

 the inertia of their bodies. A loaded wagon requires 

 more force to start it in motion than to keep it in motion 

 when once started, and if it is moving on a level pave- 

 ment, it requires considerable force in addition to that 

 exerted by gravity to bring it to a stop. The direction 

 of the motion is always in a straight line or simply a 



