4 MATTER AND ENERGY 



Matter cannot of itself cause motion ; when any body 

 of matter seems to cause motion in itself or in some 

 other body, it does so by virtue of the energy which 

 for the time it possesses. A body of matter may gain 

 or lose energy without changing in size or weight, 

 or in any other way that we should commonly notice. 

 Energy must then be something quite different from 

 matter, having no substance and occupying no room, 

 yet very important because without it no motion would 

 be possible. 



6. Force. When the energy of a body is used in an 

 effort to cause motion, we generally say that force is 

 exerted. Thus a moving body is said to " exert force " 

 upon anything that it strikes. Similarly in the case of 

 a bat acting upon a ball, a bowstring upon an arrow, 

 a hanging body upon the support from which it is sus- 

 pended, steam in an engine, or an exploding powder 

 charge in a gun, each is an example of some force 

 acting. Notice that the effect of these forces is to cause 

 motion or to make an effort to do so. A moving body 

 gives motion to the body that it strikes, even if it is 

 only the air; the ball, the arrow, and the bullet in the 

 gun move when forces act upon them. But in some 

 cases, as the hanging body or the steam in a boiler, 

 the force may not be seen to cause motion; still the 

 steam pushes hard upon the sides of the boiler, and the 

 hanging body tries to pull its support downward. In 

 these examples force is exerted; but because it is not 

 great enough it may seem to cause no motion, and we 

 say that it tends to produce motion. 



