GRAVITATION 119 



be shoved off, this energy at once becomes kinetic. If it 

 falls in the mud, it expresses itself in the depth of the hole 

 made in the mud. If it hits some one on the head, the 

 pain it gives him will be a rough measure of its energy. 

 So we see that energy when merely potential accomplishes 

 nothing. It may lie unexpressed for ages. But once it 

 becomes kinetic, something happens. Kinetic energy is 

 the energy that does things. 



The kind of energy you have just been considering is 

 due to the operation of the law of gravity. You know that 

 energy may be also due to the operation of the laws of 

 heat, or of light, or of electricity, but, whatever the law 

 of nature involved, or the form of energy under considera- 

 tion, the principle of kinetic and potential energy always 

 applies. Thus in coal there is much potential energy, 

 irrespective of its position. This energy is due to condition. 

 It expresses itself in heat when we burn the coal. Heat 

 is a kinetic form of energy. It comes from the coal, be- 

 cause the molecules of coal are in such condition that, 

 when burning, they unite rapidly with oxygen, and give off 

 heat in connection with this process. 



Gravitation. Later we shall discuss those manifesta- 

 tions of energy known as heat, light, and electricity. But 

 let us now consider the law of gravitation. We have al- 

 ready considered many manifestations of energy which 

 are due to its operation. One of these is the fact that 

 water runs down-hill. This is one of the first great facts 

 whose effects you were asked to consider. You have 

 noted its e/ects. Now let us note its cause. 



Whether it be the fall of water or the fall of an apple, 

 the same principle is at work. As you may have heard, 



