82 ENERGY. 



expended is now stored up as potential energy, ready to be 

 reconverted into energy of the kinetic type, whenever we 

 let it drop. As it falls the distance lia, in passing from b 

 to a, this reconversion is gradually going on. When the 

 pendulum reaches a its energy is all kinetic, and just equal 

 to that spent in raising it from a to b. This kinetic energy 

 now carries it on to c, lifting it again through the space ah. 

 Its energy is again all potential just as it was at b. If we 

 could free the pendulum from the resistances of the air 

 and friction, the energy originally imparted to it would 

 swing to and fro between the extremes of all potential and 

 all kinetic; but at every instant, or at every point of the 

 arc traversed, the total energy would be an unvarying 

 quantity, always equal to the energy originally exerted in 

 swinging it from a to b. 



162. Indestructibility of Energy. From the 

 last paragraph it will be seen that, were it not for friction 

 and the resistance of the air, the pendulum would vibrate 

 forever ; that the energy would be indestructible. Energy 

 is withdrawn from the pendulum to overcome these imped- 

 iments, but the energy thus withdrawn is not destroyed. 

 What becomes of it will be seen when we come to study 

 heat and other forms of energy, which result from the 

 motions and positions of the molecules of matter. The 

 truth is that energy is as indestructible as matter. 

 For the present we must admit that a given amount of 

 energy may disappear, and escape our search, but it is only 

 for the present. We shall soon learn to recognize the 

 fugitive even in disguise. 



Note. Physics may now be defined as the science of matter and 

 energy. 



