122 ELEMENTARY GENERAL SCIENCE CHAP. 



since it is a mass moving with the velocity it has gained in its 

 passage from A to N, it possesses energy of motion or kinetic 

 energy enough to carry it up to its next position of rest at A' 

 where the only energy it will have will be again potential. 

 Through the next oscillation from A' to A it will pass through 

 just the same transformations again. 



At any point in the swing the pendulum will possess a certain 

 amount of energy due to position and a certain amount due to 

 motion, but the total amount of energy the sum of the potential 

 and the kinetic energy is always the same, the loss of one form 

 of energy being exactly equalised by the gain of the other. 



Measurement of Kinetic Energy. When we wish to 

 measure the energy of moving bodies we have to find an ex- 

 pression which will be equal to the amount of work these bodies 

 are capable of performing when the whole of their energy is 

 converted into work. Such an expression is easily found from 

 first principles, as the reader will learn as he pursues his 

 studies. 



If M represents the mass of such a moving body and v its 



TWT ^ 



velocity, the expression -^ is a measure of its energy, and 



gives us a means of calculating the energy possessed by any 

 body in motion in terms of its mass and its velocity. If we 

 wish to express it in foot-pounds we shall, as we have seen, 

 divide its value by g. 



Kinetic energy, or the energy of moving bodies, is equal to 

 one-half the product of the body's mass and the square of its 

 velocity. Hence, the measure of energy is dependent on the 

 units of mass and velocity employed. 



Forms of Energy. A body may possess energy due to 

 other causes than that of the actual motion of the body as a 

 whole. When it is in rapid vibration, or when it is heated, or 

 when it is electrified, it is endowed with energy in consequence 

 of these conditions. But when a body is in rapid vibration it 

 gives out sound or becomes a sounding body, hence we may 

 regard sound as a form of energy. We shall see that work may 

 l)e done by the passage of heat from a hot body to a cold one, and, 

 in consequence, heat is properly regarded as another form of 

 energy. An intensely hot body emits light, hence it would seem 

 that light and heat have a common cause and that we must also 

 regard light, like heat, as a manifestation of energy. When a 

 body is electrified it has the power of attracting unelectrified and 



