112 HEAT. 



fore, not been converted into potential energy. But we suppose the heat 

 which has appeared to be itself a form of energy, and that the energy 

 which was formerly kinetic has taken this new form. We are strengthened 

 in this conclusion by the fact that the appearance of kinetic energy is 

 frequently accompanied by loss of heat. For example, in the steam- 

 engine, the steam cools in moving the piston and in setting the 

 machinery in motion. 



Strain Energy. Often, when kinetic energy disappears, we observe 

 the appearance of strain in matter. 



If an indiarubber cord, of unstretched length OA (Fig. 72), is fixed at 

 0, and has a mass attached to the end A, this mass has potential energy. 

 If it is now allowed to fall it acquires kinetic energy, and will move down 

 until it comes to rest for a moment at B, a point below the final position 

 of equilibrium. If it is detained in this position it has lost both potential 

 and kinetic energy, but the indiarubber is stretched, and in this stretched 

 condition we must recognise the cord as still possessing energy. 

 For, when the mass is released, it begins to move upwards, and, 

 as the cord contracts, the mass acquires both kinetic and potential 

 energy. We may term this new form of energy, recognised in 

 the stretched or strained condition of the cord, Strain Energy. 

 On allowing the mass to oscillate up and down, we have continual 

 interchanges between the three forms of energy kinetic, potential, 

 and strain. At B the energy is all strain. As the body moves 

 upwards, the strain energy is converted into kinetic and then 

 into potential, so that at an intermediate point C all three co- 

 I | I exist. When the body returns to rest at A, the strain and 

 kinetic energies have disappeared, and we have only potential 

 energy left, and so on. 



We have another case of strain energy when one end of a 

 ,_JS_, wire is fixed and the other end is twisted. Thus, if the upper 



' J end of a vertical wire is fixed, and a mass is attached to the 



FIG. 72. lower end, when the mass is twisted round and released it 

 oscillates to and fro round the wire as axis. We again recognise 

 the wire as possessing strain energy when in this twisted condition. On 

 the molecular theory of matter we may probably describe this strain 

 energy as, at least in part, potential energy of molecular grouping i.e. as 

 energy depending on the relative positions of the molecules to each other. 

 But we cannot give a complete account of it on this theory until we 

 can say how the molecules are arranged in the strain, for the energy 

 does not depend merely on the strain as a whole, but on the mode in 

 which it was effected and the time which has elapsed since it was made. 

 In potential energy due to gravitation, the forces acting are the same 

 for the same grouping at all times, and the potential energy cannot be 

 obtained in any other form, unless the members of the system are 

 allowed to move. But if a wire is strained by twisting, and kept 

 strained, the energy of the strain in general slowly decreases, though 

 the twist of the wire remains the same. If, then, the energy is 

 potential energy of molecular grouping, we must suppose that the 

 molecules do not remain in the same distorted position, but gradually 

 undergo rearrangement of some kind. 



Sound Energy. We frequently find that strain energy disappears, 



