ix WORK AND ENERGY 121 



tlitiugh tho thivad will support the weight at rest it will bo 

 broken if the weight is allowed to fall. 



EXPT. 114. Show that a falling weight attached by a string 

 to a spring balance extends the .balance beyond tho point 

 which it indicates when the weight is at rest, 



All these examples are cases of the energy of moving bodies, 

 or the energy of motion, or Kinetic Energy. Kinetic Energy is 

 the energy of matter in motion. All energy which is not 

 kinetic is known as Potential Energy. It is capable of be- 

 coming kinetic or active when the conditions become suitable. 

 Imagine a mass raised from the ground and placed upon a high 

 shelf. We know that to place it in this position we must 

 expend a certain amount of work, which is measured by multi- 

 plying its weight by the height through which its mass is raised. 

 Further, we know that just as soon as we release it from its 

 position of rest, making it free to move, it will travel with an 

 ever-increasing velocity until it reaches the ground. On the 

 shelf the mass, by virtue of its position, possessed a certain 

 amount of potential energy exactly equal to the work expended 

 in placing it there. 



Similarly, an ordinary dining-room clock, which is worked by 

 a spring, affords us an example of potential energy. The 

 wound-up spring possesses potential 

 'energy exactly equal to the amount 

 of work done in winding it up. This 

 potential energy is being continually 

 converted into kinetic energy as it 

 becomes unwound in working the 

 clock. / 



The motion of a pendulum / 



affords an interesting example of 

 the two forms of energy. At the 

 end of its swing, in the position A } 

 (Fig. 51), the bob of the pendulum "0^ 



possesses potential energy enough ~rf === * 



to carry it through half an oscilla- FlG 5L _pe ndulum in ~ 



tion, 1 that is, until it reaches its Oscillation, 



lowest position N, when the whole of 



the energy of position which it possessed at A is expended, as it 

 can reach no lower position. But though it lacks potential energy, 



1 Some physicists regard the motion from A to A* as half an oscillation. 



