With the investigation of moving bodies it became 

 clear that matter was associated with another entity nor- 

 mally — this was energy. Since a moving body did not 

 appear to differ physically from the same body at rest, but 

 when in motion it could produce considerable effects on 

 being brought to rest, it was said to possess energy due 

 to its motion, or kinetic energy. Similarly, a body located 

 at a height above ground must possess additional energy 

 as compared with that it has when at ground level, since 

 on falling to ground it becomes capable of exerting a 

 large force on the ground while it is being stopped. The 

 body at a height is thus potentially able to produce me- 

 chanical effects, and is therefore said to have potential 

 energy. The swinging pendulum is a very good example of 

 a body possessing both kinetic and potential energy: 

 at the moment of the release of the bob from a position 

 to one side of the rest position, the pendulum has potential 

 energy only; but as the bob accelerates in its arc of swing, 

 the pendulum gains kinetic energy, while losing potential 

 energy since the bob is descending towards its lowest 

 position; when the bob has reached the lowest point in 

 the swing, its potential energy is a minimum, but it is 

 now moving at maximum speed and so its kinetic energy 

 is a maximum at this moment; further motion of the bob 

 upwards to the other end of the arc of swing results in 

 the bob losing speed and thus kinetic energy, but gaining 

 potential energy. It is quite easy to show, when mathe- 

 matical expressions for the kinetic and potential energies 

 of the pendulum are derived, that the maximum values of 

 these two kinds of energy possessed by the oscillating pen- 

 dulum are the same, so that the pendulum converts one 

 kind of energy into the other continually, the total amount 

 of energy being constant. Although many other forms 



47 



