I'l* THE PROGRESS OF Sf'TFXrK n 



As a matter of fact, it is compelled to travel through 

 its left-hand half-swing, and thus virtually to go 

 up hill. Consequently, the " attractive forces " of 

 the bob and the earth are now acting against it, 

 and constitute a resistance which the charge of 

 kinetic energy has to overcome. But, as this 

 charge represents the operation of the attractive 

 forces during the passage of the bob through the 

 right-hand half-swing down to the centre of the 

 arc, so it must needs be used up by the passage of 

 the bob upwards from the centre of the arc to 

 the summit of the left-hand half-swing. Hence, 

 at this point, the bob comes to a momentary rest. 

 The last fraction of kinetic energy is just neutral- 

 ised by the action of the attractive forces, and 

 the bob has only potential energy equal to that 

 with which it started. So that the sum of the 

 phenomena may be stated thus : At the summit 

 of either half-arc of its swing, the bob has a 

 certain amount of potential energy ; as it descends 

 it gradually exchanges this for kinetic em-ruy, 

 until at the centre it possesses an equivalent 

 amount of kinetic energy ; from this point onwards, 

 it gradually loses kinetic energy as it ascvnds 

 until, at the summit of the other half-arc, it has 

 acquired an exactly similar amount of potential 

 ciM-r^y. Thus, on the whole transaction, nothing 

 is either lost or gained; the quant-ity of energy is 

 always the same, but it passes from one form into 

 the other. 



